Cheap Lithium Charger for Electronics Tinkerers

[electron_head]

This is basically what I have been using to charge lithium cells up to 1250mAH. It's simple, cheap and the components are all common as. I'm putting it here for others to veiw for ideas on building their own and take no liability for any inaccuracy or mistakes.

You only need basic electronics skills to make it but it must be checked before use with a good quality multimeter that will allow you to set the output voltage accurately and test the current limiting as well.

One of these on there own is ok but I make a bank of 4 to charge 4 flightpacks at a time. I built one with a switch to select the number of cells but after plugging in a 2 cell pack with the switch set to 3 cells I decided it was best to build dedicated charger that do one size flightpack.


Lithium Charger parts list:

2 x LM317 adj Voltage Regulator IC
2 x Heatsinks for the above
5W Wirewound resistor (1.2 ohm resistor for 1.04A current)
(1 ohm for 1.25A)(1.8 ohm for 700mA)(5 ohm for 250mA)

120 ohm resister 1/4 Watt MF 1%
Voltage regulating resistor 1/4 Watt MF 1 %

Veriboard on which to mount components and other items like cable,vented case, and capacitors if you fell they a necessary.

current regulating resistor ( do not exceed a current greater than 1C):
R = 1.25/Current in amps of current regulator.
eg. for a 700mAH Flightpack
1.25/0.7A = 1.78 ohm

voltage regulating resistor:
R = (96 x Vout) - 120
eg. for a 2 Cell pack being charged to 8.4 volts
(96 x 8.4v) - 120 = 686 ohms
eg. for a 3 Cell pack being charged to 12.6 volts
(96 x 12.6v) - 120 = 1090 ohms

With voltage regulating resistors you may have to use 2 to get the value you want.

After you have built it check that the output voltage is no greater than 4.2 volts per cell. Then set your multimeter up to read amps and short out the outputs with the probes to make sure the current is correct.

If you put a 5 Ampmeter on the input side you check the state of charge of packs and watch the amps go down as charging is completed ( For the first hour of charging a flat pack the reading
should be 1CAmp ie 0.7amp for a 700mAH pack )

As I have said above do not attempt to build unless you know what you are doing or can be helped by someone that does. Shop around for components if you can as big retailers here charge double the price of elsewhere.

I will post a pic of my charger soon.

[electron_head]

Heres the circuit it didn't upload in initail post

[electron_head]

This charger is dedicated to charging 4 X packs of 2 cell 700mAH batteries.
I generally do all my charging at home but this can also be pluged into the cigarette light socket in the car as well. The plug contains a 5 amp fuse.

[jperch]

Do I understand correctly that you use 2 LM317's per charger?

There is a simpler circuit that uses 1 LM317 and a simple NPN transistor to do the same thing. If you down load the datasheet for the 317 from TI's web site (or you can find it on Digikey's site) and look in the applications section. There is a schematic for a 6V current limited supply. If you use the theory in the datasheet, it is a simple matter to change the component values to get the voltage to 8.4V and the current to what ever you want.

There is also a link in one of the threads here to a web page that has already done this. I will try to find it and post it again.

Joe

[electron_head]

jperch:

Yes I have seen other circuits but with LM317's only costing $1.57c NZ that about 80c US cost is not an issue. This is another alternative.

I have seen many charger circuits and built a few this one has around 5 common components and works really well how many do the other circuits have?

[electron_head]

http://www.shdesigns.org/lionchg.html you are telling me this one is simpler?

[Radioguy]

Electron:

Thanks for posting that nice simple circuit ,,,, I'll toss one together tonite!

Lee

[jperch]

http://www.shdesigns.org/lionchg.html you are telling me this one is simpler?

Yes, that's the one. Okay, maybe it's not "simpler" as there are a few more components. But I like it better as there is only one 317 per charger circuit. It is not a huge expense but it just seems more efficient to me.

I just drew up artwork to put four of these circuits on a single PC board. It is a double sided board but I think I can redo it for single sided copper. This way it will be easier to construct. I have 10 2 cell lithium packs and I could use a few of these little chargers.

Joe

[tekochip]

I built Scott's charger, I suppose it doesn't matter much, an extra LM317, or a couple of transistors.

I'm still using Scott's charger, it's a real workhorse for me. I did a PCB on Eagle, if anybody's interested shoot me an email and I'll send you the files, with all credit to Scott, of course.

[electron_head]

Originally posted by jperch
Yes, that's the one. Okay, maybe it's not "simpler" as there are a few more components. But I like it better as there is only one 317 per charger circuit. It is not a huge expense but it just seems more efficient to me.


Yes it may be be more efficient if the supply voltage is close to the ouput voltage and dropout becomes an issue.

2 LM317's might be better in other cases as the load is pretty much shared evenly between the two so thermal issues are halved.

I consider the component cost minimal anyway with things like the JST connectors being far greater. I personally would not build one of these without an ampmeter on the charger or the power supply feeding it.

When I first started looking at building I saw that achieving accurate voltage was the biggest issue. At first reading the manufactering tolerance on this grade of 317's was a bit sloopy
to get really close.

Even when using 1% MF resistors you can still add another in the 1 to 5 ohm range to get even closer.

With the current and voltage parts of the circuit completely independent you can adjust either and not worry about to much crossover.

I'm not saying one circuit is better than the other as they both have merits and people can decide depending on their requirements.

One issue both have is that if a very flat pack (under 3 volts per cell) is plugged into them it may be damaged.



Regards

[jperch]

Electron Head,

You are correct about the flat pack issue. However, I intend to only use this in normal situations. The fact that it is cheap and easy means that fancy features are not included. That is a fact of life.

As for efficiency, my comment about that was not really a qualitative statement. I was merely trying to state that it "felt" more efficient. I have not done any electrical efficiency analysis or anything like that. You are also correct that the fancy connectors we use on these packs are most likely going to be as expensive if not more expensive than the whole circuit.

Oh well, it is still fun to tinker.;)

Joe

[Radioguy]

Electron:

I followed the posts from my office today and noted with great interest the reference to the LM350. After having a look at the data sheet very briefly, I see it is good for 3 amps vs the '317's 1.5 amps.

The formula for the current limiting resistor is the same, but I will be using this charger to do 3 ETec ipoly cells in series with 2 in parallel for a total of 2500 Ma.

Will the 5 watt rating be sufficient if I specify the .5 ohm resistor called for in the formula, or would a higher rating be required?

Your neat little circuit came along at just the right time, as I happen to have a couple of spare 317's and heatsinks just looking for a new home!

Going to the 350 in my next version of this charger might just be the easy ticket to the land of Nirvana though, for those parallel packs.

Regards,
Lee Smith

[electron_head]

Hi Lee,

The 5W resistor should be no problem as these hardly even begin to warm up. If the load is great they can get extremely hot so be sure to mount off the circuit board. These have been used in mechanical speed controls for 540 sized motors on which the current draw is well over the 2.5 amps your talking here.


The LM350 is a more expense chip and this little circuit is more for the indoor/parkflyer sized planes. The pack 3S2P pack your talking about sounds like it is going to power something considerably larger?

Both these Voltage Regs are virtually indestructable and will generally go out on thermal overload before any damage to them is done.

Please keep us posted on how it goes.

[jmbig]

Why not use a LDO and use a 13v car battery to charge
3 Li-Po's national will samples as requested.
http://www.national.com/catalog/AnalogRegulators.html

[electron_head]

Lee,

The lowest value you can use for the current limiting resistor on the LM317 is 0.8 ohm and for the LM350 its 0.4 ohm.

http://www.maxim-ic.com/PowerSupplies.cfm make some of the best voltage regs and charge pump IC's and there are also a few IC's about designed specifically for Lithium cells.

I think Maxim make some really great regulators with charge pumps in them as well that will let you charge 4 cell packs on a 12 volt source. But if you are going to get to complicated you may as well go and buy a commercial charger that you know will do the job right.

[electron_head]

Originally posted by jmbig
Why not use a LDO and use a 13v car battery to charge
3 Li-Po's national will samples as requested.
http://www.national.com/catalog/AnalogRegulators.html

I think thats a great idea for three cell packs as it would be impossible to flatten your car battery to the point of it not being able to start the car.

[electron_head]

Just had a look at what Maxim offer and the MAX745 Switch-Mode Lithium-Ion Battery-Charger looks to be simple and high powered (4 Amp)

Charges 1 to 4 Li+ Battery Cells
±0.75% Voltage-Regulation Accuracy Using 1% Resistors
Provides up to 4A without Excessive Heating
90% Efficient
Uses Low-Cost Set Resistors and N-Channel Switch
Up to 24V Input
Up to 18V Maximum Battery Voltage
300kHz Pulse-Width Modulated (PWM) Operation Low-Noise, Small Components
Stand-Alone Operation—No Microcontroller Needed

Maxim are going to hate me if everyone starts asking for their free samples :mad:

[jimsp]

Originally posted by electron_head
Just had a look at what Maxim offer and the MAX745 Switch-Mode Lithium-Ion Battery-Charger looks to be simple and high powered (4 Amp)

...
This chip has 20 pins and is about 5mm(.2") square. As a hobbiest how would ever solder this chip?

[Radioguy]

Electron:

Thanks for the assistance. I went to the component supply house on the way home and they didn't have any 350's in stock, so looks like tonight's project will be a 317 version....that is if I can slip away from another obligation or two this evening.

They had a good supply of 5 and 10 watt wirewound resistors on the shelf for $1.49 Canadian, so I picked up a few of various values to play with.

I'm pretty sure I have a spare 5 amp meter sitting on the shelf from my various ham radio fleamarket trips over the years.....now, what "safe place" did I store those meters so they would be safe from house-breakers in the dead of night?

I'll be using the ETec's in my Dandy Sport with a Razor 400 geared, driven by the total of 6 LiPoly cells via a XCastle Creations Phoenix 25 amp controller......should make that little puppy sit up on it's haunches and howl!

The "proper charger" has been ordered, but I like the ability to fool around a little, and I reckon I can't go wrong with having a spare charger available. Here is the one which should be here in a week or two from the Toronto area.

http://www.canadianmodelsupply.com/apache.php

I'll keep you and the others posted on my progress.

Regards,
Lee

[electron_head]

Originally posted by jimsp
This chip has 20 pins and is about 5mm(.2") square. As a hobbiest how would ever solder this chip?

If you read the pinout descriptions is really quite straight forward.
Input voltage can be anywhere between 6 -18 volts.


As far as hobbists go check out what Koichi Tanaka is doing here
http://www.rcgroups.com/forums/showthread.php?s=&threadid=24711&highlight=Infrared
this is really cutting edge stuff.

I have let myself get off the topic of this thread abit so in future try to stick to cheap and simple ideas.

[electron_head]

After using my charger from the cigerette lighter socket today I found out its weakness over other designs.

Becuase I have 2 regulators in series and each has a droppout voltage of 3 volts that means the input voltage must be 6 volts above the output to charge at full current.

So for 8.4 volts you need 14.4 volts input. (Actually 13.7 volts was enough to do all 4 cells at full current)

If you want to charge 3 cells 12.6 volts thats 18.6 volts.


Hope this informs people that may be considering building as to its shortcomings

[jimsp]

Originally posted by electron_head
If you read the pinout descriptions is really quite straight forward.
Input voltage can be anywhere between 6 -18 volts.

I think you misunderstood my question. I wasn't interested in what pins to hook to- I was wondering how do you solder to pins that only 1/2mm apart?

[electron_head]

Joe : With a very small soldering iron I suppose.

I beginning to think this approach is not the way to go.

The better way seems to be recycling the circuits from out of existing equipment.

[Radioguy]

Electron:

Well, that might explain why I didn't have any luck getting it to draw current at 16.5 volts then!

Was able to get 12.6 v at the output terminals after replacing the fixed value voltage adjust resistor with a spare 10 turn, 5K pot and twisting it up to about 4K, the voltage varied smoothly and beautifully but current remained at zero.

My variable power supply that feeds this unit can put out over 20 volts, and in a moment of frustration, I did crank it up to max value, but still no current draw observed on the meter. The current limiting resistor was a 1 ohm, but 5% tolerance, and measures 1.68 ohms. This should still draw around 800 Ma by doing some quick interpolation of values in your formula.

A check of battery voltage (no load) showed a little over 13 volts. I discharged it to around 10.5 volts and connected it, but still no go. Gave up at midnight and hit the sack.

Wiring was confirmed several times. Any suggestions?

Regards,
Lee

[electron_head]

Originally posted by electron_head
Hey I just corrected a very bad mistake on the circuit picture. Did anyone notice what it was?


Lee I hope you did not save my original picture and work off that as I had the output wire coming off the adjust pin on the voltage regulator.:mad:

Very stupid mistake on my part but what was more amazing was no-one picked the mistake up after 200 plus views.

output comes from the output pin. Thats the one in the middle (pin 2)

[Radioguy]

Elect:


Yes, Electro, I built it according to the original pictutre.

Your posted diagram still shows the output coming from the Adj pin #1. I printed the "new" picture late last night (in colour no less) ... and compared the "old" and the "new" quite closely, sort of like a "What's Wrong With This Picture?" excersize.....to what I thought was my relief, I did not note any change.

I was pretty frustrated by the end of the evening and knew I was very over-tired from the cares and woes of the week to date, so I thought a little sleep would be the answer to life's mysteries.

Perhaps you reposted the original drawing instead of the intended new one?

Do I have it right then, that the only difference required is to move the battery terminal + out lead to the number 2 terminal of the voltage regulator IC?

Take a look and set me straight. I'll be able to steal a few minutes this evening to get this operational.

Lee

[electron_head]

Yes that is correct . output wire from pin 2 not pin 1 as on the original picture.

Only reason I can see for you not getting the change is the original is on some other servers cache close to you.


uploading again with different name

[Radioguy]

E:

Thanks for the update. I'll move that lead this evening after supper and run the power supply up to 18+ volts and see how she goes.

Lee

[Radioguy]

Electro:

Seems to be operational. Getting to be bedtime, and I'll pass along an update in the morning.

Lee

[electron_head]

Thanks for getting back on this Lee

Heres a Pic of the first one of these I made up to do 4 x 2 cell 17670 laptop cell packs (inset). I built it about September 2002 and have been charging these packs on it ever since with no problems. Notice the variboard rails aren't running the best way.

I can see a definite need for a good circuit to do 3 cells packs from a car battery in the field for those that go out flying for extended periods or that take planes on trips/holiday. 3 cells packs seem very popular for those wanting that extra performance. 3DX, T3D, etc.

The other single circuit posted earlier in this thread using the LDO regulator is the only one I've seen so far.

[Radioguy]

Electron:

I don't seem to be able to obtain much more than a partial charge on 3 cells. Starting with a battery which indicates a little over 10 volts, the current starts at about 1 amp, falling rapidly to about 200 Ma over about 30 seconds.

It then trails off to less than 50 ma in about a minute. Battery terminal voltage climbs to, and matches the 12.5 volts set on the charger.

Discharging the battery into a 10 ohm, 10 watt resistor mounted on a heatsink at about 1.5 amps takes only a couple of minutes to discharge it back down to around 10 volts again.

I'm puzzled, as my converted Motorola cell phone charger takes much longer to do 2 cells and will deliver about 600 ma for quite some time before the charge rate begins to decrease, eventually stopping when it reaches 8.4 volts.

I'm wondering if I shouldn't try a different power supply to feed this?

Lee

[electron_head]

Hi Lee,

"After you have built it check that the output voltage is no greater than 4.2 volts per cell. Then set your multimeter up to read amps and short out the outputs with the probes to make sure the current is correct. "

Can you tell me what the Amp reading is after this test?
Could you tell me what current seting resistor you are using?


Arent you trying to charge a 3S2P pack of 1250mAH cells? This will require 2.5 Amps at 1C. The max current a LM317 is rated for is 1.5A

I assume your using some kind of Lab power supply where you can crank up the voltage and also adjust the current.

Also are the LM317s getting warm as they should be quite hot for the first hour then drop off to stone cold after that. I hope you aren't excedeing the 1.5 amp limit and they are going out on overload.

Thanks

[electron_head]

Sorry reading back on the thread I am now assuming you are using 2 3cell 1250mAH Etecs and paralleling them up after they are charged?

If it is a lab supply you have and you are happy satisfied the circuit is limiting the amps and the voltage never excedes 12.6 on the output side turn the amp adjust on the Lab supply to full as this is being limited on circuit anyway and then keep turning the voltage up until the ampmeter on the Lab Supply reachs the max you have set on the circuit. Note the voltage. I would go for around 20 volt input. What is the max current your supply can deliver at this voltage?

Hope we can get this sorted soon.

[electron_head]

Originally posted by Lee Smith

I don't seem to be able to obtain much more than a partial charge on 3 cells. Starting with a battery which indicates a little over 10 volts, the current starts at about 1 amp, falling rapidly to about 200 Ma over about 30 seconds.

It then trails off to less than 50 ma in about a minute. Battery terminal voltage climbs to, and matches the 12.5 volts set on the charger.


This is the bit that really puzzles me how could your cells terminal voltage rise to a fully charged state in just 1 min at less than 1 amp. This should be over 1 hour for only a 90% charge.

[electron_head]

Heres a couplle of pics showing testing

1.2 ohm resistor for 1.04A current

Connected like this the current reg will get hot very quickly but the input voltage should not alter the amperage reading (above the dropout voltage that is. And the Voltage regulator should remain cool.

[electron_head]

For a 2 Cell pack being charged to 8.4 volts
(96 x 8.4v) - 120 = 686 ohms

Using a trimpot is not good practice in my view.

Note the different connections to the multimeter.

The more I think about the problems you are having the more I think it must be the supply isnt putting out enough voltage or amps.

I'm putting these up for others as well as you Lee so don't feel like I'm treating you like you can't use a multimeter.

[JBowling]

I just built a 3 cell version, fired right up. I bread boarded it in 10 minutes... now that I know it worked well I'll build a permanent version. Very nice little circuit - it filled my needs very well.

I used a 2k pot for the voltage adjustment. I'll put in a 10-turn unit in the final model.

I'm using a laptop power supply rated at 4A at 19.7V. I have not tried to completely charge a pack yet, but my initial current draw was 1.1A with a 1 ohm resistor. I plugged in a 10 ohm to drop the charge rate to 200 ma measured, since I am charging smaller cells.

Thanks for the design!

Jason

[electron_head]

Thanks Jason

The small stuff is where this circuit will shine as then its well with the specs of the regulators.

It would be nice if some kind off fast finishing could be implemented as that second hour of charging is a bit annoying.

[Happy Hobit]

A small warning.

The LM317 datasheet specifies the “Minimum Load Current to Maintain Regulation” to be typically 3.5ma but not more than 10ma.

My 4.2 volt charger worked on 2.3ma, but it’s a good idea to test the output with no load.

I built a charger a few weeks ago, 4.2 volts @ 125ma, using two LM317’s but I used a 5k pot for voltage reg.. I had to add a 2.2k in parallel to get it to regulate with no load.

A 2k pot for a 12.6 volt supply would probably work fine, but a 5k pot on a 4.2 volt supply won’t.


Jay

[electron_head]

Thanks Happy Hobit the small load and the Low ESR capacitor are good points and should really be put on. I actually considered the battery to be load enough but if a pack is left on charge for extended periods some strange things could possibly happen.

I also see you have used a 1 watt for the current regulation 5 watt is overkill here do you think?

The thing that concerned me most when building this was the accuracy of my muiltimeter as the output has to be right.

[Happy Hobit]

Hi Electron_Head,

>I actually considered the battery to be load enough but if a pack is left on charge for extended periods some strange things could possibly happen.

A battery charged to 4.2 volts on a 4.2 power supply is no load (except battery leakage) :)

This would probably take a while because of the way the charger works and I can’t see any way to make it faster with the voltage limit.


>I also see you have used a 1 watt for the current regulation

No it’s only 1/4 watt.

The voltage drop across the current regulator resistor (10 ohms) is 1.25 volts. I = E / R, 1.25 / 10 = .125 amps

P=E*I, 1.25 volts *.125 amps = .156 watts.

A 5 watt resistor could handle 4 amps.

Like you said it makes a fine, simple, cheep, charger.

Jay

[electron_head]

Hi Jay,

Your starting to show up my lack of electrical knowlege now.

So heres the circuit with the mods as suggested by Jay. If you download a datasheet for the 317 or 350 you will see the reasons for each capacitor and the output load.

Can be omitted in most cases but much better to include for peace of mind as these batteries arent cheap.

[Happy Hobit]

You have a fine eye to pick up the capacitors I had included.

I’d suggest a ceramic for the input cap, Tantalum and electrolytic don’t usually come that small.

I bought a 5-pack of 10 ohm resistors from Radio Shack to build 2 chargers. A 125 ma, using 1 of the resistors, and a 500ma charger using the other 4 in parallel.

Here’s the corrected schematic for my 125 ma charger.

Jay : )

[Radioguy]

Electron:

Was only able to steal a couple of minutes to devote to the charger this evening.

Amps with the terminals shorted is 1.69, so it is drawing too much current for the 317 it would seem. Ther wirewound resistor measures 1.68 ohms, and is a 5% tolerance.

Voltage adjust resistor is a 2K 10 turn pot. Using 122 ohms in the output to adjust of the voltage regualting 317. In the next day or two, I'll go back to the fixed value restors I started with at the beginning.

Power supply is only voltage variable, not current. Feeding 20 volts to the input of the charger. Should be good for about 3 amps.

Will keep you posted as I get the time to tinker further. The 3 cell battery behaviour while on charge has me puzzled though.

Lee

[electron_head]

Lee

Check very closely the current limiting part of your circuit.

With 1.68 ohm resistor there you should only be getting 0.74 Amps across the output when shorted.

Don't attempt any battery charging until after you have got it to pass the two tests in the pictures above (Voltage and Current)

For 12.6volts the voltage resistor is 1090 ohms or less (1K and 82 ohm resistors in series or 820 ohm and 270 ohm) would be close enogh to begin with. Or whatever you have to add up right . 120 ohms is for the other fixed resistor and this always remains at 120 ohms. Agian whatever the input voltage is the output should never excede 12.6 volts


Regards ,
E

[Radioguy]

E:

Well, managed to miss my window of opportunity for sleep, so poked around in the basement for awhile.

I changed out the 2K 10 turn pot that I had used to adjust the voltage for 4.2 v per cell for a network of 1% fixed values totalling 1100 ohms in the voltage adjust function and shorted current came down to 1.4A.

Unfortunately, the output is now up to 16.3 v. Just to put all this in one summary, here are the values:

Input = 22 v.

122 Ohms 1% in the adjust (pin 1) to output (pin 2) of the voltage regulator 317. (closest I could get for values)

1100 Ohms 1% for the voltage adjust resistor. (should provide 12.6 v but provides 16.3 v)

1.68 Ohms 5% as the current limiter. (should give .744 A ... actually gives 1.4 when shorted)

Thanks for continuing with the use of your brainpower Electron, as this shows great promise I think!

Lee

[electron_head]

Lee you have me totally baffled and I vow not to give up until way after you have.

1/ I assume your multi meter is of good quality and is giving accurate readings.

2/ I'm sure you have gone over the circuit at least a thousand times by now.

3/ In approxaimately 24 hours I will post a pic of your circuit dead bug style( That I have tested as working )so you can clearly see all the connections.

4/ If this fails to work I shall declare myself clinically insane.

I hope this helps ;)
E

[electron_head]

Ok this is made with LM350's as you know same but more power.

Tested at 17 volts input and got the readings on pic.

Of course you dont want to use the 1.8 ohm resistor
what you want is a 1 ohm for your 1250mAH 1P packs or if they are 1250 mAH 2P you want 0.5 ohm and heatsinks and LM350's.

As stated before min current resistor for the LM317 is 0.8 ohm.

Regards,
E

[bmwcars]

Hi everybody, I am trying very hard to build a lithium charger. I hope somebody can help me.....Please somebody help me>>.

I went to the shop to get some resistor,manage to get LM317 /5W1ohm resistor and 120ohm1/4 watt resistor.
Shop owner asking what is the value of the (Voltage Adjustment resistor) and the voteageof the 0.1uf electrolytic.
Sone body please help me.

How can i tell the batteries is fully charge? Can i add a LED to monitor it?

Thanks

Raymond Han
Singapore

[jperch]

Raymond,

Unless I misread the datasheet (and I am sure Electron Head will correct me), the required value for teh voltage adjust resistor depends on the number of cells you want to charge. The equation for determining that value would be:

R_adjust= (Vout-1.25)/.0104

Vout should be the max charge voltage for your pack. This would be 4.2V for one cell, 8.4V for two cells and 12.6V for three cells.

The 0.1uf electrolytic should actually be a 0.1uf ceramic.

If you can't find the exact value of Radjust you need, you could use a trim pot and adjust the voltage using a good volt meter.

Joe

[bmwcars]

hi jperch , Thanks for your reply. ok... i am using it for my kokam 1020amp 2cell battery pack.

Can you tell me the aucture resistor to use? and the voltage for the cap.

Can i add a LED to monitor the battery went is full?

[electron_head]

current regulating resistor ( do not exceed a current greater than 1C):
R = 1.25/Current in amps of current regulator.
eg. for a 700mAH Flightpack
1.25/0.7A = 1.78 ohm

voltage regulating resistor:
R = (96 x Vout) - 120
eg. for a 2 Cell pack being charged to 8.4 volts
(96 x 8.4v) - 120 = 686 ohms
eg. for a 3 Cell pack being charged to 12.6 volts
(96 x 12.6v) - 120 = 1090 ohms


0.1 uf cap voltage doesnt matter as long as its ubove the intput voltage. Tantalum Caps are the best as the ESR is best and most stable but I dont use any caps at all.

Only has to be there if power supply is a long distance from the regulator. Ceramic caps are prone to oscillation I was reading an with Electrolytics the ESR changes in very cold conditions.

[electron_head]

People please take time to read though this thread from the beginning.

The circuit I posted was only intended to charge small Lithuim packs and preferably at home on a mains driven power supply.

If you want to charge off a car battery with 3 or 4 cell packs then for this circuit you will need some voltage step up device.

If you have big Thunderpower packs look elsewhere or buy a decent charger.

This circuit is simple and I wont be adding any pass transistors, a fast finishing circuit, or status LEDs to it. If you want to thats fine.

This is the simple KISS (Keep it simple stupid) charger . You build it with no knobs or switches for one sized pack. That way you plug in a pack for 2 hours and its charged and except for power failure nothing should ever go wrong.

[jperch]

Can i add a LED to monitor the battery went is full?

Raymond,

If you want to add a status indicating LED, you might want to look at page 1 of this thread. A few posts down, there is a link to another circuit from SHDesigns. They have aslightly different circuit that is only slightly more complicated (more resistors and a transistor, two if you want the LED, but it only uses one regulator) but does offer a version with a status LED.

Joe

[electron_head]

Thanks Joe,

If I was going to take time to build a charger I would scour the internet for every one I could find before deciding on what suited me.

The SHdesigns circuit is a good one from all accounts.

Hopefully some electronics professional will post the details for a high end charger that has all the bells and whistles. But poeple may find that it costs more to make than just going out and buying one.

[jperch]

E_Head,

I totally agree. The financials of doing it yourself drops away pretty quickly. For the most part, the idea of DIY is really more a matter of fun than economics.

In my case, I now have 10 2 cell packs of Etech 1200's. I have my Triton, an Apache charger, and a surplus car charger that I got from Troy at Bishop power products. I have already drawn up the artwork to put for of the SHDesigns chargers on a single board.

The idea is to do what I saw somewhere on this forum a while ago. Instead of getting a very expensive, multi-output, fancy charger, I am building a RAIC. Redundant Array of Inexpensive Chargers:D

This way I can maybe have a chance to keep ahead of the charge curve and keeps my packs charged.

Joe

[bmwcars]

Hi I have build this today, i am using a 12.V battery to the input for the charger. The max that i get from the output is 9.V. i am using a adjustable 1k resistor. is this ok?
How can i tell the battery is full? using it for kokam 1020 poly 2 cell.

[electron_head]

Hi BMW,

Thanks for the nice clear picture I can't see some of the wiring to well.

That current resistor your using is 1 ohm and will deliver 1.25A to the battery. You say your using Kokam 1020 mAH cells so this resistor has to be 1.2 ohm for the correct 1C charge rate.

Do you have a wire connecting pin3 of the voltage regulator to pin1 the current regulator?

Have you done the tests for current and voltage?

Go back on this post and look to see your circuit matches the dead bug picture I posted. That picture leaves nothing to chance.


You say the most you can get is 9 volts? If you use 9 volts you will kill your battery pack. The output must be set to 8.4 volts exactly for two cells

Keep all cables short as well.

For the 2 cell pack a PC switching PS works fine


How can you tell if the battery is full?
The circuit will almost stop drawing current from the power supply. Easiest way is to have an ampmeter on the input (in series of course)

[electron_head]

As I have said before I don't recommend the use of pots for adjustment of the voltage output. Buy the right resistors put them in and forget.
Pots arent very accurrate and can be bumped and turned inadvertantly causing a ruined battery or worse.

I also dont recommend having switches to change for different currents or cell voltages. Having built a charger that could do 2 3 or 4 cells and charging a 3 cell pack as a 4 cell by mistake.

Build a charger and clearly label what voltage a output it has. Also what pack it was made for would be good. e.g Kokam 2 cell 1020mAH.

[Happy Hobit]

Hi Raymond,


> The max that i get from the output is 9.V.

The Dropout voltage for a LM317 varies according to current and temperature between 1 and 2.5 volts. Charging an 8.4 volt battery from a 12 volt battery should work but not at full current capacity.

My charger needs 6.5 volts input, with no load, to output 4.2 volts.
It needs 9.4 volts input to provide 125ma output (current regulation). @ 4.05 volts.


>. i am using a adjustable 1k resistor. is this ok?

You may find it tricky to adjust your output voltage with the pot you selected. That’s why I used a 10 turn. Buying the correct value fixed resistor is probably best.


>How can i tell the battery is full?

Fully Charged
After you have set-up the charger and everything is working fine.
Measure the voltage across your ‘Current limit Resister’ with no battery connected.
This is your ‘FULLY CHARGED’ voltage.

For a 8.4 volt, 1 amp charger and a 1K volt-reg pot, it would about 8 milivolts.

This voltage will start at 1.25 volts, when it’s outputting ‘Max Current’, and end up at the ‘FULLY CHARGED’ voltage


Reguarding your ONE OHM current limit resistor.

The accepted charge rate for lithium batteries is 1C. For a Kokam 1020 is 1.02 amps. With a 1 ohm resistor your charger will output 1.25 amps.

For 1.0 amps you need a 1.25 ohm 2 watt, or equivalent, resistor. Eight (8) 10 ohm ¼ watt resistors in parallel would work, or four (4) 5 ohm ½ watt resistors in parallel.

And if you really NEED that last 20 miliamps, add another 30 ohm 1/4 watt resistor in parallel.


Jay

[electron_head]

Thanks again Jay,

I'm concerned about people building this circuit with incorrect values and ruining there expensive cells.

I have done my best to provide all the information necessary as accurately as I can.

Maybe posting one very well documented post with all the imformation and testing of circuit at the start would prevent this to some extent then this thread could be deleted.

One last thing on the use of pots. The accuracy of how you set these depends on the accuracy of your multimeter. I brought a cheap meter and found it to be so inaccurate I gave it away to some to use for basic stuff.

If you use the values specified with 1% resistors you can't get it wrong.

[bmwcars]

hi , what"s the other picture i saw that has a input voltage of 17v
and output of 12.v ?

whats that for?

[electron_head]

That picture shows the circuit setup to charge 3 cell packs at 0.65 amps.

You circuit should be the same except you will be using a 1.2 ohm resistor for the current limiting and a 680 ohm 1% for the voltage setting.

Have you read my questions for you?

[electron_head]

Originally posted by bmwcars
hi , what"s the other picture i saw that has a input voltage of 17v
and output of 12.v ?

whats that for?

Download the datasheet for the LM317
www.fairchildsemi.com/ds/LM/LM317.pdf

The input can be any voltage up to 40 volts but ideally should be just a bit above dropout voltage or the max amps you can draw before thermal overload becomes very limited.

[bmwcars]

Can i fix a bulb on the input of the charger? When the charger stop drawing current from the 12v Battery the bulb will not light up.

Will this work?

[electron_head]

Sorry BMW but I refuse to answer anymore of your questions until you respond to the ones I have asked you.

[bmwcars]

So Sorry too many questions for you. Sorry i dont get the question you ask.

[electron_head]

Originally posted by electron_head
Hi BMW,


Do you have a wire connecting pin3 of the voltage regulator to pin1 the current regulator?

Have you done the tests for current and voltage?




Also have you changed the current resistor from 1 ohm to 1.2ohm so you wont damage you battery when you charge it?

Regards

When you charge your Transmitter( TX battery) does it have a light to tell you when its fully charged?

[Happy Hobit]

Hi E,

I like your avatar.

>>I'm concerned about people building this circuit with incorrect values and ruining there expensive cells.
>> I have done my best to provide all the information necessary as accurately as I can.

You’ve done a fantastic job of presenting this charger, facilitating this thread and trying to resolve everyone’s problems in building it.

People are going to make mistakes. That’s why I noted the issue of ‘minimum current to maintain regulation’. That was a mistake I made.

Learn from the mistakes of others, you can’t afford to make them all yourself. (A sign in drafting class 40 years ago that I still remember)

>>Maybe posting one very well documented post with all the information and testing of circuit at the start would prevent this to some extent then this thread could be deleted.

I think of this thread as an example of performance art, an example of people, strangers, sharing and working together.

All the information is in the thread, all somebody has to do is read it.

I once suggested to someone, who had questions about microcontrollers, that he get a couple of books and read. He explained to me that he didn’t want to read he just wanted his questions answered. He didn’t understand that he didn’t know enough to ask the right questions.

You cannot teach. You can only present the info and allow people to learn. Some people learn much better from their mistakes.

Some parents try to protect their children from everything, that’s wrong, children must make mistakes. They must fall in order to learn to get up.

Just my opinion. We all got one. :D

Jay

[bmwcars]

Hi BMW,


Yes i have a wire connecting pin3 of the voltage regulator to pin1 the current regulator.
I am am using a Adjustable resistor. Lowest is 1.25 .V hightest 9.33 V.

I tried charging a Lithium iron 2cell at 7.2 V the amp meter does not move at all. I have to adjust the voteage to max at 9.33 V then the amp meter shows o.8amp.

I have not change current resistor to 1.2ohm, because cant find any at the shop.


Thanks for your help.

Raymond

[Happy Hobit]

Hi Raymond,

The Lithium Ion battery should be charged at 8.4 volts. NO MORE.

The output of the charger should be set to 8.4 volts with no battery.

You probably will not get full current using a 12 volt battery.

Jay

[electron_head]

Hi Raymond,

I suggeest measuring the voltage of the 2 cell Li-Ion before you put it on the charger. The closer the voltage matches that of the charger output the less current you are going to see. Cells have about 3.6 volts in them when brand new.

Regards.


Jay ,

Thanks for the encouragement. I suppose the people that build and don't have any problems never have any reason to post.

Regards,
Garry

[Capt. Electron]

Garry,
You've got it, many read and understand it, but having no questions you never hear from them. If you haven't noticed there have been over 1700 views on this thread already. There are many lurkers behind the scenes gathering info that you never hear from.

I think it's all very clear and am probably one of many that is now trying to decide how best to use it.

I think I'll make a two output box for 2 cell packs and another with two outputs for three cell packs.

Great job, it's simple, cheap, failsafe, and quick to build. And now that you've worked out all the details I won't even need my calculator, and very little brain power.

Thanks.....

Jimmy

[electron_head]

I am led to believe people would like some sort of indicator to let them know when the battery is charged. Note the picture is to demonstrate the concept. Being only just made and not completely tested the circuit may undergo some major changes before the design is right.

The LM339 is cheap and readily available. This is a quad voltage comparator with TTL outputs so can drive LEDs directly. Also it can run on voltages up to 36 volts.

If this IC is used to monitor the voltage drop across the current limiting resistor and biased using a high resistance pot (50K) it will control an LED very accurately. Also being 4 comparators built into one it can be used for multi battery pack chargers. One Lm339 could also be set up on a single battery pack charger to show 4 levels of charge. eg. Still charging at 1C, 80% charged, 90% charged, Fully charged.

The way I set it up was the LED is glowing with no battery connected. If you plug in an already charged battery nothing will happen. If you plug in a battery that needs charging the LED will go out and stay unlit until the voltage drop has reached the preset value as defined by the voltage divison of the pot. As it is getting very close full charge the LED will start to glow slighly and steadily get brighter until full brighness has been reached.

If you want to build this go to google and type in LM339.pdf and download the datasheet so you will know the pinouts etc. Remember this IC is comparing millivolts so make sure there is good insulation from stray voltages. Just putting your hand on the pot to adjust it can cause the LED to light up due to the fact you are earthing the circuit slightly. An unstable input voltage could also cause problems.

Happy tinkering.

Also remember that this charge will still charge your cells at 1C even if they have been over discharged and need charging at 0.1C!!!!


I have removed circuit picture to correct some major errors will post an updated one soon.

Thank-you Jay aka Happy Hobbit for your knowlege

[electron_head]

Second try

[electron_head]

I built one for my old charger and it appears to be working fine. Have yet to fine tune and give more testing

Set the voltage threshold by slightly discharging a fully charged pack then turn trimpot until light just goes out. When this voltage is exceded the LED will light up indicating your pack is charged.

The one you see monitors 4 packs.

[Dave Hederich]

This is really a great thread for tinkerers, with a lot of creative thought. I'm only a partial tinkerer myself, and would rather buy a good charger than go out and chase down all of the individual parts. I wonder what kind of market there would be if someone put together a kit with all of the parts and a detailed instruction sheet included? What could an individual parts kit be sold for if someone bought 100 of each and then re-sold individual parts kits? I don't imagine after all of the work involved that you could undercut the price of the least expensive ready-made chargers by much and still make much of a profit.

[electron_head]

Hi Dave,

My electrical knowlege is limited to say the least. And I started this thread mainly for self driven interests in the hope those with more knowlege than myself would give feedback on what I am doing right or wrong.

Sourcing any of the components should not be a problem as in every case only the most cheapest widely available ones have been used.
For example : The LM339 is only NZ$1.60c from the most expensive supplier so would be under US$1. An ampmeter in the circuit is by far the easiest and best solution but costs NZ$30.

The requirements for me are:

1/ Be able to charge at least 4 packs simultanously.
2/ Give indication of state of charge. Feeling how hot the LM317's are is one but far to crude.
3/ Must not require any PCB artwork.(note all construction is on variboard)
4/ And most importantly must be made using the smallest component count using the cheapest most effective components.

Hope this helps.

It appears the problems I was having running from the cars cigerette lighter socket where due to drawing to much current through wire that was to light causing a significant voltage drop. 14 volts required

All wiring on the power supply side should be very beefy and the power supply if charging 4 packs at 1amp or more should drop very little in voltage with a 5 amp load.

Note on setting up LED's. Batterys can be considered charged when the current flowing into them is between 0.1 and 0.07C milliamps. So for 8.4 volt 1AH pack the calculation is (8.4V divided by 0.07 = 120 ohms). If you short out the output with a 120 ohm resistor and adjust the trimpot so the LED lights up. Then with a flightpack connected the LED will light when the current flow has dropped to around 70mA. Waiting for it to drop to zero in theory would take forever.

[lensrc]

Electron,

A couple questions,

Are the 1% restistors critical?
Do they need to be metal film or can carbon film be substituted?

If the 1% is critical,can I do a pick and choose from a selection of 5% to get it close as possible,with the output tests ending up where I need them?

The reason I ask these questions is Radio Shack only lists 5% carbon film resistors in there online catalog.

If I have'nt worn out my welcome already,can a charger do a larger capicity pack with same cell count?IE: charger built for 2 cell 830ma,charging 2 cell 1200ma pack.I am aware that it obviously take longer,but would it be bad for the cells or have any other negative outcome.

Thanks for any info you can give me.This circuit could be an answer to a problem for me.I have 5 two cell Qualcom packs,with the charge boards still in place,using the Qualcom charger.I want to get some poly's but the cost of another charger and batts,plus having yet another charger laying around has kept me from bothering.This could let me get rid of the charge boards on my 830 li-ion packs,and have a poly charger also,without spending a mint.

[Radioguy]

Hi Lensrc:

I can jump in with a note of encouragement on the use of the Qualcomm charger for 2 cell LiPoly batteries. I have been using mine for many cycles on my Plantraco 850 ma packs.

The charger was "modified" in accordance with the instructions on http://www-personal.umich.edu/~aroger/qbattery/qbattery2.htm and it works perfectly to charge to LiPoly cells. Charge rate is about 700 ma initially, decreasing as the cell voltage increases. I added a JST connector, and as the expression goes "Bob's your uncle!"

So, you already have a very good charger suitable for your needs for 2 cells with just a tiny bit of tinkering.

Regards,
Lee Smith

[lensrc]

your doing this without the center tap for individual cell voltage read?
I slightly modified mine to get rid of the ribbon, thermister and resister in the pack,but left the charge circuit board on the pack.I put the resister and thermister in the box.Mine would not trigger just by hooking up the pack,but I found that if I cycled the AC power to the charger it would.So I put an on-off switch in the AC line.So if I put on of the safety circuits in the box with it will I need the center tap?

[Radioguy]

Lens:

I left the protection circuits on the batteries and also had a second lead and connector feeding the ESC, so I could discharge directly from the cells bypassing the protection circuits on the Qualcomm 830 packs. Charging was done through the other lead and connector via the protection circuits.

The packs would run out of sufficient power to fly the airplane before the cell voltage became depressed sufficiently to be a concern before recharge, although I always used to check the voltage before putting it on the charger.

So, no .... I don't use the middle cell sensing lead, just the + and - output from the charger board. I hooked the meter up to it and monitored many a charge before developing any degree of trust in it when I started charging the poly cells.

There is also a "voltage set" pot on the board which may be tweaked slightly to obtain a true 8.4 volts. I think mine was set around 8.15 volts "out of the box."

Charging my 2 series, 2 in parallel packs it works just fine to bring them up to full charge. The light functions normally.


You can PM me if you need any more details, so we don't divert the thread.

Regards,
Lee Smith

[electron_head]

lensrc:

For the question on how critical the 1% resistors it all comes down to how accurate you want the output voltage to be. Others have used a 10 turn trimpot. 4.2 + or - 50mV is what the datasheets for most cells seem to say. As long as you get the voltage right. This is especially important if your going to leave your packs on the charger overnight.

The voltage checking of each individual cell is (and charging individually) is ideal but not practical. I wonder too what happens when one cell is discharging to a lower voltage and then the pack is placed on charge. The manufacturers spec for overcharge is anything up to around 4.4 volts. With packs like 3cell Etecs they come all wired up and theres no way to check individual cells without cutting into the pack.

I would concur with Lee on the use of the charger that came with the cells if you can. Why go and build something new when you already have something purpose built to do the job!

Lee:

Don't worry about diverting the thread as I would like it to be open to any ideas.
Thanks for your input I would probably use some old laptop or cellphone chargers if I had them at hand.

I'm currently toying with the idea of one very powerful regulated supply using a pass transistor. The idea is you have your very accurate voltage 4.2,8.4,12.6 available at almost unlimited amps. Then plug in your packs with a series resistor only to cut down the amps. The flightpacks will take longer to charge this way as the 1C charge rate will start to drop from the beginning of the charge cycle. The series resistor would pass 1C amps at the difference between a flat pack and fully charged one. ie. for a 2 cell 8.4V - 6V = 2.4V. If the pack was flatter than 6 volts a warning buzzer or Red LED would come on indicating the pack needed to be charged at 0.1C.

Any Tech types see a problem with this apart from the extended charge time?

[electron_head]

lensrc:

One more thing on the large capacity packs. The LM317 is only rated at 1.5 amps so if you had a 2s2p pack of 1250mAHs you would be excedeing its limits needing at least 2.5 amps current.

[lensrc]

I think you mis understood part of what I was asking.Say I build a charger to charge 2 cell packs of 500ma capicity,will that same charger charge a 2 cell pack of 700 ma capicity? Or,put another way,can lithiums be charged at less than 1C?

[electron_head]

For sure lensrc. With a normal charge cycle the C charge rate drops from 1C to almost 0C. You can build a charger to charge 2 cell 500mAH battery packss at 1C and use that charger to charge any 2 cell packs that are larger (they will take longer to charge of course) But you definately can't use a charger that is built for 700 mAH packs to do 500mAH packs as the 1C limit will be exceded.

In some datasheets the max charge rate is listed as being 0.7C!

[lensrc]

Thank you thats what I needed to know.

[Radioguy]

Lens:

Sorry, I neglected to answer your question. Yes, it is just a function of time on the charger at the lower rate.

Electron:

"It is a poor workman that blames his tools" is the proper expression ...... I was using a cheap digital voltmeter in my construction of your charger. The little beast reads almost 2 volts high when compared to another meter!

All seems well with the world now.

Regards,
Lee

[lensrc]

I still think I'll build one or two of these things.Looks like it will cost about $10.00US each circiut.I have a pair of larger ion cells from the bigger Qaulcom pack,and a coulple prismatic ions I'd like to try,but I have read that the prismatic arent very good for our purposes.

[Radioguy]

Lens:

I can offer encouragement. My charger is working just as it should now ... after one pack anyway.

Finding that my el cheapo voltmeter was way off calibration was a revelation!

Lee

[electron_head]

If you are only going to charge at currents up to 800mA (800mAH packs) then the LD1117V regulator would be worth looking at. Its a direct pin for pin replacement with the same reference voltage. Dropout voltage is under 1 volt as opposed to the 2.5 volts of the LM317. So charging of a 12 volt power source would be possible.

Also the 5 watt resistor could be replaced with a 1 watt in most cases saving circuit board space and a small sum of money. See Jays charger and calculation back in this thread.

Remember the accuracy of the output voltage is only going to be as good as the equipment you are using to measure it! Also the input voltage must measure around 14 volts under full load for things to work as they should.

Do you people in the US and Canada shop at Radioshack and is their online catalogue all they stock? If so we are spoilt for choice here.


Here's another homemade charger thread for more ideas.
http://www.rcgroups.com/forums/showthread.php?s=&threadid=50838

[lensrc]

I have an expensive multimeter I bought a few years from the Snap-on man.It also has 10amp range.Radio Shack is the easy place to get components,and maybe some peoples only choice.The city I live near has a large electonics supply company I can go to,but that means going "downtown".Some of us dont like to go into the heart of the city,traffic lights and one way streets and all that.

[bmwcars]

Hi everbody This is the first charger i build , i think it works.
In at 12V /adj output at 8.4V without fly pack(lith poly 1020)
After plug in fly pack votage drop 6.++V and amps does not stay too long, amps drop very fast. Is this ok?

Raymond

[jperch]

Raymond,

I think that Electron Head mentioned earlier in this thread that the drop out voltage for the 317 is about 3V. This means that the input voltage needs to be at least 3V higher than the output voltage. Since this charger has two 317's, then the main input voltage should be at least 6V higher than the pack voltage.

If your pack voltage is 8.4V, then you need an input voltage of at least 14.4V. A 12V supply just isn't going to cut it.

Joe

[bmwcars]

I am trying to post the photo of my charger and there are some problem with the computer

[bmwcars]

Hi , thanks for your reply. I am trying it now with 15V input.
Do i adj the output voteage with or without pluging in the fly pack? because before pluging the fly pack i adj voteage at 8.4V and after pluging the fly pack the voteage drop to 7.6V with input votage at 15V. Is this ok?

[jperch]

Raymond,

You check the voltage without the pack in place. If the pack is in a low charge state, then the voltage will drop when the pack is connected to prevent too much current from flowing.

If you want to test the circuit without a chance of damaging your pack, you could connect a resistor in place of the battery pack. The value of the resistor should be such that it will not pull more current than you are regulating to. The voltage across this resistor should be your pack's fully charged voltage.

For example, if you are planning to charge a 1200mah 2 cell pack, then you will want to set up the charger to regulate the current to 1.2A max and the voltage to 8.4V max.

To test the circuit, you will need to connect a resistor larger than 7 ohms in place of the battery pack. For power reasons, I would use something like a 1k resistor. You should be getting 8.4V across the resistor.

Next, you should remove the resistor and connect a current meter in place of the battery. This will, in effect, short the output of the charger. This should put the charger circuit into the current limit mode. The measurement on your meter should be no more than 1.2A.

If this works as described, then your charger is working. Connect the battery pack and monitor it for the first few times until you are confident everything is as it should be.

If things don't work as described, you need to double check your circuit to make sure it is properly connected. If that checks, then you can start looking into damaged components.

I hope this helps,
Joe

[bmwcars]

I have just finish charging a pack of 1020 2cell fly pack. voltage goes up slowly to 8.3V ,the amps remain at 0.8 amp.
that's ok? Can i join the two heatsink together , because is very hot. the current regulating resistor is hot too.

[Happy Hobit]

Joe,

You shouldn’t need to add a resister to set your output voltage.

Set the output voltage with no load. You want the voltage to be exact when the battery is nearly fully charged, minimum load.

Short the output. This will simulate a normal discharged battery. You will go into current limit and the voltage drop across the ‘current limit resister’ will be 1.25 volts.


Raymond,

Just because the battery voltage is 8.4 volts, with the charger attached, doesn’t mean that the battery is charged. Wait until the current goes down to at least 60 or 70 mili-amps.

Jay

[jperch]

Jay,

You are correct. I was trying to explain a way of simulating the battery pack with a resistor. The problem is that this would require a very high wattage variable resistor to do properly. So, it is not a very good method unless you know what you are doing.

The idea of connecting a current meter in place of the battery would have the effect of shorting the charger and putting it in current limit. The meter should read the level of the limit.

However, your method is a little more straight forward. If you measure the voltage across the current sense resistor, and it is 1.25V, then you are limiting. You know the current by dividing the 1.25V by the current sense resistor value.

Yes, much easier.

Joe