Cheap Lithium Charger for Electronics Tinkerers - RC Groups
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Jul 02, 2003, 09:41 PM
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electron_head's Avatar

Cheap Lithium Charger for Electronics Tinkerers


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.
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Jul 02, 2003, 09:44 PM
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electron_head's Avatar
Heres the circuit it didn't upload in initail post
Last edited by electron_head; Jul 03, 2003 at 09:05 PM.
Jul 02, 2003, 10:02 PM
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electron_head's Avatar
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.
Jul 02, 2003, 11:32 PM
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jperch's Avatar
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
Jul 02, 2003, 11:56 PM
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electron_head's Avatar
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?
Jul 02, 2003, 11:59 PM
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electron_head's Avatar
http://www.shdesigns.org/lionchg.html you are telling me this one is simpler?
Jul 03, 2003, 07:35 AM
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Electron:

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

Lee
Jul 03, 2003, 09:40 AM
Too lazy to repair!
jperch's Avatar
Quote:
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
Jul 03, 2003, 09:45 AM
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tekochip's Avatar
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.
Jul 03, 2003, 11:22 AM
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electron_head's Avatar
Quote:
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
Jul 03, 2003, 11:49 AM
Too lazy to repair!
jperch's Avatar
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
Jul 03, 2003, 05:11 PM
Registered User
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
Jul 03, 2003, 05:31 PM
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electron_head's Avatar
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.
Jul 03, 2003, 05:35 PM
Registered User
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
Jul 03, 2003, 05:49 PM
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electron_head's Avatar
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.


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