Cheap Lithium Charger for Electronics Tinkerers - Page 2 - RC Groups
Thread Tools
Jul 03, 2003, 04:56 PM
Registered User
electron_head's Avatar
Quote:
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.
Sign up now
to remove ads between posts
Jul 03, 2003, 05:15 PM
Registered User
electron_head's Avatar
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
Jul 03, 2003, 06:01 PM
Registered User
jimsp's Avatar
Quote:
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?
Jul 03, 2003, 06:02 PM
Registered User
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
Last edited by Lee Smith; Jul 03, 2003 at 06:10 PM.
Jul 03, 2003, 06:12 PM
Registered User
electron_head's Avatar
Quote:
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
https://www.rcgroups.com/forums/show...light=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.
Last edited by electron_head; Jul 03, 2003 at 07:55 PM.
Jul 04, 2003, 01:28 AM
Registered User
electron_head's Avatar
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
Jul 04, 2003, 06:25 AM
Registered User
jimsp's Avatar
Quote:
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?
Jul 04, 2003, 07:05 AM
Registered User
electron_head's Avatar
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.
Jul 04, 2003, 08:19 AM
Registered User
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
Jul 04, 2003, 01:11 PM
Registered User
electron_head's Avatar
Quote:
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.

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)
Jul 04, 2003, 01:29 PM
Registered User
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
Jul 04, 2003, 01:34 PM
Registered User
electron_head's Avatar
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
Jul 04, 2003, 02:07 PM
Registered User
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
Jul 04, 2003, 10:40 PM
Registered User
Electro:

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

Lee
Jul 04, 2003, 11:15 PM
Registered User
electron_head's Avatar
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.


Thread Tools