# BATTERY CHARGER HELP! Ideas?

#### WonHungLo

Joined May 9, 2011
22
i have 4 of the 14500 lithium batteries
http://www.dealextreme.com/p/trustf...7v-900mah-lithium-batteries-2-pack-blue-19626

i have them wired in parallel to keep the same voltage and increase the capacity.

i am trying to figure out a way to charge these batteries as i will use them as a pack inside of a sealed enclosure.

anyone have any ideas for a simple charger for these in this wiring configuration. its around 3.7V - 4.2V and ~3600mAh.

i want to be able to use the device its powering and charge at the same time if possible. just think of it like a cell phone. you can charge and talk at the same time. i am just worried about over charging and simplicity

#### WonHungLo

Joined May 9, 2011
22
something like a lm317 would be great but i want it to cut itself off when charge is complete

#### #12

Joined Nov 30, 2010
18,224
I was going for Battery University as my first choice, too. I suggest picking a voltage regulator with an internal current limit in the right range and designing it as a voltage regulator set to 4.20 volts. My last design was a 78L12 which limits the current to a little over 100 ma until the battery is nearly full, then goes into constant voltage operation. A 78M3.3 (do they make those?) designed for 4.2 volts would limit current to about 1/2 amp. See where I'm going? Try to find a regulator that limits current to about 1.8 to 3.6 amps (C/1 to C/2).

The shut off part requires another circuit. Say, an op-amp sensing the current and latching off with positive feedback if it ever sees less than 108ma. That's a 1C charge rate hitting the 3% level described at battery U. I think I'd set the current sense point at 200 ma to allow for changes in the battery behavior as it gets old.

Adding the "auto-off" circuit is not as simple as you could hope for, but I don't know about any single chip that can do constant current to constant voltage to auto-shut off. Then again, that's why hundreds of people watch this site. One of them might just name the right chip in a few minutes.

#### WonHungLo

Joined May 9, 2011
22
ok then if i used a similar regulator as you mentioned (the 78L12) what would happen when the battery is fully charged. im decent at building passive electronics and am a beginner at microprocessors (AVR or arduino) but the battery and charging stuff im new to. would i simply have to calculate the charge time and disconnect. im really looking for something to charge the battery and light a led when charged. this too far out of reach...

#### #12

Joined Nov 30, 2010
18,224
A 7812 can not be easily taught to deliver 4.2 volts. You must start with a lower voltage chip like a 317. Calculating time is not reasonable because overcharging a Lithium battery is not allowed. You must measure current flowing into the battery to determine when it is full.

What happens when the battery is full is that it accepts less current from a fixed (4.2V) voltage regulator. That is how you can tell it is full. You measure the current, compare that to an amount you decide on, and then switch the charger off and the LED on. My best method is as I described, an op-amp. This is not a passive circuit and I have no hope of telling you how to use a microprocessor to do this job.

Other people are welcome to add to this discussion.

Joined May 28, 2009
514
Have you looked up the battery data sheet to see the recommended charging voltage and charging current?

Joined May 28, 2009
514
This will give you some ideas. Found it a while ago. I built some chargers for DuraLife UBBL20 type rechargeable batteries.

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#### WonHungLo

Joined May 9, 2011
22
is there any problems with charging batteries wired in parallel.

i.e. 2x 18650 cells (3.7V @ 2000mAh / ea. ) = 3.7v @ ~4000mAh.

since the voltage is the same it should just take a little longer to charge right.

i have been using these batteries and chargers for years i just dont know if i can charge i parallel

these are the batteries...
http://www.dealextreme.com/p/trustfire-protected-2000mah-3-7v-tr-18650-battery-2-pack-12396

and this is the charger...
http://www.dealextreme.com/p/ultrafire-3-6-3-7v-battery-charger-1251

***just wanting to know if i can make some leads off of one of the charger chanels and connect it to 2 of the batteries wired in parallel and it charge completely and safely.***

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#### hupa

Joined May 19, 2011
8
something like a lm317 would be great but i want it to cut itself off when charge is complete
Lithuim ion batteries are much easier to charge than NiMh. LiI's charge well with constant voltage charge. NiMh dont.
An LM317 voltage regulator can therefore be set up to charge your 3 or 4 cells very nicely. Check out the LM317 application notes.

As #12 says, the batteries will take charge until they reach the voltage set by the charging circuit and that is usually 4.2 volts.

You say you want to leave the charger connected to your batteries all the time - In that case I would set the charge voltage a little lower than 4.2 volts, my choice being 3.9 Volts. Your batteries will then have a much longer lifetime, but not quite as much capacity between charges.

There is no problem having cells in parallel.

I did something like this recently. I expect these batteries to last 5 years.

Happy to pass to you the circuit I invented for my charger - a discrete circuit of 3 transistors and a few other bits.

cheers