Charging circuit for two 3.7v batteries

Hi Guys,

Is there a charging circuit for two 3.7v @ 1200 mAh batteries connected in series. I need the 7.4v @ 1200mAh to feed another board.

I used the below board

to charge one 3.7v battery and it worked fine. when I connect two batteries in series it killed the board ...funny thing is the guy at the store told me the board would be able to charge the batteries in series and parallel.

any recommendations please.

by Rhine
October 25, 2021

I have no problem, yet, using the "LiitoKala Lii-402 Smart Battery Charger", yet. Available at Walmart, EBay, Bang good, ... It automatically detects the safe voltage for the specified batteries. And it works with Ni-MH, Li-ion, and Cd types.

The charger that you presented does not seem as well adapted, at first glance. The (limited) documentation does not even mention that it works safely for 3.7 volt Li-ion batteries. Furthermore, I would not be ready to recommend charging batteries in series without extra details.

by vanderghast
October 25, 2021

1 Answer

Answer by Rhine

Hi Vanderghast, Thank you for your reply, I looked up the charger you suggested and I wont be able to use one like this unfortunately reason being is because of the size and cost.

I basically need the series combination of the batteries to power a dc/dc converter that doesnt work with one 3.7v battery the board which is a LED driver is to power a prototype design for a wearable device so the LiitoKala Lii-402 will be to big and expensive.

I was thinking along the lines of: 1.) Finding a charger (with overcharge and discharge protection) that will be able to charge the batteries, 2.) Look for another battery in the range of + - 7v @ 2000mAh that will be able to power the board and not be huge and bulky (Battery size at the moment is 16340)

Please see driver board below.

+1 vote
by Rhine
October 26, 2021

With a drone or even with a wearable, it may be important to reduce the weight to be carried. Have you considered to keep the charger "outside" the "object"?

After all, why the end user would not find preferable to "just" change the batteries than to "plug" the object to a domestic electrical wall plug? Lighter solution and lesser dead time.

by vanderghast
October 26, 2021

Thanks Vanderghast sorry for the late reply. I came across this 7.4v 2s charge

that will seem to charge the batteries however requiers 8v so from the 5v micro USB pin out I will connect a step up converter to bring the voltage up from 5v to 8v. I have already tried this and its supplying power to charge the batteries. However with this circuit there are no LED displays to indicate when the batteries are charged, Also something confusing to me is the the power output + & - share the same solder points as power input so what will stop the battery from discharging back to the power source once charged. But ill post this on another question. Thank you kindly again for your responses

by Rhine
November 06, 2021

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