Using two 3.7v batteries with protection circuit in series?


Thread Starter


Hi Guys! I'm doing a little project (portable nintendo 64) here and have a question.

I need 7.4v to power up my console. I have 4 galaxy nexus batteries which output 3.7v each and 1750 mah. I want to solder them in two packs of 7.4v (in series) than in one pack (in parallel) so it gives me 3500 mah. I know how to do it, the problem is that each battery has a protection circuit integrated so it does not overcharge... In parallel there would be no problem, but if I solder them in series. I will have a 7.4v wall charger that tries to charge a 7.4v battery pack which have two protection circuit for 3.7v... follow me? So the big question is, is it a problem? Will my batteries will just charge fine or how will it react? Thanks!
These sound like Lithium batteries. Whenever you connect them in series you need a balance charger to charge them not just a wallwart charger. These chargers have 2 plugs: one is connected to the outside terminals of the battery(7.4V) the other plug is connected to each battery cell individually. The charger checks the voltage of each cell and attempts to balance the voltage of the cells before entering full charge mode. It also handles the phases required to charge a Lithium battery properly.
I would, for myself, buy a 3000mah LiPo battery and charger,rather than fiddle with Lithium batteries. They can catch fire during charging and even with a balance charger should never be left charging unattended. In your case you could not be sure what the state of the different cells are, they might hold totally diifferent amounts of charge, which makes a balance charger imperative, so you would really only save on the batteries themselves.

Check around on the web, a few houses have been burnt down with chargers going wrong. Lithium batteries are a fire hazard when used incorrectly (ask Sony about their laptops LOL).

If you really want to do this, you should centre tap the cells and bring out a lead and then charge each 3.7V individually. This still poses a risk as if they charge unequally, the current spike when the 7.4V is connected could still start a fire, but it is less likely.

Also, you would need to check the current rating of your Galaxy batteries vs the current requirements of your console, they might not be able to cope as mobile phones have VERY low current requirements.