Lithium ion cells in parallel.

I searched this and found a couple of threads, but they are old (want to avoid necroposting) and they don't exactly address the question at hand.

I have some 10440 Trustfire "600mAh" Li-ion cells (3.7V), the cells are protected. Is it safe to run a pair of these in parallel? The laser's current draw will not exceed 200mA, and the cells will always been charged separately, never together or as a battery pack.

I'm asking because I would like to replace two AAA batteries in series (3.0 volts) with two 10440 cells in parallel (3.7 volts), for double the mAh. It should be easy enough to make a battery holder that connects the cells in parallel while allowing them to fit in the host and properly connect to the laser circuitry.

It should be okay as long as you maintain the same charge level in both cells

Yep, as long as you don't use 1 really old battery and a new one, or one charged one and one flat one, it will be fine.

The only risk is over discharging one. LiOn cells don't like to get bellow 3.0 volts. If one cell has a higher internal resistance (see: http://laserpointerforums.com/f67/h...es-how-measure-internal-resistance-57576.html) than the other, is will discharge slower.

I would recommend checking them with a voltmeter on occasion right before you recharge them to make sure they are balanced.

1) Always charge and discharge the batteries at the same time.
2) Check them with a voltmeter when they are discharged

Thanks for the replies. I would hope that the protection PCB would cut off one of the cells if its voltage dropped too low, and if it were any brand other than *fire, I might put faith in that. Unfortunately, the selection of 10440's is sparse.

It seems to make sense, if one cell drops too low its protection PCB cuts it off, while the remaining cell is still able to run, until its PCB also cuts it off. But, I could be wrong.

Marco Polo said:

Thanks for the replies. I would hope that the protection PCB would cut off one of the cells if its voltage dropped too low, and if it were any brand other than *fire, I might put faith in that. Unfortunately, the selection of 10440's is sparse.

It seems to make sense, if one cell drops too low its protection PCB cuts it off, while the remaining cell is still able to run, until its PCB also cuts it off. But, I could be wrong.

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That would be correct if they are protected with decent PCBs. If you have room, you could also put an external protection circuit in.

Contact Boeing. They should really have the up to date info on these things!

Jacob32123 said:

The only risk is over discharging one.

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That is incorrect for two reasons. 1) they're in parallel, so they will have the same voltage at their terminals - law of physics. 2) they're protected.