I recently needed to charge up 3 of my li-ions fairly quickly. So I put them in a 3S battery holder and hooked that up to charge in my hobby charger. There weren't any balancing leads, the batteries were all the same voltage and capacity.
So my question is: How stupid was I? I understand that there is the danger of overcharging if they become unbalanced, but if they are the same start voltage and same capacity (identical use) is this safe?
Currently I'm leaning towards safe, but not recommended but I'm curious of everyone else's opinion.
I do this occasionally but just to charge them to maybe 4.1V each. Then I put them into Ultrafire chargers. This was done in full attention just to speed things up. The reason is I am lazy with balance leads as I haven’t built a proper jig or holder for charging. Now still use magnet to make contact.
Other questions include whether they all still have the same resistance and current state of chemistry (possible mfgr defect, drop, dent/puncture, unseen vent, etc - not design chemistry)?
I have done plenty of attended series-charging, experimented with old laptop cells. Even without the balancing leads attached, it’s not as unsafe as most people think it is, IMO. I use charging rates at 600 to 700ma only, up to 3S. Never had cell ending voltage higher than the terminal voltage of my mode setting. The resting voltages will however be sometimes vary between cells if the cells are quite old, but usually by only .01V.
note: All my laptop cells that I keep for experiment have all been discharge capacity tested, and have passed my torture test (which is to fully charge the cell, put it my SC600, engage the turbo mode and see if it can finish the 5-minute run without stepping down to medium) If it does not, they are discarded.
More complicated Answer: Really bad idea. There are at least a dozen possible failure modes, and none of them are pretty. The absolute worst case scenario is if one of the cells happens to be shorted out - in your scenario, you’d then charge two cells to 12.6V. That’s 6.3V EACH. It’s just an all around unsafe/bad idea.
I would not recommend paralle charging at currents higher than the 1C rate of the individual cells. There is no way to guarantee that the current is being split properly.
When charging Li-Ion, avoid shortcuts to save time. IMO, charging in series (preferably with balancing) at 0.75C would be safer than charging in parallel at higher than 1C.
Another note: make sure that the initial state of the batteries being put in parallel are very similar. If a charged cell gets in parallel with a discharged cell, the weaker cell will most likely be ‘charged’ by the good cell at a very high current.
Good point. With the development of higher capacity like the Panasonic 3400mAh cells, the term “C-rate” has become more or less inappropriate.
For example, it’s not recommended to charge the 3400mAh cells at 3.4A, which translates to 1C, whereas 1C would be a perfectly fine rate to charge lower capacity cells.
Over at CPF, there has been tests of the caveats of parallel charging, and it has been deemed reasonably safe to combine cells with different voltages.
4.25V is not a good termination voltage for any LiIon cell - except those rated for 4.35V.
I sometimes charge parallel. Mainly my 14500 IMR cells on the 500mA setting of my Xtar WP2 -> 250mA each. I make sure they are within 0.05V of each other and I dont see how that could possibly be of any harm.
Series charging.. I dont know. Brand cells, same usage, I wouldnt be too paranoid about that. Not recommended for anyone, but scaru is no newbie. ;)
Early power tool battery packs were built that way and they didnt fail..
