Low current charger for individual cells?

[davekra]

I just bought three 3s1p 2000mah Lipo’s that I’m going to parallel so I can draw enough amp’s.

Once I give each cell a full charge and run them can I then charge all the packs together in parallel?

I’d heard you shouldn’t charge packs in parallel but isn’t that what your doing with a 3s2p?



Another thing I was thinking, is there a slow charger I can build that can charge each cell individually? I found a simple circuit a long time ago that is easy to expand to multiple cells. Could I just expand this circuit and charge all the cells at the same time or would they not be isolated so as to not sense the voltage of each cell to charge properly? I’d only be doing this occasionally to re-balance all the cells.


Thanks,
davidk

[Terry S]

I dont like the circuit you posted. I would not trust it for LiPos.

Terry

[oldphart]

Terry S.
Could you explain why you dont like Davekra's circuit? It seems reasonable to me,but my electronics knowledge is of the 'hobby experimenter' standard,and some of the subtler details escape me.

[davekra]

I dont like the circuit you posted. I would not trust it for LiPos.

Terry
Anything more constructive than that? I've built this for 3 LiMetal cells several years ago and it worked well. The opamp compares the refference voltage and stops the charge at just the right voltage.

The second part of my question was is there a way to charge all the individual cells in a pack at the same time to rebalance the pack? With 9 cells doing it one at a time will take quite a while. Also, what about charging the three packs in parallel? They will all be used at the same time so shoudl be at the same level of discharge, right?

Thanks,
davidk

[Dan Baldwin]

One problem with making individual chargers as shown in the circuit above is that you would need an individual-isolated power supply for each cell being charged. It is not a constant current/constant voltage Lipo charger, so you will be missing out on part of the charge, but as long as it's adjusted correctly, and working properly, it should be safe to use. The only thing that might be unsafe about it is that if you charge at a low rate, the packs would have to charge for many hours, so you almost certainly wouldn't keep an eye on them while they're charging. I prefer to charge at or near 1C to keep the charge short, so I can watch the batteries.

I personally am not a fan of charging cells individually. I don't think it has any advantage over using balancers. There are many balancers available that are inexpensive, and accurate.

I don't think there's anything wrong with charging all 3 packs in parallel. If the packs were used separately, they might be at a different state of charge, so there could be a high current flow when you first hook them together, but in this case they are already hooked in parallel, so they are at the same state of charge. If you were charging with balancers, you would need a balancer for each cell, so 9 (or 3-3 cell balancers) if you charge all 3 packs in parallel, while you would only need 3 (or 1-3 cell balancer) balancers if they are charged separately.

Dan

[Terry S]

Sorry for the lack of info I was in a hurry ( slap wrist for me ). It seems to me that it will only charge the cell to the first phase ? also the general lack of protection built in is my main worry. No protection from the relay spikes or capacitors to keep things under control. I am shaw it will charge a cell but I would prefer a more belt and braces circuit. Just my opinion, its up to you at the end of the day.

Terry

[davekra]

Thanks Terry and Dan,
I've been doing a bunch more searches and discovered comercial balancers are not that expensive and neither are dedicated Lipo chargers. I'm also glad charging the packs in parallel is not unheard of. I'm using a Triton and wasn't thrilled at the prospect of having to purchace more of them. Now that I can charge all three at once it should be fine for field charging. Every once in a while I'll check the voltage of each cell and re-balance them. I'm going to stop by the LHS and see what solutions they offer.

Thanks again.
davidk

(A little knowledge is a dangerous thing ;) )