A Question of Balance

[rhitee05]

Unfortunately, I don't think there's a simple passive circuit that you can hook up to balance your cells. If there were, I suspect GGoodrum/Fechter would have been all over it a long time ago... If you just hook up a string of resistors along your 5S pack, one across each cell, the cells will just discharge individually with nothing to guarantee they end up balanced. That's why the BMS uses a transistor to actively control the current and balance the pack.

If you don't have or want to invest in something that will actively balance the pack, my suggestion would be to use your Meanwell to fully charge each one to your normal charge voltage. You should check the voltage on each cell, so if it's all you have a DVM will work if tedious. Like dogman said, I think +/- 0.05V is fine, so I wouldn't be surprised if the cells end up within those limits on their own. If not, you can manually use a resistor to discharge a little from the high cells. Then I think you'll be fine to just connect them together, either permanently or with a pigtail.

If you can't exactly match the individual cell voltages, I think it's a little safer to connect them at full charge rather than near midpoint. My reason is that the voltage/charge curve for LiPo is fairly steep here. That is, there's not a lot of energy stored between 4.2V and 4.15V as the surface charge dissipates. There is a lot of energy stored between 3.85V and 3.8V. Less of an energy difference means that it'll be easier for the cells to equalize after you connect them.

If you want to be really cautious, build up one of those parallel pigtails with resistors between each pack on the balance leads. Something like a garden-variety 1k 1/4W resistor will be fine. If you plug the packs into that first, the resistors will allow the packs to slowly equalize. Let them sit like that for a little while, then you can plug them into the real pigtail without resistors. I think this method should work even with packs that are fairly out-of-balance since the resistors will limit the current even if the cells are substantially different - you'd just have to let them sit for a while.

[Holocene]

Uh, bulk-charge the pack with the Meanwell. Until you can source a Battery Medic, you can top balance individual cells with a cell-phone charger. Limit balance charge rate to Max 1 amp through the little balance leads.

[Kingfish]

Uh, bulk-charge the pack with the Meanwell. Until you can source a Battery Medic, you can top balance individual cells with a cell-phone charger. Limit balance charge rate to Max 1 amp through the little balance leads.

I sourced a Battery Medic yesterday from A Pro Hobbies. Actually the call it a "3in1 2S-6S Lipo Medic System - Discharger / Balancer / Voltage Meter".

I also have the Battery Medic Booster kit from TPPacks which I plan to integrate with the above device for improved performance.

It's the best that I can hope for at this point
~KF

[Spacey]

Are you referring to parallel balancing?
If so, this is what i have my 5s packs plugged into:



And then the charge/discharge leads are all plugged into parallel harnesses too.
They are thusly, for reasons of charging, balancing, and discharging, large 20 amp hour packs. The cells keep each other even, and the pack voltage as a whole keeps itself even too.

The only thing that can throw them severely out of balance is if there is one cell in the parallel pack that is super wonky. In that case, i disassemble the pack and cycle each one to find the weak pack.

Trying to find these plugs that are for 8 cells i.e 9 wires... nowhere sells them

[scoot]

ProgressiveRC will make you a custom one pretty inexpensively if they don't stock it. EP Buddy is another supplier that may do so it as well.

[neptronix]

Good link, kingfish. Let me know how that battery medic works for ya.

Maybe the ones hobbyking sells are the rejects that weren't calibrated so well. You never know...!