Self balancing (newbie) question..

If two (nominally) identical cells are left paralleled together, for a month say, without load or charging cycle, to what degree will they self balance?
Any thoughts on whether this might be helpful before doing a 'bottom balancing' on a subsequent 2P16S pack? If i can get each pair(2P) in perfect balance, then an active cell balancer can handle the 16S side of things???

Yes parallel cells will be in balance. Make sure they are close to the same voltage before connecting them, or connect them with a low ohm resistor so they can get to the same voltage.

Are they connected in series yet? If not yet in series, you might connect them all in parallel, get them all the same voltage and then build your pack.

Thanks DThames. So irregardless of whether both cells were 'fully' charged or both 'depleted', so long as they were matched 'voltage wise' to start with, the cell balancing process would be achieved? If left long enough, would this balance down to three decimal places? Is two decimal places considered close enough in balancing? Opinions welcome.

So if I have all cells around 10% soc and I parallel them they will then all be basically bottom balanced after connecting them back in series? My question is sort of the same as Solarfun.... I'm sitting there and trying to get them perfect and burning a lot of time. I haven't seen a statement like " bottom balance should be within XXX Mv at 10%SOC".

Thanks

reply to toothy...I am working to balance some used 24v batteries that the cells are welded together, so I can feel your pain about burning a lot of time. I am using 2.95v as my bottom balance voltage and have a discharge circuit with a low voltage cutoff. So I am discharging one cell at a time after the lowest cell gets down to 3.0v or so. If you can parallel them with a series 1 ohm resistor on each of them you should be able to get them close to the same voltage without too much struggle. Then remove the resistor and let them settle together. You could have 2 or 3 in parallel and add others one at a time with a resistor to protect from too much current and then remove the resistor when that one is close to the parallel pack. Keep repeating (or get more resistors) until you have them all at the same level. You could then bleed down the entire parallel pack if you want to get a lower balance voltage.

Note my electrical and electronics experience is much more than my battery balance experience.

Yes it works, but it's sloooooooooooow when your voltage is only .02ish apart. A MUCH faster method is to parallel the cells and then charge them.
Take a look at this article that goes over cell balancing in detail:

One other tip: I have used my dc power supply to add voltage to a cell when I had 5 cells bottom balanced pretty well and one cell with lower voltage than the rest. It was quicker to raise the low cell voltage a bit than lowering the other five to meet the low voltage cell. I am no expert but I went slow adding voltage (2 amps max) then let the cell rest after disconnecting it from the power supply a hour to see where it settled.

Thanks all,

I had read the linked article and used my IMAX charger to raise a cell that had gotten a bit low. Had to put the IMAX on NIMH to trick into working.

So, using the KISS principle, since I have the second S down pat! Parallel all cells drain to 2.___ V, rest, connect in series, charge. Or have I missed something?



Thanks
Wade

toothy said:

Thanks all,

I had read the linked article and used my IMAX charger to raise a cell that had gotten a bit low. Had to put the IMAX on NIMH to trick into working.

So, using the KISS principle, since I have the second S down pat! Parallel all cells drain to 2.___ V, rest, connect in series, charge. Or have I missed something?

Thanks
Wade

Click to expand...

Imax B6? Go to the User Program and you can select 3.7 (LiPol), 3.6 (LiIon), 3.3 (LiPo4). So then when you select Li Charge it will say, LiFe (I think) and it will charge to 3.3 nom, 3.6v cutoff.

DTames is dead on from everything I know about!

Parallel balances cell voltage, but it's not top or bottom 'Balancing'.

Top & Bottom 'Balancing' is kind of a misnomer,
With 'Top' it's actually limiting the charge to the battery that reaches charge FIRST.
trying to charge the rest past what the first cell that reaches 100% SOC will damage the first one to reach 100% SOC by over charging it.

With 'Bottom' is the first cell to reach the cut off voltage.
Even through other cells might be more 'Healthy' and don't discharge as fast, you have to stop all discharge when the first cell reaches say, 3 Volts.
Both top & bottom limits are determined by the WEAKEST cell, stay within it's limits.

It's not all bad, group like cells together and use a top voltage programmable controller,
And use a programmable low voltage disconnect.
With the lifespan of LiFePo4, and the power they can still exchange at say 80% capacity, the battery is still VERY viable with it's charge density, still 4x what of lead/acid.

Self balancing also depends on battery chemistry. Lead acid batteries drop in voltage a fair amount as they discharge, so if 1 cell is discharged more, it's voltage will be less and therefore it will supply *slightly* less power to the load than will a cell that has a higher charge state...so the higher charged cell provides more power and discharges faster, until they are the same voltage.
LiFePO batteries stay at a much more constant voltage during discharge, so you get less of this 'self-balancing' occurring...if they're all at the same voltage, they all supply the same power, regardless of state of charge.