running batteries in parallel slight voltage difference?

[drifta303]

hey,

i have 3 powertool batteries at 58v.. i want to run them in parallel, 2 of them are 58v or 57.9 on my multimeter and the other one is around 57 or 57.5v... will this be a problem? ive put them all on the charger but this one battery just doesnt get as high even when left on for a while... im not sure what the internal bms is like on these or how that effects things as i havent opened them up.


will the slight voltage difference be a problem? in theory the two higher ones may shove power into the smaller one if i connect them all together right>?

 

[fechter]

When you connect them, the higher voltage one will try to feed the lower voltage one. If they are close enough in voltage, the current will be small and everything balances out quickly. If the voltage is too different, the current might be higher than the cell rating and things burn up.

1v difference is kinda borderline. If they both finished on the same charger, they should be OK. The super safe way is to connect them with a light bulb or resistor in series. After a while, they will equalize, then you can make a solid connection.

 

[amberwolf]

It also depends on the reason for the difference.


Hypothetically: If there are some cells (or a cell) in the lower voltage pack that simply aren't charging up to full, vs the others in the same pack, then the higher voltage pack will basically be attempting to charge those cells at whatever the highest current possible from that pack is, which will be too high for the BMS to "bypass" the already-full cells, so those already-full cells will be charged further. With only about a volt to divide across the total series of cells, that shouldn't be a big deal, and the BMS would probably just bleed them down again over the next few hours anyway. The only issue I see, potentially, is that that charging could occur at a rate that is probably a lot higher than the original charger, and as Fechter says, stuff could be damaged.



I'd recommend checking the packs for the reason for the voltage differnece before connecting them.