At what voltage will a 100v Monster balance charge?

[JeremyB]

Like the title says, I need to know what voltage my 100V Monster starts balance charging at. I built myself a 6amp charger and I'd like to set the voltage halfway between where it starts to balance charge and the max 100.8v

[mrelwood]

While I don’t know the exact voltage, what I would consider is that:

- On BMSs that the threshold has been examined on, the balancing threshold seems to be at or very close to 4.2V per cell.

- The components that determine the threshold have large manufacturing tolerances.

- The equipment you will use to set and measure the threshold also have large tolerances. It’s not uncommon to have a tolerance of +-0.5V or even +-1V for 100VDC measurement on a $100 multimeter.

(Edit:) - So far no one has reported a wheel being rendered useless due to aged cells. But there are numerous cases of dead battery packs due to insufficient balancing of cells.

Edited June 17, 2020 by mrelwood

[Chriull]

6 hours ago, JeremyB said:

Like the title says, I need to know what voltage my 100V Monster starts balance charging at.

There is not really such a thing as the BMS would start balancing the pack.

It is just a resistor in parallel to the cells at and above 4.2V. So if all cells are balanced it starts at 25*4.2V=100.8V. The more the cells are out of balance, the earlier it starts.

The current these resistors can bypass is not too much - so if the cells are too much out of balance the first cells getting the bleeding resistor activated have good chances to be slower, but still charged up to ~4.25V. At this point the cell overvoltage protection kicks in and cuts off the charge input - so no charging or balancing happens anymore.

One can try in such cases to discharge the battery a bit and charge it again. With some luck after a couple of such small discharge/charge cycles the battery could get balanced again.

But this balancing system is very limited and can only balance small differences each time - once cells differ too much they have to be replaced. (Or the whole pack)

6 hours ago, JeremyB said:

I built myself a 6amp charger and I'd like to set the voltage halfway between where it starts to balance charge and the max 100.8v

?

I could not see why one would like to do this? But maybe i'm missimg something?

More details on battery packs/bms:

 

[RagingGrandpa]

16 hours ago, JeremyB said:

I built myself a 6amp charger and I'd like to set the voltage halfway between where it starts to balance charge and the max 100.8v

Far better to let it CV at 4.2V/cell and then to stop charging at a specific current value.

I suggest stopping at [P-number x 50mA], to provide ample opportunity for balancing.

[Chriull]

4 hours ago, RagingGrandpa said:

Far better to let it CV at 4.2V/cell and then to stop charging at a specific current value.

I suggest stopping at [P-number x 50mA], to provide ample opportunity for balancing.

Batteryuniversity.com suggests 3% C - which would be 3500mAh * 3%= 105mA per paralleled cell.

I'd assume that this number from batteryuniversity is just a rule of thumb and so anything in about this magnitude should be ok - according to ones personal preferences.

Edit: this post is my way to agree to yours .

Edited June 17, 2020 by Chriull