## At what voltage BMS cuts battery?

Batteries, Chargers, and Battery Management Systems.
Motormark   10 mW

Posts: 27
Joined: Jun 23 2016 2:03am

### At what voltage BMS cuts battery?

I have a 72v (20x13)battery pack. 44ah
But after using 27ah (voltage around 72V) the voltage drops fast when using more then 15A, and the bms cuts the battery pack.
-At what voltage of the pack the BMS suppose to cut?
-At what voltage of individual cells the BMS suppose to cut the pack?

The fact is that i dropt the pack when it was new, and had a shortcut thru 1 individual cell (corner cell) . It melted down. I replaced the cell, and his neighbors.

Motormark   10 mW

Posts: 27
Joined: Jun 23 2016 2:03am

### Re: At what voltage BMS cuts battery?

jonyjoe303   100 W

Posts: 146
Joined: Aug 31 2017 9:44pm

### Re: At what voltage BMS cuts battery?

This something I copied from the internet
At a 2 amp current draw

cell voltage--------------for 72 volt pack
4.20 volts – 100% -------84 volts
3.97 volts – 80% --------79.4 volts
3.85 volts – 60% --------77 volts
3.77 volts – 40%---------75.4 volts
3.72 volts – 20%---------74.4 volts
3.58 volts – 0%----------71.6 volts

I know with no-load the lowest a cell can go is about 3.3 volts, you can go lower to test capacity but once you reach 3.3 volts the voltage goes down quickly.

When you mention the voltage drops fast, your pack might have lost capacity. Have you checked the balancing on it? if you have a weak (lower voltage) parallel group that will cause your problem also. Over time the the bms will not balance a battery that goes severely out of balance.
The battery pack will not get a full charge because of an unbalance condition, and you will notice the reduce capacity.

brumbrum   100 kW

Posts: 1360
Joined: Oct 30 2010 3:29pm
Location: Wales U.K

### Re: At what voltage BMS cuts battery?

2.8v is the usual value per cell for bms cut off. You probably have a week cell/cells in one of your parrallel groups which is causing a large voltage sag as there is less energy in that combined cell group.
Sounds to me that you need to remove the series connections and get a multimeter on each parrellel group to measure the voltage on eaxh to make sure they are all similar.
Btw, under load 2.8v is fine for 18650 cells. Some can be taken down to 2.5v. Obviously at rest the voltage will be higher at around 3v - depending on the amount of amps being drawn from the motor
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