At what voltage BMS cuts battery?

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

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

At what voltage BMS cuts battery?

Post by Motormark » Mar 27 2018 8:54pm

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

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

Re: At what voltage BMS cuts battery?

Post by Motormark » Mar 27 2018 8:57pm

IMG_20180122_105600_resized_20180328_035601105.jpg

jonyjoe303   10 W

10 W
Posts: 91
Joined: Aug 31 2017 9:44pm

Re: At what voltage BMS cuts battery?

Post by jonyjoe303 » Mar 28 2018 1:17am

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.

User avatar
brumbrum   100 kW

100 kW
Posts: 1356
Joined: Oct 30 2010 3:29pm
Location: Wales U.K

Re: At what voltage BMS cuts battery?

Post by brumbrum » Mar 28 2018 4:11am

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
My NYX and cromotor build
viewtopic.php?f=6&t=74615
My LMX 161 dirt bike build
viewtopic.php?f=6&t=94074

Post Reply