# Advice for Li-ion Battery Cutoff Voltage

#### electromeow

Joined Oct 12, 2021
9
I'm trying to design a BMS board for my huge battery pack consisting of Molicel P42A 21700 batteries. I designed the over discharge protection part of the circuit using a simple comparator. But I don't know how to choose the cut-off voltage for my battery.
According to the datasheet, the cutoff voltage is 2.5 volts, the nominal voltage is 3.6 volts and the maximum voltage is 4.2 volts. But I've run into too much conflicting information about all those over discharge protections on the internet. Some designs cut the battery connection when the voltage drops under the nominal voltage, some of them cut it under 3.3 volts, and so on. So I found myself in a dilemma:
What cutoff voltage should I choose, maximizing the capacity I'm making use of, also avoiding from damaging the battery pack? What does "nominal voltage" actually mean? Should I choose the cutoff voltage of my over discharge same as the cutoff voltage on the datasheet of the cell, or same as the nominal voltage, or something between those two?
BTW, my battery pack is a 4S6P pack, so I'm going to multiply this cutoff voltage with 4 since this is the number of cells in series.

#### Alec_t

Joined Sep 17, 2013
14,009
BTW, my battery pack is a 4S6P pack, so I'm going to multiply this cutoff voltage with 4 since this is the number of cells in series.
Each cell should have its own cut-off arrangement for safety, because cells can become unbalanced. Why not use an off-the-shelf BMS designed for a 4S pack?

#### electromeow

Joined Oct 12, 2021
9
Because the BMS circuits alreadily made don't fill my criterieas. My output current may increase up to 150 amps.

#### camerart

Joined Feb 25, 2013
3,600
I'm trying to design a BMS board for my huge battery pack consisting of Molicel P42A 21700 batteries. I designed the over discharge protection part of the circuit using a simple comparator. But I don't know how to choose the cut-off voltage for my battery.
According to the datasheet, the cutoff voltage is 2.5 volts, the nominal voltage is 3.6 volts and the maximum voltage is 4.2 volts. But I've run into too much conflicting information about all those over discharge protections on the internet. Some designs cut the battery connection when the voltage drops under the nominal voltage, some of them cut it under 3.3 volts, and so on. So I found myself in a dilemma:
What cutoff voltage should I choose, maximizing the capacity I'm making use of, also avoiding from damaging the battery pack? What does "nominal voltage" actually mean? Should I choose the cutoff voltage of my over discharge same as the cutoff voltage on the datasheet of the cell, or same as the nominal voltage, or something between those two?
BTW, my battery pack is a 4S6P pack, so I'm going to multiply this cutoff voltage with 4 since this is the number of cells in series.
Hi E,
My understanding is that these cells must not be charged above 4.2V and best not discharged too low, as it wears the cells out 3v is lowish 3.5v is highish, so somewhere in between. I generally use 3.3v.

Try to charge all cells at the same voltage (balanced charge) or they wear each other out.

Use a temperature probe when charging, not too warm, and don't leave them unattended, or they can burn the house down.
Camerart.

#### LowQCab

Joined Nov 6, 2012
3,584
It's a balancing act between Battery-Life and maximum Battery-Capacity.
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#### Audioguru again

Joined Oct 21, 2019
6,445
The RC airplanes circuits that use my Li-PO batteries produce a low voltage warning when the battery cell voltage drops to 3.2V and disconnects the main motor but allow the control servos to work when the battery cell voltage drops to 3.0V.
The manufacturer advises not to frequently drop the cell voltage that low because it will reduce battery life.
Most lithium batteries are sold and stored at 3.7V which is halfway between 4.2V when fully charged and 3.2V when the load should be disconnected.

I think manufacturers of protection circuits who use a 2.5V cutoff are selling many more more replacement batteries.