White Paper - Low voltage cut-off: a false sense of security

Without a BMS, even with a low voltage cutoff, cells in an high voltage battery pack will be over-discharged

Whether a cell can stand being fully discharged depends on its chemistry:

• some cells like to be discharged once in a while (e.g.: NiCd)
• some don't mind (e.g.: NiMH)
• some do mind but survive it, though not in as good a shape (e.g.: Lead Acid)
• some are irreversibly damaged (e.g.: Li-Ion, which cannot be at below 2.0 V)

No cell can stand being reverse charged, regardless of its chemistry.
Once a cell is discharged, continuing to run discharge current through it will reverse it (the polarity of its voltage will be reversed). This is not likely to happen with a small pack, but becomes increasingly likely in batteries with more and more cells in series.
In a high voltage battery, one with many cells in series, there is a much greater chance that the overall pack voltage is not evenly divided among its cells.

In theory, if the pack starts perfectly balanced when full, all the cells will discharge evenly, and all their voltages will remain in lockstep, all the way down to discharged. In reality, the capacity of the cells will be mismatched, and the least capacity cells will reach the low voltage first. (Unless cells were pre-selected and matched to have exactly the same capacity.)

A battery with 10 LiFePO4 cells in series (whose minimum safe voltage is 2.0 V) may very well have a total voltage of 20 V. But there's no telling if some cells may be at 3.0 V, while others at 1.5 V.

LiIon cells do not deal well with over-discharging; once discharged, they cannot produce more current as the other cells in series are still doing so. Their voltage drops rapidly once discharged, so it's very easy to bring their voltage to below 2.0 V, or even reverse them.

Overdischarging a high voltage battery: the most discharged cell is under-voltage (left) or even reversed! (right)

Various battery loads handle low battery voltage differently:

• Some battery loads continue to draw current as long as there is some battery voltage (e.g.: a lamp)
• Some shut down when there's not enough voltage for them to run (e.g.: a DC-DC converter),
• Some shut down at a programmable voltage (e.g.: better quality motor controllers),

Even a load with a low voltage cut-off cannot prevent this problem: the total battery voltage may very well be above the cut-off point, yet an individual cell may be over discharged. You may feel comfortable using a 10-cell LiFePO4 pack with a motor drive with a low-voltage cut-off of 25 V, a full 5 V above the 20 V minimum. But, in reality, you may be damaging the most discharged cells and not know it!

Discharging with a low voltage cut-off load: the most discharged cells are below 2.0V!

This is why a BMS (Battery Management System) is essential when discharging high voltage LiIon packs. Not only will the BMS tell you if a cell's voltage is too low. When properly connected to the load, as soon as any cell reaches its minimum discharged voltage, the BMS will turn off the load.

Discharging with just a BMS controlling the load (left). Discharging stops (right) when the least charged cell reaches its minimum voltage, even if the total pack voltage may still be above the low-voltage cut-off of the load.

In particular, in a vehicle, the BMS may gradually tell motor driver to the reduce the available drive, enabling the user to still get home.

An advantage of using a BMS is that a programmable low voltage cut-off is not required: any load will do, even a resistive one, as long as the BMS can turn it on and off.

"Low voltage cut-off: a false sense of security" by Davide Andrea is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Permissions beyond the scope of this license may be available by contacting the author.

Davide Andrea, Elithion, 1/8/09