LiFePo4 cut-off under load?

[crookedspoon]

Hi all,

Often enough, it's said that the cut-off point for a 25.6V (24V) LiFePo4 battery is at 24V to 25V as to not discharge the battery completely.
What I wonder, I haven't easily found an answer to: Does the cutoff voltage value also apply when the battery is under load?

When my battery is at about 27V, then put under heavy load, the voltage easily reaches 25V. Should the load then be removed to avoid damaging the battery should I only consider the voltage when no load is applied?

thanks for your insights!

 

[JJJJ]

Just a thought. Add a second battery in parallel to share the work?

 

[Mattb4]

Voltage droop occurs when heavy loads are placed on a battery. This is a function of battery total capacity and design. The battery is not necessarily damaged nor is it as discharged as the voltage reading would indicate. You will see the voltage rise back up immediately on removal of the load. The ability of the battery to sustain a minimum voltage under load is not often mentioned for batteries rated in ah but it is a function of batteries rated for CCA. https://www.batteryequivalents.com/how-to-convert-cold-cranking-amps-cca-to-amp-hours-ah.html

What catches up a lot of folks is low voltage inverter trips during heavy loading. They see the power go away and run to look at their system. Battery voltages look plenty good (load is now gone) so they do not think that they encountered insufficient battery capacity.

 

[Steve_S]

I run 24V LPF, several packs in one battery bank. I am often running at 10A-40A draw average but can spike up to 200A Draw and even higher occasionally. Voltage Drop always happens on a heavy load but even when doing the Nasty 200A Pull it's usually only around a 1V drop seen across the pack during that load. Again though I run 6 Packs in Parallel as One Battery Bank. There were a few occasions when the voltage drop took packs below the cutoff point which of course dropped the inverter.

I have my Undervolt Protection set at 2.700 Volts per cell. 21.6V @ Pack
Undervoltage Release is set to 2.800 Volts per cell. 22.4V @ Pack.
BMS Pack Power Off is set to 2.600 Volts per cell. 20.6V @ Pack. This leaves room above 2.50V to restart the battery packs & apply a charge without ever bringing the cells down too low which can harm the cells.

Some BMS' have Low Volt Cutoff delays which can be an advantage when you have a Low Voltage State pack that is still within operational voltages but at the bottom end. This allows them to still service the demand and handle the "momentary surge drop" even if it drops below operational parameters for the few seconds required. But if that timer is exceeded and the cells do not recover above the trigger value, then the BMS will shutdown the pack.

Battery Packs operating in Parallel within a Bank divide & share both the Charge & Discharge proportionately to their capacities.

Hope it helps, Good Luck.