LifePO4 batteries - what is your cutoff voltage?

LydMekk · Mar 9, 2023

I am running my 4 big batteries with Voltage settings only as the BMS don't communicate to the Deye (Sol Ark) inverters I'm using.
The MFG does not list any voltages except 48V nominal (this is a 15S2P config) and charge and float voltages (I am running 54V and 52V at the moment).

What is YOUR cutoff values? Atm. I have mine at 47V (3.13V) but thinking I might try 46V (3.06V)...again this is 15S so if you are running a "normal" 16S battery your values might be 49-50ish for cutoff.

Let me hear your thoughts please.

SeaGal · Mar 9, 2023

3.0V per cell is a good safe choice for LiFEPO4. Obviously voltage is not a reliable measure of SOC, so you may hit 3.0V when running a load without the SOC being that low, but sounds like you have no option unless you invest in an external shunt or similar.

dmsims · Mar 9, 2023

Worth repeating

wattmatters · Mar 9, 2023

dmsims said:
Worth repeating

I find voltage-SOC curves to be pretty useless as they either require batteries to be at rest or charged or discharged at a constant controlled rate and under standard conditions.

Which just makes voltage an impractical measure of SOC for everyday use.

LydMekk · Mar 10, 2023

I agree with you @wattmatters.
But, as my BMSs don't communicate with my inverters, Volts are the most precise of otherwise bad choices.
Percentage is more crap than volts.

Good point further up in the thread of trying to avoid the "knee" for prolonged battery life.

Ampster · Mar 10, 2023

wattmatters said:
Which just makes voltage an impractical measure of SOC for everyday use.

Yes but at the bottom of a discharge curve it can be a useful tool for the safety of a pack. My Coulomb counter is fairly accurate but if I don't reset it occasionally it can drift. That is why I have a hard Low Voltage Disconnect number.

callmeburton · Mar 10, 2023

Using the default "long life" settings for batrium which places the low shunt cutout at 50.4 for discharging. It does have a low SoC cutout as well but it isn't enabled right now ... default is 20%, resume at 25% (meaning it will allow discharging at 25% SoC or greater.

Ampster · Mar 10, 2023

Yes, I also use an 50% SOC number to tell my inverter to charge from the grid and that stops battery discharge. That way I have a reserve if there is a power outage. This is done when rates are lowest.

RCinFLA · Mar 10, 2023

If battery is 15s it might not be LFP cells. They make NMC packs but they are 14s.

LydMekk · Mar 10, 2023

This type atleast is a normal LifePO4 15 in series and 2 rows - 15S2P.

brb58 · Mar 10, 2023

wattmatters said:
I find voltage-SOC curves to be pretty useless as they either require batteries to be at rest or charged or discharged at a constant controlled rate and under standard conditions.

Which just makes voltage an impractical measure of SOC for everyday use.

The purpose of this discussion is what to set the BMS shutoff voltage to. How else would you do that without voltage on a cell by cell basis?

Sabre36 · Mar 10, 2023

Charge = 3.45VPC
Cut Off = 3.6VPC

wattmatters · Mar 10, 2023

LydMekk said:
But, as my BMSs don't communicate with my inverters, Volts are the most precise of otherwise bad choices.
Percentage is more crap than volts.

I use a good shunt for the regular SOC monitoring.

Ampster said:
Yes but at the bottom of a discharge curve it can be a useful tool for the safety of a pack.

Oh for sure.

The charger and inverter should already be managing the system based on upper and lower voltage limits set inside the BMS voltage cutoff envelope, while a good shunt should also recalibrate itself when the opportunity presents.

brb58 said:
The purpose of this discussion is what to set the BMS shutoff voltage to. How else would you do that without voltage on a cell by cell basis?

Is it though?

I agree that BMS settings are for protecting cells/battery from damage but the OP didn't ask about cell protection specifically. Hence the discussion has been a mix of thoughts on protecting cells vs managing battery life and other things.

The BMS settings really should only be the outer envelope settings for cell protection. Everything else should be managed by charger, inverter etc.

Ampster · Mar 10, 2023

wattmatters said:
Is it though?

One would hope that the discussion follows the title of the thread, but forum topics have a life of their own.

wattmatters · Mar 10, 2023

Ampster said:
One would hope that the discussion follows the title of the thread, but forum topics have a life of their own.

Agree, however the OP didn't actually state what the purpose of the cutoff voltage they were asking about was. Hence the ambiguity in subsequent discussion.

A cutoff voltage can be to prevent cells from irreparable damage, or it could be for keeping a reserve capacity, or it could be for extending the cycle life of a battery, or whatever other reason.

brb58 · Mar 10, 2023

wattmatters said:
Is it though?

Yes...it is...hence the title of the thread.

wattmatters · Mar 11, 2023

brb58 said:
Yes...it is...hence the title of the thread.

Which was:

LifePO4 batteries - what is your cutoff voltage?

Again that's doesn't tell us which cutoff they are talking about, and the OP's initial post muddies it further. Later they mention "avoiding the knee", which implies they means a different cutoff to a battery protection shutdown.

IOW we don't know if they talking about BMS protection cutoff settings, or charge controller cutoff and/or inverter cutoff settings?

brb58 · Mar 12, 2023

wattmatters said:
Which was:

Again that's doesn't tell us which cutoff they are talking about, and the OP's initial post muddies it further. Later they mention "avoiding the knee", which implies they means a different cutoff to a battery protection shutdown.

IOW we don't know if they talking about BMS protection cutoff settings, or charge controller cutoff and/or inverter cutoff settings?

LOL....but you were the one bringing up SOC out of the blue which had NOTHING to do with cutoff voltages.

kampto · Mar 12, 2023

Running this on my 24v system cant say if its right but been no issues over a year after switching to LiFePO4:
Bulk/Absorb: 3.5v (28V)
Float: 3.375v (27v)
Re-bulk: 3.2v (25.6V)
Low Voltage Cut off: 2.96v (23.7V)

During winter with snow I hit the low voltage cut off often.

wattmatters · Mar 12, 2023

brb58 said:
LOL....but you were the one bringing up SOC out of the blue

SOC and battery life was injected before I commented: