SOC reads 54% but battery reads 46.4v

[SallenB]

Need some diagnostic input.

48v system, two 5kwh lifepo batteries in parallel connected to 5kw all in one inverter with 1600w pv array. Supplies a 120v critical loads panel, grid isolated.

3 months old. Used to work fine but now has some issues. Battery discharge cutoff is set to 48.8v and charge/discharge at 20%/95%.

Inverter shut down when reading 54% SOC and battery voltage at 46.4v. It is set to shut off at 20% SOC although I do have voltage cutoff set to 48.8v, I was understanding that SOC% and voltage correlated to one another. All 16 cells read about 2.9v.

How could SOC% and voltage drift apart?

54% SOC should be around 52.2v when referring to the Lifepo voltage chart.

Any input is appreciated.

 

[Tomthumb62]

SOC meters will drift from accuracy over time. On my battery, the instructions to recalibrate it is to fully discharge to 0% until the BMS cuts off, then charged back up to 100% until the BMS cuts out.

I’m not familiar with your battery but many SOC meters, whether built in or external (such as the Victron smart shunt), need some sort of similar process to get the meter to be accurate again.

I believe on the Victron smart shunt, you can manually set the SoC, but you’d need to let the battery rest for 24 hours, disconnected from everything to find the real voltage. Or do what many of us do if the accuracy isn’t crucial - wait a couple of hours before measuring voltage.

 

[SallenB]

Thank you Tomthumb62. I'll do that and report back.

 

[Zapper77]

Need some diagnostic input.

48v system, two 5kwh lifepo batteries in parallel connected to 5kw all in one inverter with 1600w pv array. Supplies a 120v critical loads panel, grid isolated.

3 months old. Used to work fine but now has some issues. Battery discharge cutoff is set to 48.8v and charge/discharge at 20%/95%.

Inverter shut down when reading 54% SOC and battery voltage at 46.4v. It is set to shut off at 20% SOC although I do have voltage cutoff set to 48.8v, I was understanding that SOC% and voltage correlated to one another. All 16 cells read about 2.9v.

How could SOC% and voltage drift apart?

54% SOC should be around 52.2v when referring to the Lifepo voltage chart.

Any input is appreciated.

Fully charge the batteries. Try to reach 56.5v twice per month and hold for 1-2 hours.

What bulk voltage were you previously charging at?

 

[SallenB]

Thanks. It's set to charge at 56.8v. Charging is set to 95% and cutoff at 20% but I can have it charge to 100% twice a month if that benefits longevity.

What are the thoughts on cyclical discharge? For example, using a washing machine in the agitate cycle where the drum motor will cycle on and off loads of roughly 500w? Will that intermittent loading cause any harm to battery/inverter as opposed to a continuous electrical load?

 

[Tomthumb62]

What are the thoughts on cyclical discharge? For example, using a washing machine in the agitate cycle where the drum motor will cycle on and off loads of roughly 500w? Will that intermittent loading cause any harm to battery/inverter as opposed to a continuous electrical load?

In my limited knowledge on lifepo4, it shouldn’t matter much. Lifepo4 is very robust and handles voltage drop like a dream compared to lead acid.

Can it affect the longer term health of your battery and inverter…maybe, probably. What’s the alternative, to not use the washer? Have you ever tried washing clothes by hand?

 

[SallenB]

Thanks. Washing by hand is an option and could be done. I've been learning a lot about solar power/inverter capabilities and it's limits of use. Much appreciation for your input.