How to know Lithium battery charge percentage

Plucka · June 2018

I have 360amp/hr lithium battery and I run everything on 240 volts .After toast and a cup of tea in the morning the battery is 12.6 to 12.9 volts .What is the percentage of discharge? It is winter here with lots of cloud and the battery only gets up 13.3 volts after running fridge, freezer etc during the day .In the summer the solar input shuts down around lunch time as the battery was full @ 13.6 volts

BB. · June 2018

What is the exact brand/model/chemistry of your Li Ion pack (4x cells in series?)... For example LiFePO4 (lithium iron phosphate) batteries are commonly used for home and vehicle battery banks--More stable chemically and less likely to fail with fire--But they charge at a substantially lower voltage than many other Li Ion chemistries.

https://www.powerstream.com/lithium-phosphate-charge-voltage.htm

-Bill

mike95490 · June 2018

Li batteries have a nice simple voltage curve / state of charge. It gets wild at the upper & lower ends, but you should never go there. Your battery mfg should have a graph of what Voltage:SoC is.

Plucka · June 2018

My battery is LiFePO4 4 cells in series and I'm just after someone elses opinion if I go below 12 volts .

BB. · June 2018

The "generic" LiFePO4 cell should not be discharged below 2.8 volts per cell (4x2.8=11.2 volts)... If you look at the curves in my link above, there is not much capacity left below ~3 volts per cell (12.0 volts).

Generally, the longest life for Li Ion chemistries is to run them between 20% and 80% state of charge. Running high or low voltages (high or low State of Charge) generally gives you a shorter life (high SoC gives both a shorter cycle and aging life).

And remember that voltages are "per cell"... If you have once cell running below ~2.8 volts and the others are at 3.0 volts (=11.8 volt battery bank voltage)--It is that low cell that is "at risk".

If you have a BMS (battery management system) that can measure/control per cell voltages/state of charge--That will help. Otherwise, you should measure (and log) the cell voltages (like once a month) to insure the batteries are "balanced" (there is a whole bunch of discussions about "top" and "bottom" balancing (full charge vs is discharged balancing).

The closer you run to the "limits" (3.6 volts 100% and 2.8 volts 0%), the "more" cell balancing matters.

-Bill

Plucka · June 2018

I have a battery managemant system. My new 3000 watt inverter arrived and is performing great put out 242 volts. The cables that came with it aren't long enough so I had to join them but I'll put new ones on tomorrow. Thanks to those that responded to my question.

karrak · June 2018

Due to the very flat charge/discharge curves for LiFePO4 batteries it is not practical to use voltage to accurately determine the SOC. On the other hand due to their very high coulomb/current efficiency of around 99.5% you can get very accurate SOC readings by keeping track of the current going into and out of the battery. Victron make the BMV700 battery monitor or you can get some cheaper units on ebay . Things to look for are

Automatic reset to 100% when the battery voltage reaches the full voltage and the current has tapered off.
Ability to adjust the reading to take into account the battery efficiency.
Ability to calibrate the device
Devices using a current shunt will be more accurate than one that use a magnetic current sensor

Simon

Plucka · June 2018

Thanks

jonr · June 2018

The only reasonable SOC you can get from voltage is at the extremes - say > 90% and < 10%. But best life is obtained when you avoid the extremes.

How to know Lithium battery charge percentage

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