For example I have Lead acid battery of 150 Ah (C20), 12 Volt and it is discharging with flat rate of 15 A/s.
The initial voltage level at (t=0) is 12.2 volt.
Then What will be the SOC level at t=0 and t=20 time instant.
Please share the formula to calculate the SOC level for both the condition. I will be thankful to you for this act of kindness.
How can we calculate the state of Charge (SOC) of the battery in on load condition?
When discharging with C/10 (15 A), the capacity C10 available is lower than the C20 value. Typically your battery data sheet will give a good estimate for C10.
The rest is quite simple: SOC = C10 * (1 - t/10 h)
https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery and
https://de.wikipedia.org/wiki/Bleiakkumulator are great sources for this. And they hold good info links too. (Though you may need an translation program for the German content.)
BTW: The best method to measure the SOC of a lead acid battery is to measure the acid density which has a direct correlation to the SOC. Calculating the SOC from the open circuit voltage (OCV) is possible (with some constraints), but clearly inferior.
When having a variable load, it is possible to compensate the effect of the serial resistance (measuring voltage and current) - at least for higher currents (C/5 and beyond), but is clearly inferior to the OCV measurement. (OCV varies by about 0.2 V from 0 to 100 % OCV. The exact value will be battery-specific.)
Regards
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