Hi Folks!
I have a 2x120 Inverter/Charger with Cerbo and Smart Shunt.
As the batteries are draining, the SOC hits 50% and the voltage shows 11.41v
According to the manufacturer, 50% dead should show a voltage of 12.10v.
So, Victron thinks that 11.41v = 50% dead. But the battery chart from the manufacturer shows that 11.41v is more than 90% dead.
This causes me to wonder, should the SOC roughly follow the battery manufacturer’s chart?
Nope. LFP batteries don’t really work that way. Besides you would need the battery to sit for a significant period of time with no charge or load to get a stable resting voltage that you could somewhat reliably use.
This is why a shunt is your best tool for measuring SOC.
Someone on this site know the science why and will likely chime in.
OK, thanks. I should have mentioned, these are lead acid batteries and that the load was consistent during the discharge cycle. I do hope someone comes in and clears this up for me as it has me very curious. The SOC is coming from the shunt as the internal one for measuring SOC on the inverter is turned off.
The SmartShunt measures the charge taken out of the battery and compares that to the capacity you put into the settings to give the SOC. If you have a 100Ah battery and the shunt sees 5A for 10 hours, that is 50Ah so it will display 50% SOC. Whilst discharging the batteries there will be voltage drop in the battery due to resistance inside the battery as the current flows so the voltage at that point will be less than 12.10V. 50% and 12.1V are the resting voltages, so once you get to 50% you have to stop discharge and wait 24hrs for the battery to rest, at which point the voltage recovers. The SmartShunt can not account for this.
The thing is, as a lead acid battery ages it loses capacity and the SmartShunt can not account for this so the SOC is lower than indicated, if the 100Ah battery is now 75Ah then the SOC after 50Ah is discharged would be 33% resulting in a lower voltage. Additionally, as the batteries age the internal resistance increases so the voltage under load reduces to an even lower voltage.
Finally, the SOC voltage curves are typically generic, there are small variations depending on battery type, alloying elements in the lead etc.
Depending on your battery age, capacity and current draw your values may be acceptable (high current draw from small battery bank) or they may indicate that your batteries are getting tired (low current draw from large battery bank). What you need to do is compare the battery voltage at 50% when new with the current voltage at 50% for the same current draw to see how they are ageing. If they used to be at 12.0V and now they are 11.4V they show loss of capacity.
All this is based on the SmartShunt being installed correctly with all negative connections to the system side of the SmartShunt and no connections to the battery negative other than the SmartShunt. If you have unmeasured loads all bets are off.
For reference I have just looked at some of my data when I had a 690Ah AGM battery bank, after 3 to 4 years use at 50% SOC and a low draw of around 10A the battery voltage was 12.0V. For me that meant the batteries were still in good condition after 3 to 4 years and had lost little capacity. If I drew 150A from the batteries at 50% SOC then the voltage would fall to 11.5V showing how sensitive measured voltage is to current draw.
This is fantastic information, thank you pwfarnell, this site would not be the same without you! I’m going to do some testing based on your info. Some of the Victron gear (like the cerbo etc) are direct wired to the batteries and not the other side of the smart shunt so I will investigate the wiring as well.
Told you there was someone on here who new the technical stuff who would chime in.