Measuring a battery state of charge (SOC)

[GeraldTheBonzai]

As part of my ongoing fiddling with my home solar rigs, I was doing some experiments with my alternative rig that uses AGM batteries and noticed some oddness. Immediate thought was that my batteries were on the way out but did some research and thought I would share, as it might come in useful.

State of Charge (SoC) is a measure of how "full" a battery is. It can be measured by taking the voltage of a battey at rest. However different battery chemistries have different voltages relating to their SoC. Heres a summary:

SoC, Flooded Lead Acid, AGM (Sealed), LiFePO4
100%, 12.60V – 12.70V, 12.80V – 13.00V, 13.50V – 13.60V
90%, 12.50V, 12.75V, 13.40V
80%, 12.42V, 12.65V, 13.30V
70%, 12.32V, 12.50V, 13.20V
60%, 12.20V, 12.40V, 13.15V
50%, 12.06V, 12.30V, 13.10V
40%, 11.90V, 12.20V, 13.05V
30%, 11.75V, 12.05V, 13.00V
20%, 11.58V, 11.90V, 12.90V
10%, 11.31V, 11.80V, 12.00V – 12.50V
0%, 10.50V, 10.50V, 10.00V

(these values have come from various docs online - should be correct but per normal, always check. )

In my main system, which is LiFePo4, the battery has a BMS. There is also a shunt, which is an accurate way of measuring SoC that isn't voltage based, and finally the charge controller, that gives an estimated SoC by reading the battery voltage. My second system is AGM with just a charge controller, and it was this one that was looking odd.

The CC was showing a SoC of 100% and had just been charged via a panel. I put on a load of 60W (at 12V, so around 5A) and the SoC dropped tp 99%. However, within a few minutes the SoC was 78% and dropping, so what appeared to be very quick discharge. Initially assumption was that the batteries were on the way out (they are about 4 years old, so shouldn't be). So this is where the research started, and it all comes back to chemistry.

When the battery is charging, the CC is applying a bulk charge to the battery, so in the region of 13.8V. The CC sees this voltage so estimates the battery is at 100% SoC. Looking at the current going into the battery its low - less than an Amp. When the panel is disconnected, due to the battery chemistry, it retains a surface charge - it hangs on to the 13.8V. The CC knows there is no current going into the battery but still sees the 13.8V so reports 100% Soc. After a bit of time, the surface charge dissipates, and the voltage drops to the resting voltage - around 12.7V but the CC has seen a voltage drop, so it now thinks the battery has discharged. Its basically confused.

Putting a load on the battery when it had just been charged causes a similar effect. The surface charge is keeping the voltage high, but when the load is put on, its immediately absorbed by the load, so the SoC appears to drop rapidly and make it look as if the battery is discharging very quickly.

Rebooting the CC, and the readings persist. The CC is seeing a voltage of 12.7V but reporting a SoC of 75%.

So bottom line - if your Charge Controller reports SoC, don't necessarily believe it - measure the voltage of the battery whilst it is at rest rather than being charged.