Why is SOC dropping while not using my battery?

Modified on Wed, 14 Aug at 12:06 PM

You may notice that your battery State of Charge (SOC) is decreasing while you are not actively using your battery.

Depending on how fast SOC is dropping this could be perfectly normal, but it deserves an explanation.

How is SOC calculated? When your battery is fully charged SOC is set to 100%, then BMS monitors discharge current over time and decrements SOC value according to amount of discharge current passed every hour. Even if you are not actively using the battery it is still being slowly discharged internally by BMS's own activity and externally by any always-on devices such as safety gas leak monitor, inverter's idle load, any displays or LEDs, etc.

Why SOC is decreasing while Lithionics app show zero Amps? SOC and Current (Amps) are reported in 1.0 resolution, but internally tracked in 0.1 resolution, with some margin of error due to limited measurement precision. Fractions of 1A are counted by BMS software when small background load is present, even if BMS is reporting 0A. So, when you think there is no load, there might be some background load presented by some always connected devices with total consumption less than 1A, which still shows up as 0A in the app.

It's also possible that you don't see a load when you are looking, but some intermittent load might be registering during certain hours, at night, etc. when no one is looking, but BMS is accounting for such load and reduces SOC counter.

How much daily SOC loss is normal? It depends on the size of your battery and how many always-on devices you have in your system. Let's calculate an average example for a single 320Ah battery and idle load of 200mA, or 0.2A. An average load of 0.2A over 24 hours would consume 24h x 0.2A = 4.8Ah. Let's convert this into percentile value 4.8 / 320 = 1.5%. So, every day your battery is expected to lose 1.5% of SOC, but since SOC is rounded to a whole percentile you may see between 1% and 2% loss per day, averaging to 1.5% per day.

Please note that we are considering a battery which is turned on and connected to an electrical system, even if the main disconnect switch is turned off, some safety devices are wired in bypassing the main disconnect switch. This is why Lithionics battery has it's own On/Off switch which allows a complete battery shut off when your vehicle is in long term storage mode. However, even in storage mode when BMS is turned off there is still a small internal discharge going on due to electro-chemical nature of batteries where small amount of charge is lost due to self-discharge. This is why it's important to fully charge your battery after long term storage to re-sync BMS's "remembered" SOC to an actual full charge SOC at 100% which is confirmed by briefly reaching the full charge voltage at the end of charge cycle.