SOC calculation when charging

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I am using a Quattro and Smart Shunt on a yacht. I use the data from the Shunt to display the SOC.

When I am off the grid with engines off obviously the SOC reduces over time. This makes sense to me.

My question is around what happens when I switch on the engines. Obviously this charges the battery bank again. I would expect the SOC to slowly increase again as a result. However, what happens is that the SOC moves immediately to 100% again, even if the batteries are clearly not charged full again.

Am I doing something wrong, or am I thinking about this wrong?

[M_Lange (Matthias Lange - DE)]

That’s a frequently asked question.
Please read the manual and also take a look at this:

Battery monitor jumping to 100% or reporting a high SOC with low Volts / the BMS shuts down

This topic covers the reported SOC being out of sync with the actual battery voltage or state. It covers BMV battery monitors, SmartShunts, Lynx BMSs and Lynx Shunts and will just be called shunt below. It is also assumed that the shunt is monitoring the domestic / house / cabin battery bank and not engine start / bow thruster / winch / caravan mover battery banks. The problems may be one or more of the following and is often linked to mobile applications with small solar systems. The SOC jum…

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Danke Matthias,

That helped me solve it.