Proper battery management is essential for the long life of your lead-acid or lithium battery. With a lead-acid battery, the voltage is more or less related to the percentage: take a healthy battery where you read 12V. You can then assume that it is approximately 50% discharged.
Nowadays, we want to know more accurately how the energy management is performing: this can be done with a battery monitor or potentially via an app (built into the lithium battery). Every accurate measurement is performed via a so-called “shunt.”
The shunt registers the incoming and outgoing current and calculates this against the battery capacity to arrive at a percentage. There will always be a small deviation: a difference will arise every time there is partial charging or discharging. Over time, these differences accumulate. The discrepancy increases! The percentage, also known as SOC (state of charge), then no longer corresponds to reality.
Solution: by fully charging the battery, the battery monitor will reset itself and provide a correct reading again. If, for example, charging is only done via solar panels and the batteries never reach a 100% charge range (winter), a large margin of error can occur in the SOC reading on the battery monitor.