How do you measure the battery’s charge rate?

For batteries with free acid, the specific weight (density) can be measured.
It is about 1.10-1.12 for a discharged battery and 1.26-1.28 for a fully charged battery.
For Gel/AGM battery, the density cannot be measured to determine the charge state.
Instead, measure the battery’s rest voltage is a good way to get an idea of the charge rate.
Voltage measurement means measuring the battery voltage after the battery has rested for a few hours.
Resting means that no current is removed or charged into the battery. Simply put – the battery is switched off.
Voltage measurement is usually of no use if the battery is connected to an existing system and always only gives an approximate perception of the condition.

Tip:
Approximately 12.8V indicates a fully charged battery and approximately 11.8V is discharged and the ratio is linear.
In other words: 0.1V corresponds to about 10% charge rate.
A battery ( in rest – remember ), measured to 12.2V, is thus charged to about 40%.

What is the efficiency of a lead acid battery?

Any energy (Ah) charged into a lead acid battery can then not be used when the battery is discharged.
Some of the charged energy is instead converted to heat.
It is said that the battery has an efficiency.
For lead acid batteries it is about 85%.
Therefore, charging a 100Ah battery requires approximately 1.2 x 100Ah = 120Ah.
This fact is important to consider choosing the size of charger and the expected recharge time.