How charged are my lead-acid batteries?
The voltage tells you how charged your lead batteries are, no matter how big your battery bank is. To find out how big your battery bank is, you must perform a load test.
The size of your battery bank is measured in amperage hours (Ah). It is a measure of how many hours you can consume a certain number of amps with your batteries. If you have a 200 Ah battery, you can theoretically use 20 A for 10 hours. (Which only works in theory, because lead-acid batteries die at too many deep loads).
The table shows the voltage across lead-acid batteries at different levels of discharge. The table applies to consumer batteries during rest, or very low consumption. A starter battery need to rest for several hours after an engine boot for the value to be reliable.
When using more power, add approximately 0.2 V to the instrument's value before comparison. Thus, if your instrument shows 12.1 V in consumption, it should be read as 12.3 V.
Note that the table is approximate, but after a while you have learned how your lead-acid batteries works. A gas tank gauge is also not accurate but still provides a sense of calm. If you have a battery monitor that uses current measurement to estimate the charge rate, you should still keep track of the voltage because the voltage never counts off.
At charging, when the voltage is over 13V, it is fully charged. When the voltage is over 14 V, the charge current has decreased properly as resistance increases.
