Ever wondered why your battery doesn’t retain charge anymore? Maybe the voltage is so low and you don’t know how to take the voltage readings, well battery voltage is like the pressure in a water tank pushing the water to it’s destination, have you ever imagined what happens when there is no pressure in the thank? Your guess is as good as mine. A solar system owner as you should know how to test the voltage of your battery.
With simple tools like the voltmeter or the multi-meter you can test the voltage and do it yourself