Battery testing method in a layman's language
Could you first explain what IS an Inverter battery. In a layman's language. A
I assume an SLA battery that acts as a backup for the inverter in a solar powerd system
When a lead-acid battery goes bad, in your car or in an inverter, they will put out 12 volts to a digital volt meter BECAUSE there is no load on the battery. (because)... A voltmeter only loads the battery with a TINY fraction of an ampere called a few "micro-amps". So the trick is easy. Just connect a REASONABLE load to the battery. Something that may draw an amp or 2 if it's a small battery... or a heavier load for larger batteries. A small light bulb may be sufficient. As you connect the load, watch the voltage on the meter. If the voltage drops by a volt or 2, then the battery is bad (or really needs charging). If the voltage holds pretty steady, the battery is likely OK.
A standard lead acid battery won't stand being discharged below 11.8 volts (very often) if you d it won't recharge with a standard charger.