It should not be over 70% capacity (12.32 volts on the multimeter).
charge
Yes and no! it is possible to charge a battery from an inverter but more energy is drawn from the source than the energy that is saved, so no it is not possible.
An inverter typically converts DC power from a battery into AC power for use with electrical devices, while a UPS (Uninterruptible Power Supply) is designed to provide backup power and often includes a built-in inverter. However, if you connect an inverter to a UPS, it may not charge the UPS's internal battery effectively, as UPS systems are designed to charge from AC mains power. Instead, a UPS usually requires a direct AC input to charge its batteries properly.
Most inverters will not be harmed by charging the battery [running the engine while the inverter is connected]. However, for safety [inverters are expensive] it is best that the Owner/User Manual be consulted to make sure that it is safe to charge the battery while the inverter is connected.
12 volt of an ordinary inverter battery
To store energy produced from a dynamo, you can incorporate a battery into the system. The dynamo generates electricity, which can be used to charge the battery. An inverter can then be connected to the battery to convert the stored DC energy into AC power for use when needed.
Batteries are storing charge or free electrons. The state is by nature a fixed voltage rather than a varying voltage. You can convert the battery charge to AC by using what is called an inverter.
An inverter simply takes a DC voltage, usually from a battery, and converts it to AC for use by standard appliances. A UPS does the same thing, but has added circuitry to charge the battery when AC utility power is available, and to automatically switch the inverter on when utility power fails.
Yes, Its good alternate solution but by using non-solar inverter it requires manual switching. (Solar charge controller will no longer works) & In Solar inverter, its easy switching & more life (solar inverter manufacturers gives max 5 years warranty) As we use more panels in parallel it charges battery quickly.
Yes, it will hold it's charge for at least 6 months in storage if it is a very good battery to start with.
Hydrometer
In UPS ,the main power comes to the UPS and is used to charge a battery inside. The output from the battery is fed into a sine wave inverter which converts DC into AC and feeds to the computer in case of a power failure. In such a scenario, the battery is stopped from charging and instantly starts to supply power to the computer. In Inverter ,AC is converted into DC and then is used to charge the battery. When the power supply goes off, the relay triggers the switch from mains to inverter. It is the use of a sensor and a relay that is the main difference between a UPS and an inverter, otherwise the two are same. And the use of relay and sensor cause the time delay in power supply from an inverter.