A generator (or car) battery discharge rate is about 1% per day at room temperature, 0.25% per day at 10 °C (50 °F), and 1.5% per day at 30 °C (86 °F). Self-discharge happens without anything connected to it. When your generator is on "standby," the controller is using battery power. It will discharge even faster. But how long will it last?
How long the generator battery lasts depends on the battery's age, the climate (heat speeds up the process), and how often you run your generator.
When your generator is running, the engine alternator recharges the battery. This quick charge is a bulk charge up to about 80% of its capacity. It is due to a powerful amperage and voltage. To fully charge the battery requires a more extended period. This absorption charge stage is at a much lower current rate. Exercising the generator is often not enough to ever do this.
The solution is to use a smart charger. It can stay plugged in all the time. The bulk, absorption, and float charge cycles are all taken care of for you.
It will save you fuel, prolong the life of your engine and battery.
When the battery is at rest, not under load, or being charged, the voltage can tell you what the state of charge is. Here is a chart you can use below. The best way to measure the battery voltage is with a voltmeter. The voltage shown on the generator controller can show 0.1 volts lower as the actual measurement is at the controller, not directly at the battery. (Something to keep in mind)