I have the following setup for solar power, which worked well until recently:
solar panel <--> solar charge controller <--> small (car) battery <--> converter: 12V DC to 220V AC <--> my power tools
Last year, on a full battery I could power heavy tools such as a belt sander for many hours before it ran empty. But now, when I use power on a seemingly full battery (see video below, the battery indicator is full at the start) and 13V on the battery which should be good, the voltage drops immediately after starting the tool - and the battery indicator drops to zero. The converter beeps (low voltage warning). After a few seconds, all power supply stops. When I wait about half a minute, the battery indicator jumps back to "full" again, and I can use the drill for another few seconds until it drops again.
What could be the case here? The battery seems to have enough liquid. My initial thought was to replace the battery, but this strange behavior with the indicators on the solar charge controller makes me think the problem might be in another component.
Video of it happening: