With the engine running, the red light indicating battery overload turns on in the alternator state. However, this cannot happen because the battery would have exploded. It was measured in two cars and showed the same results. Their batteries are four years old in one car and six years old in the other. With the engine off, they appear as new, the battery state shows full (green). The conclusions are up to you. I don't consider it reliable though. At such a price, much lower than the equivalent for measuring household batteries, it is probably just a toy for children.