Unhook the cable- - - - -then go buy an alternator. That test works- - - -sort of- - - -on generator-equipped vehicles. Solid state rectifers and voltage regulators really don't like the 100+ volt spike that happens just as the cable gets disconnected from the battery. A battery acts somewhat like a shock absorber in a charging system- - - -helping to dampen out sudden surges from things like solenoid contacts making and breaking, and other voltage surges. Parts dealers love it when people repeat "old mechanics' tales" that went obsolete decades ago, like "Keep a hot engine running while you pour water into it"- - - -that one quit being relevant when they put a water pump on the Model A Ford in 1928.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!