You are probably right about it showing for more than a split second. You aren't right about the alternator needing the battery once its running (the battery actually becomes a load and not a source since its being charged by the alternator) but now we are getting off topic lol. Just for shits and giggles though it wouldn't take more than a few seconds to clamp one end of a jumper cable to the neg. stud (giggity) and the other to clean ground on the motor and try it. Possible that the spike is only at the initial hit with the sol. engaging and spoolin up from a dead stop?
Shit I dunno I'm outta guesses but hey I tried
lemme drink on it awhile longer whilst I wrench on my tr....
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Shit I dunno I'm outta guesses but hey I tried

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