The primary cause of overcharging in car batteries is the alternator producing excess voltage, which can lead to battery swelling, overheating, and electrolyte loss. Undercharging, on the other hand, does not damage the starter but prevents the vehicle from starting. Overcharging affects the battery's electrolyte rather than significantly increasing voltage levels. Understanding these dynamics is crucial for maintaining battery health and vehicle performance.
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from: https://itstillruns.com/causes-alternator-overcharge-7391992.htmlphinds said:I think you're likely to ruin the battery by overcharging long before you could get to where it would hurt the starter (BUT ... I'm not an auto guy so take that w/ a grain of salt). I don't think overcharging raises the voltage level much, it just screws up the electrolyte.
If you're undercharged, I don't think that hurts the starter, you just won't start.