Q: Passenger side battery boiling over

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Passenger side battery boil over. Post and crossover cleaned. Tested cold nonrunning 12.5 volts. Ran 15 minutes passenger battery started to boil. Read 15.2 volts. Same as driver side. Battery tested good.

My car has 170000 miles.
My car has a manual transmission.

This may suggest you have a voltage regulator that is allowing the alternator to overcharge the battery causing it to boil as you describe. The voltage regulator is a unit that regulates the charging of the battery by the alternator. When the voltage regulator is not working properly, this may result in the alternator allowing too much power to be delivered to the battery resulting in damaging wires and prematurely sometimes destroying the battery. A common sign of this is usually the acid inside the battery boiling causing the battery to swell (as in your case). You may also smell a bit of smoke due to things potentially getting too hot. In other cases it may result in the alternator not supplying enough power to the battery, resulting in undercharging the battery or not charging it at all. I would recommend having an expert from YourMechanic come to your location to diagnose your vehicle’s charging system.

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