Battery boiling = over charge condition for whatever reason.
Over charging is most often a result of voltage drop problems in ignition & charge circuit, or occasionally an old battery or one that has been severally discharged for a period of time won’t take a full charge.
Get a voltage meter; take a reading at battery when engine is running should be 14.4 to 13.8 volts depending on under hood temperatures. If over 14.4 V, start cleaning all connections at battery, engine ground strap, bulkhead connector, starter switch etc. Remember a little bit of corrosion on several connections adds to make one big resistance in a circuit that has to be removed.
Big picture is if voltage regulator is detecting lower than actual harness voltage from defective ignition circuit, it will control alternator to increase its output an equal amount to bring phantom miss read voltage up. In other words if voltage regulator is reading 13 volts, 0.8 v low, the alternator will put out an additional 0.8 volts on top of existing harness voltage to battery which could be as high as 15.2 v in this example. Don’t forget to make sure you have zero voltage between negative battery post, and bases of voltage regulator, spark controller if equipped, and alternator case, as voltage drop can occur on the ground side of these circuits.
