It may be fine and just a low battery.
But here is a scenario when there are bad connections or terminals.
Lets say all of the bulkhead connections have lots of resistance.
As a result, the voltage regulator sees a lower voltage than the alternator is producing.
So the regulator lets more power go through to create a stronger magnetic field.

Result is something like this.
The alternator is putting out power at higher voltage. Lets say 16.5 Volts
Some of the voltage drops as current flows through the resistances.
Regulator lets enough current through that it sees 14.0 Volts
The battery, like many electric devices, draws more current at higher voltage.
While it would have drawn no current at 14 Volts, at 15.5 Volts, it draws 16 amps.
It can't transfer any more electrons chemically, so it turns that energy into heat and boils off the acid.