Two realistic possibilities:

#1 A dead cell.
#2 A faulty charger.

I have two ctek chargers (supposed to be the best right?), which overcharge car batteries, in the one it's like something had gone out of calibration and won't stop charging until it reaches nearly 16v (instead of 14.4v), and it does this indefinitely at around 4A.

What I always say with car batteries though (and having experienced an exploding battery), it is NOT worth f***ing about with them, the risks are too high and it'll likely leave you stranded one day, so just get a new one.

 

Trying to make this story short but I pulled a battery off my step son's Mustang that has been sitting behind my house since summer. Back in July my wife bought a brand new battery for it.

I pulled the battery off it the other day since the car is just sitting there and he won't pick it up. I did not check the battery voltage but assumed it was probably a bit weak.

I connected it to one of those "smart chargers" and at first the lights lit up but then none of the lights on the charger were lit. So I checked to make sure the charger was getting power (breaker didn't throw). So I grabbed another battery charger. It acted like it didn't work either. So I plugged the first charger into a different outlet. Same thing. I decided to check the voltage of the battery. It's at .5 volts!

By all accounts, this battery could be a bit weak but should not be dead. Apparently this car has some electrical issue and is draining the battery. It should not have went dead in 4 to 5 months. The car is in bad shape anyway. It would not pass inspection in a state that did inspections.

Anyway. I have done this in the past and it may or may not be a good idea.

I have one of those "old school" 6 amp battery chargers that was my grandfathers, Sears Craftsman, probably made in the 1960s. You know the kind with an analog meter, 6/12 volt switch, non-polarized power cord and non-grounded metal casing! I did replace the 120 volt and 12 volt leads on this thing.

Anyway, I keep this charger for one reason. Whenever I have a totally dead battery, one of those new "smart chargers" will not charge a battery. But this thing will.

I connected this charger to this battery and let it charge for about 12 hours. I don't use this charger normally because it lacks the technology to turn off when the battery no longer needs to be charger. If left on a battery for days it will boil the acid out of the battery.

After 12 hours I connected my smart charger to it and it stayed lit for about 2 or 3 hours and eventually it showed "charged". I have not checked the voltage in a few days but so far it seems to be holding.

 

I have seen it once, a battery exploding. The only result was the acid burned winter coat and bad spots on the cars fender paint. But this was from trying to jump a frozen car battery. Not from charging one with a dead cell.

One day (back in the BIG 80s!) dad was coming home from work and stopped at a gas station. I can't remember if this happened as soon as he shut the engine off or after he had filled up and was going into pay but he said the battery exploded on his '79 International Scout II for no apparent reason. The engine was not even running at the time. He said it sounded like a bomb going off.

The only thing I can think of was the lead plates inside the battery collapsed and shorted the battery out. I don't think any of the leads were chapped against the chassis.

Since he was at one of those gas stations, back when gas stations actually sold more stuff for cars than they did food, he was able to buy a new battery right on the spot. Today it's the opposite. Gas stations sell more snack items than they do car related stuff.

I remember "back in the days" gas stations had a Pepsi or Coke machine, a Lance snack machine and maybe a machine that sold cigarettes.

 

One day (back in the BIG 80s!) dad was coming home from work and stopped at a gas station. I can't remember if this happened as soon as he shut the engine off or after he had filled up and was going into pay but he said the battery exploded on his '79 International Scout II for no apparent reason. The engine was not even running at the time. He said it sounded like a bomb going off.

The only thing I can think of was the lead plates inside the battery collapsed and shorted the battery out. I don't think any of the leads were chapped against the chassis.

They explode because they "gas" during charging, producing hydrogen gas, the older they are and the more you try to overcharge them, the more they "gas". This gas is extremely volatile and explosive (Hindenburg anyone?), it normally requires a spark or flame to ignite hydrogen, BUT it auto-ignites at around 500°C or 930°F (even hotter on cat equipped vehicles), which is just about within the range of the temperature of a hot exhaust manifold.

It's more than possible that the old battery was gassing due to a faulty alternator, when moving the gas would disperse quite quickly, but when the vehicle was stopped it was trapped under the bonnet/hood until it came into contact with the exhaust manifold.

 

They explode because they "gas" during charging, producing hydrogen gas

Worse - they produce the exact explosive mixture of hydrogen and oxygen.