My battery popped today, it vented it's internal fluids abruptly. The battery was very warm to touch.

Any suggestions as to why?

Under what circumstances? What was the car or yourself doing when it popped? How old? A few more details may help people to offer suggestions. Hope you kept your hands free from the contents when feeling the warmth! Heat usually equals excess current draw over a period of time, short circuit, or boiling the contents with excess charging current or duration, or an internal collapse. Hope you find cause and solution, and avoid/minimise effects of spillage on other areas.

2 year old car low miles

Engine struggling to start numerous cranking

1 hour journey

After journey observed fluids emanating from bonnet shuts

2yrs is young for a battery problem to appear unless a manufacturing fault or external influence (lack of maintenance, loose battery terminal connection, etc) has given rise to poor performance. Slow to start suggests battery failing and thus it is likely to demand higher current draw through the regulator (this can worsen the situation when battery is worked hard and generates heat). The battery will generate more explosive gas (hydrogen) which may or may not be vented (depends on battery type) and could ignite through external, or even internal spark. I would suggest you get it checked and replaced a.s.ap. by a reputable supplier (or better still under your warranty through the dealer). Good luck.

If the battery acid got on the paint work, then you're going to have problems there.

FWIW a leak/short in the battery between a couple of cells could have caused the poor starting and potentially lead to this, but that would be a manufacturing fault or old age.

Certainly I'd be replacing the battery and I'd be talking to the dealer.

If your car is under warranty, get it done that way, as the acid on the paintwork will cause the paint to degrade.

Thanks for the input. Glad this is a lease car, it's had an awful life so far. I pity the future owner of this one.

Any of the above, or possibly a failure in the alternator control box causing the alternator to overcharge the battery (and possibly fry the ECUs). I would advise verifying that you have no more than 14.4v across the battery terminals with the engine at about 2_500 RPM before driving anywhere.

Any of the above, or possibly a failure in the alternator control box causing the alternator to overcharge the battery (and possibly fry the ECUs). I would advise verifying that you have no more than 14.4v across the battery terminals with the engine at about 2_500 RPM before driving anywhere.

Also very true..

All of which would be a warranty job.

Also very true..

All of which would be a warranty job.

Oh yes, unless you drive a car with a failed control box, in which case it can be (and has been) argued that the damage to the ECUs was deliberate misuse and not covered by warranty.

Any warning lights?

Nothing at all.

It is likely that an internal short has caused one of the cells in the battery to explode. I would expect battery warranty to replace the battery, consequential damage will need to be sorted out.

We used to run a dairy with a fleet of electric milk floats. It wasn't uncommon for cells to explode. Typically first thing in the morning when the driver put his foot on pedal to reverse out the charging shed there would be bang from the battery pack behind the cab. The hydrogen gas released during charging was ignited by a spark caused by a internal failure in the 2v cell.

I used to test and certify battery packs for use in coal mines. The ventilation design was very important to keep the Hydrogen concentration down to 2% i.e. half of the lower explosive limit and prevent such ignitions under normal charging and discharge.

The ventilation is normal yes, but not acid/water with it to the extent that is can come out of the cars bonnet shut lines.

if you have had battery acid on the paint and bodywork it'll need washing down with a solution on bicarbonate of soda - otherwise the acid will continue to eat the paint and bodywork...