Exploding Battery

BigJohn

It has been my habit to change the leisure battery on my caravan at 3 monthly intervals whilst it is in storage to make sure the alarm still works.  I do this by swapping between two batteries and charging the spare in my shed.  Last weekend while loading
the newly charged spare into my car it exploded, injuring me and damaging my car.  I estimate the battery to have been about four years old and it has been used in this way for most of that time.  I am obviously keen to avoid a repeat performance.  Any advice
would be appreciated.