I have to agree (almost) with XYM's last paragraph.
His contention that the Exploadig is not all that bad is less than factual. They can be bad. I have done boat work in this area up until the depression shut it off. In that time and in my small business area (~300 non-trailerables), I got to deal with at least three and often many more (the high being about eight) battery explosions a season.
Yes, some were little. They only blew the caps or top off one battery in the set. Some were not so fun. This was so regular that the motorhome that was my office/tool crib/meeting room/electronics shop and occasional motel had an inventory of baking soda.
Explosions of single batteries are very rare.
Explosions of identical batteries in parallel are pretty common.
Explosions of batteries in parallel that are in any way different are no surprise.
Explosions of batteries in parallel that are vastly different can be expected.
Simply, Lead Acid batteries do not play well in parallel.
Paralleled batteries of any sort will be very difficult to charge to full density. - Just the way that they are.
Matt