Auxilary Battery - Toast

Help guys, 99 Serria 2500 with the aux battery. After a 4 hour trip I smell rotten eggs in the cab thinking something is rotten. Turns out the aux battery is spewing smoke out of the vents and i very hot to the touch.

Any ideas as to why? Maybe relay in alternator sticking?

Thanks

If the battery is smoking you'll have to replace it, but first I would take your truck to a parts store and have the charging system tested, maybe a bad regulator.