I've seen forklift and pallet jack battery packs made up of multiple automotive deep cycle batteries in parallel. They don't usually develope a problem untill late in their life.

You hit it on the head.. basically stuff happens at the end of the battery life..

There are parallel plates inside each of the six cells of a 12 volt battery to yield the needed surface area to make a given amp hour / CCH rating. If you get a dendrite growth through the separator (causes high resistance short) on two adjacent plates, it's going to bring down the whole cell. This is rather easy to detect on stationary batteries on constant float charge.

We routinely parallel commercial batteries for the same reason as above to yield the needed AH rating.

Some customers insist on parallel strings for reliability reasons. We have some commercial sites with 3 and 4 battery strings paralleled together. The biggest site (battery wise) has with 2 strings paralleled with each string containing twenty four 2 volt batteries in series, each weighing right at 550lbs each to make up a 48 volt system.. 26,400lbs of batteries. <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/zombie.gif" alt="" />

Of course these systems are either on charger or, if commercial power goes out, carrying the load.