Chassis Batteries

[Keith Moffett]

New Interstate batteries were installed in 2016. 
There are two banks of two 4D batteries.  Up to a month ago when last checked there had not been a hitch.  no boiling over or off gasing to notice.  The charging has always been right on as expected.  This has been checked by Aladdin and a VOM.
Three days ago I checked the main house batteries because the AH had a low voltage light on.  One battery had exploded.
The battery shop showed me that the innermost battery had both of the cables on it, with the two smaller cables going to the other battery.  Basically, the inside battery was doing the work and the outer battery was helping, when they should have been equal.
Pos on one and Grnd on the other with jumpers to go between.
This caused the failure.
Just a heads up!  See pic!

[Fred Brooks]

    Keith,
It is difficult to understand the failure by your description of the cabling. The only time I have seen a battery explode like that is when there is a massive short, like dropping a wrench across the 2 posts. Perhaps there was an internal failure between the plates? If the cabling was the cause it would not take 4 years to become apparent. I must be missing something. Perhaps if you could make a sketch of the cabling and gauge of cables that connected the house batteries. Please advise, Fred

[Eric Maclean]

Keith
The damage in the picture was caused by an explosion with in the battery this happens when a cell connector comes loose  or breaks causing an ark or spark within the battery which in turn ignites the gases ( hydrogen).
This would likely have happened during a period of high charge ( equalization) or high discharge ( inverter load)
As a battery gets old or inactive the sulpher in the sulfuric acid tends to attach itself to the surface of the lead plates in the battery this is known as sulphating this shows as a calcium like coating on the plate .
As this coating builds up it covers the active surface of each of the plates effectively making the working area of the plate and there fore the battery capacity smaller ( less ability to convert chemical energy into electrical energy).
It also creates hot spots on the plates because of the unequal activity these hot spots will eventually result in warpage and swelling of the cell bundle and in some cases cause a shorted cell or a  cell connectors to break.
However I have seen brand new batteries with broken cell connectors so this could happen at any time if there is a defect in one of the cell connectors.
Bottom line is that battery is junk.
The tech is correct the batteries should be connected as you discribed to help distribute charge and load evenly over the battery bank but this would not cause the battery failure you have .

PS
Battery sulphating is common in batteries which are inactive for longer periods of time IE ( six month storage periods)
This is why full timers seem to get better life from there batteries .
To help prolong battery life they need to be cycled through charge discharge regularly and not over discharged or run flat.

Equalization charges are designed to help bring all the batteries in the bank to an even state of charge so that they can all perform equally under discharge instead of having one battery do the heavy work while others lag behind.
And of coarse the quality of the battery to start with dictates it's life cycle each manufacture has an expected number of cycles for a given battery in there specs.
Rule of thumb the more lead in a battery (the larger and more plates in the batteries ) the better the battery is .
So when comparing batteries always buy the heavier of the two batteries

Batteries should always be set up in equal banks meaning when you replace one battery you replace them all with identical batteries never replace one battery as that battery will end up caring most of the load untill it is brought down or degraded to match the older batteries.

Sorry for going on so long
Eric