Cart laid over on it's side. Battery issue now

[Drewfus]

Hey all,
Background:
T-875 batteries, new 4/15
D&D motor, FSIP controller, Solenoid, DPI carger(thanks ScottyB!)
Volt Meter, 2ga cables crimped and soldered.

Son and friend tipped cart on it's passenger side back in April this year performing some stupid tight turn maneuver. No one got hurt thankfully.
Cart was on it's side for about 5 minutes until they managed to upright it.
A bit of acid obviously came out and I neutralized and rinsed accordingly. Plates were not exposed, so I just put a bit of distilled water in and charged up with DPI charger.(outside)
Drove the cart about a mile the next day to check things out and all appeared ok. Figured at worst battery life was reduced some.
Cart is always on charger when not used.

Replaced a failing key switch(no relation to accident) Tuesday afternoon and checked operation by just bumping forward and backward a few inches in the shop to verify proper operation. Voltage at ~52.5 VDC. Tightened cables up.

Got in to drive to a neighbors house last night. Disconnected DPI,(52.6 VDC) backed out of shop, put it in forward and BOOM! Got off the cart pretty quickly and gingerly raised the seat and was met with a cloud of smoke/mist.
Battery 5 top was blown off.

Around a minute later, I quickly started disconnecting battery cables to isolate anything, lifted all the lids, and left cart there for about 2 hours. Came back and started taking measurements. Boom battery was 8.5 volts as was all the others.
Retook measurements this morning and all were still good at ~ 8.4-ish volts.
No leaks or cracks in battery case.

So, connections were tight, batteries were charged and healthy what now?
My confidence level has dropped way down...
Has this happened to anyone else? Internal shorting of plates? Trying to rule out a spark as all connections were tight.
Oh, and BTW, prayers to CTD!

[NoleFan4Ever]

Never had that happen, but it sounds like gasses built up and the vents were plugged and finally the pressure overcame the caps......

[kgsc]

Could have sparked inside the case as well.

[scottyb]

Like he said, maybe in internal spark because those caps are vented and they would not pop from pressure unless there was an explosion, even a small one.
I would consider new batteries. These are 3 yrs old with incidence. Or one new one, but it will be equal to the old ones in a few months use.

[Drewfus]

Thanks for the replies guys!
The more I think about it, an internal spark seems more likely to be the culprit for the top to pop off with the amount of force it did. That and the smoke. I'm assuming it was smoke and not just mist from a pressure pop, say like a soda.
Crazy thing is that battery is still holding at 8.5 volts like the others. Has been disconnected since the 5th. If it were to "hard over" fail in a cell, I wouldn't have any doubts to replacing at least that battery. I'm not sure I want to play Russian roulette at this point..
Question: With a single battery replace, any thoughts on using Sergio's bank balancing circuit as discussed in the battery sticky? Since it will be a different brand likely, and newer, would that make any difference to help keep charging rate anomalies in check?