Deep cycle batteries

[The Artist]

Anyone know anything about deep cycle batteries?

I am looking for some indepth information about them, in particular, damage which can occur by full 100% discharging.
Google throws up minimal information, basically saying replace them or they might work.

The situation is, I have 8x 6v deep cycle batteries, they were fine and left fully charged for about 4 months. I checked the voltages and all were ~6.2v so all good I though. I put them on a machine, and I needed to discharge them to perform a test in which I attached basically a giant resistor which gets hot. Draws ~50A. I left this going and forgot about it. I came back and the batteries were totally dead. They powered a little LED on a relay but wouldn't actually power the relay. I put them on charge on a special charger with some new prototype charging profile that is supposed to know what is going on. Usually it goes to full ampage which is about 25A and then near the end it decreases slowly.
It went straight for the lowest setting and stayed on there for 2 days solid and it says it is full and ready to go now. The batteries are reading 27V which is good. The machine works.

Now, without spending 5 hours driving around and using the machine, can someone tell me what is going on and if there is likely to be any damage? I know the basics of how they work, but like I said, I can't find anything in depth.

[pits]

Hmmm, bit rusty on batteries, generally though it is very hard to recover a deep cycle battery.

What you can try and do is kick the battery back into life with a set of jump leads a new battery and a very intelligent charger that can charge in series, chances are though, I would say you have fucked them up.

What batteries are they? I will ask my mate as he has done shit loads with electric machinery with 6 volt deep cycles
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[hmmmnz]

have you tryed innox (i think thats what its called) basically it a de-sulfate additive, i ve used it on my old lead acids before i changed to agm's it worked pretty well, batteries that i thought were beyond saving managed to live again,

never used smart chargers, so can't comment on all that
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[The Artist]

This is the battery data sheet

https://www.dynobattery.com/SpecD260.pdf

The machine is a 24v system so 2x 4 sets in parallel and the charger is programmed for this. It has a custom profile which has been modified for these batteries.

I just looked for innox and it threw up some information.