Fluid is right (naturally),

Modern NiMH need to be run warm off the charger, or the performance will suffer.

I once had a cooling system on a set of NiCad batteries, connected in-line; over the shrinking tube was a thin tube (grp), marginally larger then the batteries, both ends closed with silicon and a tube feeding water during the run.
On a 12 cell boat this was a major hassle with all the tubing required.

The cells never got hot, but it didn't extend the life span either.

Haven't used it since I went to run the NiMH.

Regards, Jan.