Are you talking about a NiCd or NiMh?
I've seen well over 6 volts when charging a 4 cell NiCd receiver pack...something like 6.3 or 6.4V.
You might not see quite that high on a NiMh? I typically fast charge my 700 MAH NiCd Rx. packs at the lowest setting on my MRC 969 Super Brain which is 200 MAH. Ther 1500 (and more ) MAH NiMh I use 400 to 500 MAH. because I don't like to push 'em that hard.

You are right in asking though...I have read of a couple other guys that had "issues" when fast charging with a Hobbico fast (field ) charger.

Maybe try a search in the Batteries and Chargers forum?