it's probably - F*CKED !!!!
Nimah's generally die the older they get, even Vapex's
often either:
not holding charge for very long or even overnight
(charge them - light goes green, take them off charge & pop them back on hour or day later - takes ages coz its lost it charge that quickly)
or refuses to charge at all and getting a bit hotter than "hand warm"
A good nimah should still hold charge for at least a few days heck even a week it should have most of its initial charge
normally add an extra volt or so onto the rating as it comes of charge...
8.4v - often is 9.4v -ish fresh off charge - see how long she stays at that voltage - not long in your case I think
might be the cheapy crappy one chucked in with some cheaper guns and at best these free batteries are not all that great
Main thing is if gun is ok, a battery especially nimah's can't really carry much warranty second hand
second hand laptop's for example will rarely have any warranty on the battery itself
Could look around for a nimah on the cheap - but tbh if gun is ok and battery is crap then make the jump to 7.4v lipo's
even cheapy B3 lipo chargers and some cheapy lipo at hobbyking or componentshop.co.uk
deffo go lipo if you gotta start buying new, they are cheaper, more powerful and the size/capacity options are far superior to nimah's I'd say