caveman

hi everyone.. lately i came across some sub c nimh rechargable 1.2v batteries 6000mah.. i was wondering if i could replace the my dead battery pack (2,4mah) with these batteries but will the charger charge them up or i need to do a modification to the charger?? any help appreciated

maybe it can sound stupid ...but can i change the subc batteries inside my 18v pack to li-ion ones to imprrove performance