go get a better charger.. they are pretty cheap and will save you alot of hours
if you ask me no battery should be left unwatched when you are charging it.. but well.. that's mostly for LiPo

and matching the battery? noo.. not needed. it will charge anyway.. just slow..

hmm correct me if im wrong..

2500/70 = 35,714285714285714285714285714286
2500/100 = 25