Hi Rob
i use also np1 but custom made
i had try in the past 14.4 v 3.2 a (0,900 Kgr)
it was wonderful battery but when i use it with SRIII (i don"t try 435)
the camera doesn"t switch on, and that bechause when the batterry is full
it have name voltage power about 16.6 volt so both of them is 33 volt SRIII have cut off on 32 V iam not sure but if i remember good in 435 manual says also 32V !
now i use 13.2 V /3.5 A (0,900 Kgr) and iam still verry happy with them
you can easy reccel the old batterys and now i believe that you can made more powerfull about 4 ah (new cels on the market)
Good Luck