Not sure if i can help you at all here, but i have a friend with a r33 and the way his car is tuned allows flames to be shot out of the back during hard driving, i.e. under high revs (burning off excess fuel?) so maybe remapping your ecu will fix it. Do you drive your car hard? and as for the battery it sounds like your altinator is faulty atm. Most cars running an rb25 backfire and gargle a bit anyways as you let off the throttle(my r34 is running perfect and i actually like the gargle/popping noise it makes; this could just be due to the HKS exhaust tho) I have also heard that if you are running boost that is a bit higher than factory it makes the car backfire anyway. As long as it is backfiring as you let off the throttle only (not as you put your foot down), it should be fine (my bro's s15 is running 14psi and backfires quite regularly as he lets off the accelerator). Not sure if this is of any help lol, but i suppose an opinion never hurts