Battery Relocation

Not sure why you would even bother relocating the bat. You are adding more weight because you now have to run a thick [ to minimise voltage drop over the long distance ] copper cable from bat to starter. Copper is heavy. You can use alum, but it has to be thicker because it has more resistance....

As for the extra weight over the rear wheels. Calculate the rear end weight proportion & you will find the extra bat weight is insignificant.