it's only necessary as far as battery life goes between charging. I have the 900mAh batts and they last me all day without needing to be charged, and I
vape constantly. Never had a 902 so I cant speak to it. The eGo batts are more cigar sized. I had a 510 and the batts would last a couple hrs... way not good enough for me, so I upgraded to the eGo mega, now I upgraded again to the
tanks on my mega batts. The eGo-T kits come with the 650mAh batts, but you can also get the 1000mAh batts which of course I recommend.