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My son has a bandit and it is a pretty stout car. He has gotten used to it on 2S and I had a large 3S battery that I was not using (I crashed the airplane it came from) and I wanted to put it to use. I bought the 3725X battery expansion kit and it on it says it is good for the Bandit (and Rustler and Stampede).

Long story short, I tried to install it and the silver battery posts were too high/hit the body! I was annoyed and when I called Traxxas at first they seemed confused as to why this might be and after a brief hold the customer service guy came back and told me that it doesn't fit the newer long wheel base Bandits - only the old one so I was out of luck. I for sure was annoyed but figured I might still be able to fit it with some modifications.

The battery I got is rated at 5,200 mah (it is less than that - it is a cheap $22 Ovonic unit from Amazon and it is lucky to make 3,500 mah but I don't care - it is good enough and not terribly heavy at 370 grams vs my 2S shorties that are more like 220 grams) and the dimensions are 139*47*39MM. It only needed the middle option in the 3 heights the 3725x expansion kit offers so I tried cutting it down to just above the 2nd pin hole (hack saw and polished it on a grinding attachment I put on my drill), I drilled out the plastic area where the front silver battery holders screw into to get an extra few mm of space (as there was a small gap between the grey plastic battery holder that goes across that battery and the top of the battery), and I cut down the metal ESC mounts that came in the kit (I removed about 1/3rd of the total length) as the ESC was too high too and now everything just fits! Ideally there would be some more space between the body and the ESC but we will just keep an eye on the temps and make sure it doesn't get too hot (no way a fan would fit on top without cutting the body). I did not need to cut the black screws that hold the ESC mount in place (there was more than enough space to screw them in but of course you could trim them if you were OCD).

I am excited that I was able to get it to work and ready to let this car rip! Here is a video of how it looks/fits. https://www.youtube.com/watch?v=kUqCkIX0cr4 It will no doubt be quicker than my 45mph Ultima Pro XL now (I am guessing it will do about 55 but stock it is probably more like high 30 mph on 2S - I will have to GPS it). It is still on the stock gearing but I think it will be plenty fast (he doesn't go WOT that much even on 2S but the extra power should be fun.... also, this battery fits perfectly now and my 2S shorties were really too small for the Bandit/they didn't fit snugly).

I just figured I would share in case this is helpful to someone else! I can post pics if anyone wants to see it installed. 3S for under $30s (3S battery is $22, the expansion kit was $6 at my local hobby store and I already had a Traxxas to XT60 adaptor in the car) I not too shabby and the ~3,500 mah battery is good enough as we usually only run for 10 or 15 min and he doesn't go wide open throttle for more than a few seconds here or there.