[Al Skierkiewicz]

First has changed batteries in the past. Initially, (1996) the batteries used were drill batteries and plugged into the drill housing for each drive motor. As things progressed and First adopted the IFI controllers, we started using the motor cycle battery in the size which we are now using. However, over the years, the battery has changed vendor, terminal arrangement, and chemistry (gel vs. AGM), each of which produces a slightly different size. Current batteries expand with heat, so if you make the tolerances small, it is possible you won't be able to remove the battery as temps flucuate.
As First and their vendors change, so to will batteries. There has been some talk for a year or so, that MK is working on a newer battery. It is more powerful and I expect it to be a little larger then the current battery.
I have seen many battery carts, but designers should remember that hot batteries do not charge properly and although the battery can be used in any configuration, charging is best done with battery terminals up.