In what situations would you need to use the battery you want in the cart? In your build space or gym, you should be able to get to an outlet, even if you need an extension cord. In the pits, power is provided for you. Personally, I don't think the possible gain from having the battery is worth the headache of safety concerns.
That said, here are a few pictures of our battery cart:
http://www.chiefdelphi.com/media/photos/35787
http://www.chiefdelphi.com/media/photos/35786
http://www.chiefdelphi.com/media/photos/35785
Features:
- Charging for both robot and drill batteries
- Main power switch that kills power to the entire cart
- Relays between the chargers and the Anderson connectors to prevent draining the batteries when the power is off (the chargers will slowly drain the batteries otherwise!)
- External power strip
- Lockable wheels to give a sturdy base (two fixed, two caster)
- Bolted down vice
- Sturdy work surface - you can really pound on something here, unlike on the tables usually included in the pits at events.
- Not only fits through standard doors, but it can be laid flat on its back to fit in the storage compartment beneath most coach buses (this was really important, until we got our trailer).
- Additional storage room, sized to fit a standard FRC tote, on end (generally PR stuff for the stands goes in here to be carried up later)
- It's heavy. Very heavy. Fully loaded, it takes 4 adults to lift it into the trailer.
Note that we have a foll-sized tool chest (on wheels) for all of our tools that gets wheeled in everywhere with us, as well as a set of cabinets (on wheels) that holds other tools and supplies (The cabinets come apart when we get to the pits and go in either corner, and include supports for our pit banners).