They sell those same batteries, for the same price (minus shipping) at radio shack. They are sold as alarm backup batteries. So don't wait on Tower--go get one locally.

Don't get a wet cell battery to keep in your field box--you are asking for trouble. If you want a wet-cell, then get a car battery or lawn-mower battery with a handle and carry it separate from your box. The advantage of bigger batteries is their ability to deliver many more amps to a starter if you are trying to start a large motor.

The disadvantages are obvious.