Spare Battery For Magnum Cruiser

I have a Magnum Cruiser with a 48V, 13ah battery driving a 500w motor. I was wondering if there were non-OEM batteries available? The factory replacement is around $680. It would be nice to have a second battery to extend my range beyond 45 miles. I have never run my battery completely down, but 45 miles looks close.
My concerns relate to warranty conflicts, compatibility with internal electronics, and of course connector match. I would be interested in reputable companies located in the United States. I am located in Central Florida.