I picked up an EGO brand Lithium Ion 56v 2.5 Ah battery and charger at a garage sale for next to nothing. It is small, light, and appears to be a well rated popular heavy duty battery for large yard tools like lawn mowers and snow blowers. I see they make them up to 80v and 10Ah, and many other variations. I am going to fabricate a battery cable so I can plug into my RadRunner (I have a 35Ah controller and true 750w Bafang motor, if it matters). I realize I cannot run it in parallel with the stock battery, but otherwise, can you see any issues with utilizing it as a back up battery? I anticipate it should be good for 5 miles of above average speed backup support?