20 miles on your 7.5A battery? I bet you did a fair amount of pedaling. As mentioned above, I would be cautious about running your battery to death - both by using a lot of current, and by running it far down each time you use it. Just because you can, doesn't mean you should. But maxing out a battery is a sure way to shorten its life span.

Ideally you probably wouldn't run it at a current much more than its AH, and not charge it more than 80%, nor deplete it more than 20%, but that is probably a bit too idealistic (although some cars do that to ensure those expensive batteries last 10 years).