Hillisboro Oregon 4th of July Parade.

On the 4th we had our Goldwing trike entered in the parade. With all the problems I heard, I had none. Never took it out of 1st gear. Never overheated. Once I got going I didn't need to touch the throttle once. But, when we got done, I shut down the trike and took all the 4th stuff off. Tried to start it again and the battery was dead. Looks like it didn't rev up enough to charge. After a jump it worked fine. Next morning it started up, no problem.
Thinking I would like to have a emergency starting battery. Any suggestions?