Larry, we appreciate you asking. It's good that the power seems good at night, but the capacity of the battery is still very limited. In fact, 91 has its computer turned off so it sends no telemetry and does not send every 2 minutes if not being used, all to save the battery. If you heard it at night, someone must be using it without paying attention to our request.
The reason for all this is that If the battery gets drawn down too far too many times, it will fail totally--often shorted--and that would mean no transponder in the daytime either.
Burns, WB1FJ
AMSAT Flight Software Lead