Related to this discussion. I noticed when I was testing with a 5 watt solar panel from Zamp and a medium sized motorcycle battery, that the solar panel would bring up the battery during full sunlight to about 12.9 volts but as soon as the sun went down, I could see the voltage dropping on the battery.
Was the solar panel because of no controller actually drawing energy away from the battery during these "no sunlight" hours?
Sorry, still trying to learn.
You may just be seeing normal activity. In general, a fully-charged battery has a 'resting' or 'no-load' voltage of something like 12.6 or 12.7 volts. The solar panel applies a higher voltage (when the sun is shining) to counter-act the battery's tendency to self-discharge. When the sun goes down it can no longer apply that voltage and the battery's voltage will drop back to its resting voltage.
You can see this same thing in your car's charging system. If you measure battery voltage after the car sits overnight, you'll see its resting voltage. Measure again with the car running and you'll typically see voltage in the 13s or 14s. That's the alternator applying a charging voltage. Once you turn off the car, the battery's voltage will go back down to its resting voltage.
I used the word 'may' to start this post because we don't quite know enough about your Zamp maintainer setup. According to this Zamp maintainer page, those panels are supposed to be paired with a controller. In some cases, manufacturer's put what are called 'blocking diodes' in panels to prevent the very problem you're asking about and those panels wouldn't drain the battery. But Zamp may not do that, particularly in their panels specifically designed to include a controller. If that's the case, it would indeed drain the battery.
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PS- I see ntsqd's reply came in while I was writing this up but I decided to post my wordier version anyway.