If your Tempest is not running the right firmware version, it will continually drain the batteries in the PBA when you are not running it on AC power. Power Booster - Firmware Number? - #2 by eric says you need to be at firmware 165 (or higher) on the Tempest. Many units out there are at 156, not 165 or higher … and, at 156, if you are running the PBA on batteries only, you will see continual draining of the batteries.
If you are still at 156 (or some other version below that), open a support request to get it upgraded.