"on other devices, the backup battery charges from AC power" Is that true? Could you tell which devices?

Christian; There are only two Collection Metrics in the Collection Category Battery.Status.  One is the battery level and the other is the change in battery level since the last time battery level was sampled.  Of these two, only one is available for the backup battery, which is backup battery level. It is possible to define XML to add a new Collection Metric BackupBatteryLevel to MSP and to add a new Collection Category for BackupBattery.Status. that contains just this Collection Metric. But in the real world, I am not sure how much real good it would do for you.  The backup battery level may or may not have any real meaning depending on the type of device.  On some devices, the backup battery automatically trickle charges from the main battery and on other devices, the backup battery charges from AC power.  In most devices, the level of a battery cannot be determined when it is charging.  So, depending on the device, you may never see a meaningful change in the reported level of the backup battery. You can use Remote Control and go to the Device Info page and look at the Backup Battery section and look at the value displayed for Life Percent.  If you find that for the device(s) you are looking to work with, that value displayed in Remote Control varies in ways that seem useful and interesting, then I can show you how to configure MSP to collect it.  But it would be a waste of time to do so only to find out that what you end up collecting never provides any meaningful or useful data.