When my app attempts to read the battery level or the low power characteristics it always gets an error. (HMErrorDomain error 12.)
It happens with an Elgato Degree as well as with a Samsung motion sensor (the only accessories we have with battery services). The app can read the temperature and the status of the motion sensor, but it always fails when doing a "ReadValue" on the HMCharacteristicTypeBatteryLevel or ...BatteryLow characteristiccs.
Apple's HomeKit app doesn't have the problem as I can see the battery level in the accessory details.