Use APM_UNKNOWN instead of 0xff.  Use ~0U instead 0 for the number of batteries.  This should be -1 to match other code, but since the api is unsigned ~0U is the same thing.
Use APM_UNKNOWN instead of 0xff.  Use ~0U instead 0 for the number of
batteries.  This should be -1 to match other code, but since the api
is unsigned ~0U is the same thing.

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