Battery state cannot be read

[Henrik Blidh]

Reading battery state with

libmetawear.mbl_mw_settings_read_battery_state(c.board)

leads to exception in the C++ library:

terminate called after throwing an instance of 'std::out_of_range'
    what():  _Map_base::at

[Henrik Blidh]

Added issue in C++ API: mbientlab/MetaWear-SDK-Cpp#2

[Henrik Blidh]

This was due to error in my code. Can be done with new pygatt solution.