Returns the battery level retrieved by the battery related API.
"-1" will be returned in case of failure to get the battery level.
The battery value/level returned by PD40-100 device refers to:
0 - Low power and battery level is almost 0%. The commands can't be executed with the level, charge the payment device.
1 - The battery icon display 1 grid. The battery level in percentage between 1-25.
2 - The battery icon display 1 grid. The battery level in percentage between 25-50.
3 - The battery icon display 1 grid. The battery level in percentage between 50-75.
4 - The battery icon display 1 grid. The battery level in percentage between 75-100.
5 - The battery is charging state, the indicating lamp is red.
6 - The battery is fully charged (100%), the indicating lamp is green.