Increase precision in voltage reading

[JosephMarinier]

• Two bits were ignored at the end of the first byte (less significant byte). Adding them now makes the tension units 4 times smaller and more precise.
• Now no longer limiting the last byte (most significant byte) to the last 4 bits. Theoretically, the other 4 bits allow room for tension higher than 18,6 V. Haven't had the courage to this it. :)