Voltage drops when amps increase. The voltage regulator will call for more volts as the current draw increases. With only the minimum draw, alternator output should be 13.8VDC to maybe 14.6VDC If it isn't the alternator, it's the battery.

 

That is normal, the wipers only work with the key in the run position.

 

Volts = power
Amps = amount of current being moved.

With high amp draw at low rpm, the voltage will drop.