How to measure DC voltage without knowing which pin is ground
I would like to find out which pin carries the battery voltage, and which one is the ground. The three other pins are much lower voltage, sending 52V to them would cause damage.


Modern digital multimeters typically have an input impedance of 10 M&Omega;. They are so sensitive that they can for many practical applications - such as your battery measurement - be considered ideal in that they pass no current while measuring the voltage. This means that the points being measured stay at their own voltage and are not shorted or pulled to the higher voltage while making that measurement.


Switch to DC volts and plug the leads into the correct sockets.
Connect the black lead to pin 1 and probe the others with the red lead. Record your measurements. 
The pin with the lowest reading is most likely to be the battery negative.
The pin with the highest reading is likely to be the battery positive.
Subtract the lowest from the highest (do it properly if the low reading is negative) and the difference should be the battery voltage.