APM Mission Planner is adding more support for the AttoPilot Voltage Sensor. Thus the Open Hardware Store has introduced a voltage sensor kit that is ready to be used and you just need to plug it in and configure Ardupilot. It comes with XT60-connectors and a 15cm long servo cable to connect to the IMU.
The kit is called the OHS APM Voltage/Current Plug-in Kit and compliments the other OHS APM Plug-in Kits. Using the OHS APM Plug-in Kits you don't have to solder anything on the electronics. They are tested and ready to be used.