Kevin,
There is more to your question than you think. First, we have used 3 different GPS receivers over the last 3-5 years as technology changed and improved. Two (2) of the receivers have an internal battery.
G35 Receiver - looks like a doorstop and has several wires that must be connected to power, ground and the speedometer head unit (via the white wire on the gauge). This receiver is powered externally by 12 volts (hence the power and ground wires) and does not have an internal battery. This is the older style receiver.
Nordskog Receiver - black with the Nordskog "N" logo on the top and a cable exiting the side of the unit ending in a PS2 connector (like the plug for a pc keyboard). This receiver plugs into the back of the speedo head unit via the PS2 connector and is powered by the gauge through that connection. This receiver does have an internal battery that is charged by the 12 volts fed to the gauge and then on to the receiver. These are not batteries that need to (or can be) changed. They are lifetime batteries that only supplement the power supplied to the gauge. If the GPS unit is not powered over an extended period of time, these batteries will require a few minutes to charge before the receiver will communicate with the satellites and relay a signal to the gauge.
G18 Receiver - round with a PS2 cable similar to the Nordskog receiver above - works exactly like the Nordskog receiver.
Gauge and receiver operation are identical regardless of receiver type, the only difference is that the older style G35 requires more wire connections to operate.
I hope this helps and let us know if you have further questions.
Thanks,
Mike McCarthy
Nordskog Performance Products