Marine Rugged Mount Power Suppply for Garmin 650t

I'm exploring using my Garmin 650t as a data source for a chart plotter based on a Raspberry pi. The Li battery in the Garmin is a nominal 3.7V so I assume the GPS needs something like this to function. However the Marine Rugged Mount power input requires a nominal 12V so it appears that there is a some voltage stabilisation used to drop this to the required 3.7V.

I've been trying to use the mount in conjunction with a 5V supply but this seems not to do anything, which I find a little surprising.

Is my observation that trying to power the GPS with 5V via the rugged mount cable is not an option? I suppose I could used the 5V via the USB socket, which is how I normally charge the battery, but this would introduce yet another cable into the mix, and would expose the Garmin internals to a salty atmosphere. I don't understand why Garmin would use a voltage converter that cannot handle less than 12V, even though it is comfortably more than the device needs.

 

Thanks. I understand why the mount or, more accurately, the Garmin contacts to the mount can accept these voltages. It enables them to be powered by lead-acid batteries etc. My point was that it should not be necessary for the power source to be that high, given that internally it is being reduced to around 3.7V to drive the unit. Most voltage regulators will happily stabilise a broad range of inputs provided it is at least a little over the required final value.