dei 502t, battery backup, will it work?

Thanks for your answer.

I am grounding the Dei520i and the Gps unit to the main battery '-' terminal (twisted both wires together then connected them to the main battery w/ a loop connector) and not to the 'chassis ground' as indicated in the instructions.  Once the negative of the main is disconnected, my GPS switches it's ground to the backup battery instead.. This how it seems to work when I tested it w/ a voltmeter.

Does this sound correct to you or am I missing the big picture here?? Thanks again for the help.

ground your gps and back up to chassis ground.... most good thiefs know that disconnecting ground is the easiest way to shut all systems down... if your back up has it's own ground... it will pull power more effectively with less resistence... the last thing u want any 12 volt component to do is to have to search for a ground because the primary has been disconnected....

otherwise.. what u are doin is fine.. but i would ground it separate from the battery if it was my car......

First up this GPS unit is going in a boat made out of fiberglass. Sorry for not explaining this before hand.

Secondly, If the Dei520t is grounded to the chassis ( if this was a car...) and the GPS is also grounded the the chassis as per it's instructions, the GPS would work because it was indirectly picking up the ground connection of the Dei520t. through the chassis.??  Am I all wet on this..??  The cicuit just needs to be complete..

Was initially thinking of wiring both the main and the backup batteries in parallel so in the event of the main being taken out, the GPS would pickup the 12v '+' and '-' from the backup.

I do appreciate all the helpful advise.