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It's pretty simple to do what you originally asked.
All you need do is install a single-pole-double-throw (SPDT) switch to switch the positive power lead from your VHF between the house batteries (normal position) and the new emergency battery.
A VHF radio draws about 5-8 amps maximum, so the switch needs to be capable of handling that amperage @ 12 volts DC. Appropriate heavy duty switches would be like: Cole Hersee Co. 5584 or 5586 SPDT On-On or On-Off-On Toggle Switches.
First, ensure that the negative pole on the new battery is connected to the boat's negative ground.
Now, carefully remove the radio's positive (red) power lead from the existing switch or breaker and attach it to the center pole of the SPDT switch. Then, run a new AWG10 or 12 wire from one of the end poles on that switch to the original switch or breaker.
Test that out. Your radio should work as before with the new SPDT switch thrown one way or the other.
Now, simply attach the positive pole of the new battery to the other end of the SPDT switch, using a new AWG10 or 12 wire.
It would be good to include a 10A fuse in that wire, located close to the battery.
That's all that's needed to wire in a spare battery (emergency power source).
HOWEVER, now you have to find a way to keep the new battery charged. How you solve this will depend on how your boat is set up at present and what type/size of spare battery you use. Also will depend on the type of sailing you do. If you just do weekend sailing or short trips, you could charge the spare battery from shore power using a small smart charger. If it's a gel, you'll need a charger capable of charging gelled batteries. Gels have a very low self-discharge rate, so you wouldn't have to worry about charging it on a short cruise.
Last edited by btrayfors; 12-16-2011 at 07:05 PM.