Mastiff

I have a Chevy Sub, so I had room for a second battery tray. I put one in, and a battery, and hooked it all up to a battery separator (one of those thingies that looks like a CPU cooler, with one input and two separated outputs). But the extra battery gets drained. I think I know why as well: Right now I have the regular battery hooked directly to the alternator (since it wouldn't charge when it didn't have access to battery power), but the tank battery gets it's charge from that separator. The voltage loss in that separator is around 0.6 V, and that's obviously exactly the loss that will stop it from working. I have an idea, but I wanted to ask if this will work first:

I can hoop up a thin cable with a relay and a fuse on it from the alternator to the tank battery. The relay will be wired to something (perhaps the driving lights) so it's only open when the keys are in. Then it won't drain the start battery while the keys are out. That will keep the charging voltage up. But will starting the car make that cable burn up or the fuse blow? My main problem and cause to get that extra battery was that every time I started the car the computer had to be shut down (into hibernation). With the extra battery it won't. But that doesen't really help if I can't go into the gas station to pay with the computer running since the tank battery is almost empty. Any help would be appreciated!

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Tor - The MediaSUV