I wouldn't mess with a bypass wire. You could put in a 25Amp fuse which is a "lot" of juice, and would allow you to "charge" the battery for a while, before attempting to start it. For jumping, I'd just carry some 8Gauge wire that could be hooked up from one battery to the other straight. The initial pull of current should be very quick when the starting is pulling, and then when the engine is running, the pull will be reduced. The 8G will handle that pull for a short time.