Q: Can I jump start my camper using the auxiliary battery? How?
We get this question pretty often. It seems that once a person has two battery systems, they think to themselves, "Great! Now if I ever need a jump start, I can jump start myself!" The truth of the matter is that having an auxiliary battery system will make it less likely that you'll need to jump start your vehicle. In fact, the whole point of running an auxiliary battery system in a camper is to isolate the starting battery when the engine is not running—which prevents it from being drained and unable to start your vehicle. With the engine off (read: alternator not producing power), the auxiliary battery is used to power all of your interior accessories (interior lights, stereo, 12v fridge/freezer, etc.), thus preserving your starting battery for... well, for starting.
However, if you ever find yourself with a dead starting battery and you want to charge it back up using your auxiliary battery, it is quite easy. All you have to do is put a jumper cable across the battery isolator solenoid—that's the small silver cylindrical device that comes standard on all Eurovan full campers (EVC) and is included with all GoWesty auxiliary battery kits for Vanagon. On the EVC, it is located on the firewall just above the starting battery under the hood. The GoWesty auxiliary battery kit for Vanagon locates it beneath the driver's seat. One of the big posts coming off the solenoid is hooked to your starting battery, and the other is hooked to your auxiliary battery. The "guts" inside the solenoid join these two posts when the engine is running, so that the alternator can charge both batteries. When the engine is shut off, the solenoid does just the opposite, thus isolating the batteries from one another. To join the batteries manually, all you have to do is bypass the "guts" of the solenoid. Just connect the red jumper cable clips to each of the big solenoid posts, give it 15-20 minutes to allow power to flow from the auxiliary battery to the starting battery. If the main battery is not completely dead, that should be enough time to get enough power in it to crank the engine over. Make sure you remove the jumper—this is very important!—and start 'er up.
The other alternative, of course, is just to use a set of jumper cables between your auxiliary battery and main battery, assuming the batteries are close enough to each other/you cables are long enough.
But like we said... you should never have to do this!