My dual battery setup works decently on my Silverado pickup truck for producing camp power for use under the truck cap. For the first night at camp, it provides more then enough power at night, including lots of lights, alarm clock, and moderate use of a portable fan, and 90 watt laptop charger.

After the first night, the deep cycle battery starts to wear down. The inverter will show lower and lower voltage, until it starts beeping every 15 seconds, and eventually shuts off at around 11 volts, to protect the battery. Yet, it can be continued to be run, all you have to do is turn on the engine, and let the truck run at idle speed for like 10 minutes, every 1-4 hours, depending on how heavy your electrical load is. I used an average of 1/3 gallon of fuel for every night after the first night parked, without driving.

As the battery is isolated from the starting battery, running the deep cycle down to the point where the inverter shuts it off is not a big deal. Your starting battery is still fully charged, and the deep cycle will after all get recharged as soon as truck restarts, and the inverter prevents you from going below 10.9-11 volts, a point where damage can occur, even with a good deep cycle.

For most moderate lighting demands — let’s say 75-100 watts — enough to run a couple 26 watt florecents, your pretty close to the 4 hours. But when you start adding a laptop charger or fan, and after the third day without the truck moving, you start to drop things down closer to once an hour. Reducing the load when charging, also helps the truck charge faster, as does swapping out the 60 amp fuse between the batteries with a 100 amp fuse, to send any excess amperage to the deep cycle as fast as possible.

The system originally came with a 80 amp fuse, but I cracked that fuse, when I was re-tightening the connections on fuse holders. Apparently, the fuse holder between the deep cycle battery and the starting battery was loose, and on the bumps of dirt roads would become disconnected. While ultimately, I fixed it by retightening the connections, I ended up breaking the 80 amp fuse, and could only get a 60 amp fuse locally. I plan to buy a 100 amp fuse over the internet at some point.

I rarely stay in the same campsite more then one night, at least without driving somewheres in the truck. Even relatively short periods of charging the battery at above idle speed, e.g. driving rapidly increases the rate of charge. It does however use much more fuel to drive the truck then staying idled.