After installing a 12 volt outlet inside my camper, I use a battery monitor that plugs into the cigarette lighter style power receptacle. This allows me to conveniently monitor available battery charge without crawling under the slide-out bunk, opening the battery box, and checking the battery directly with a multimeter.
The chart above shows how much charge you have left for a given voltage reading. That’s right – a 12 volt battery is actually dead at 10.5 volts and fully charged at 12.6+ volts. And to prolong battery life, you don’t want to go below 50%.
You’ll want to take readings after the battery has had a chance to sit dormant for a while. Measurements taken while a device is drawing from the battery can be wildly inaccurate. For me, this means measuring after having been out of the camper all afternoon on a hike.
Some RV owners will use more elaborate solutions like the Tri-Metric, monitoring not just battery percentage but also charging and discharging amps. This device truly is amazing and I encourage you to also check it out to determine what is best for your use.
Personally, I get by just fine with the simpler and cheaper cigarette lighter electric meter option. I simply plug it in to check the voltage and then unplug to a