Here is the monitor panel after I installed a good quality voltmeter to monitor the coach battery's state of charge. The meter is wired with the panel so that it only works while the levels switch is pressed. To the left of the meter, you can see the standard LED level monitors, showing, from left to right, an almost empty holding tank, an empty freshwater tank, a full propane tank, and a battery that's fairly well charged.
Side note: you can see where I swapped the hinge on the refrigerator door. It now hinges from the right, and opens on the left, where the latch is.
The problem with the built-in LED battery meter is that its scale is terribly imprecise, with LED's for 9, 10, 11, and 12 volts. But a fully discharged lead-acid battery has an open-circuit voltage of about 11.7 volts, which is enough to keep all the LED's lit. By the time the top LED goes out, your battery is pretty well completely discharged. It's too late. You should have started charging (or cut down on your electrical usage) a long time ago.
Lead-acid batteries will last longer if not discharged more than 50%. This meter has an expanded scale, covering the range from 11.6 to 13.6 volts. It also translates that into percent of charge for wet cells and gel cells. Since the EuroVan has a wet coach battery, I just ignore the middle scale for gel cells. You can see the meter is indicating the battery is about 95% charged here, with a voltage of 12.65 volts
The meter only correctly indicates state of charge when the coach battery isn't under a load, and isn't being charged. If you've got all the lights on and the furnace blowing, the voltage will temporarily drop down even with a fully charged battery. Conversely, if the battery is being charged, the meter may indicate off-scale at the high end, even though the battery is only 60% or 70% full.
This meter should help me avoid discharging the battery, and therefore should increase the battery life, as well as saving me from an early morning dead battery surprise when I try to use the furnace.
The meter was purchased at West Marine, made by Blue Sea Systems, and designed for marine or RV deep cycle battery applications.
A few people have emailed me regarding the installation of the meter. Two common themes emerged.
The first is that it's a lot harder to drill the hole in the panel than one would initially think. That panel has a plastic covering, but there's some fairly solid metal behind it. A standard household wood-cutting hole saw like one would use for installing door locks will probably have trouble cutting through the metal. I spent plenty of time using a Dremel tool with metal cutting bits to do the hole, shaping the cut by hand. Fortunately, the meter covers the edges of the hole, so if the cutting is just a little bit sloppy, nobody will notice.
The second note is that the electrical connections aren't completely straightforward, though they're not too difficult. As shipped from the factory, the battery's voltage goes through a resistor on the LED meter circuit board before it goes through the pushbutton "levels" switch and on to the rest of the LED meters. If you want to get accurate voltage readings while using the existing switch to control your new meter as well as the existing LED meters, you must rewire it so that the switch is the first thing in the 12V line, before the resistor on the circuit board. This involves cutting the old wires between the circuit board and switch, soldering and taping the ends of the two wires from the circuit board together, and then cutting the 12V supply line to the circuit board, and inserting the switch in this line. Then connect the + side of the voltmeter to 12V downstream of the newly relocated switch, and connect the - side to ground. This is the way I installed mine.
An alternative is to connect the voltmeter directly to the + and - sides of the cigarette lighter wires. You may wish to install a switch to prevent the voltmeter from drawing any current when you don't need it, even though the current draw will be small.