AGM state of charge.pngI agree that battery charge management is a PITA and the last thing that we want to be hassled with when all we want to do is enjoy the outdoors in our FWC. So to keep things as straightforward as possible, here are some things to keep in mind.
It is important to monitor your camper battery's state of charge but there is no simple "gas gauge" way to know how full your battery is like there is for your truck's gas tank. You can, however, come close to a "gas gauge" for your battery with an inexpensive digital voltmeter that you touch to your battery's terminals for a voltage reading. BUT, and this is a big BUT, to make an accurate one-time state-of-charge voltage reading you have to make sure that the battery is not being charged nor is a load connected to it. AND, for maximum accuracy, the battery should have been disconnected from a charging source or a load for an least a couple of hours! (And who can afford to do that when camping?)
Nevertheless, I strongly urge everyone to regularly monitor their camper battery's voltage even though it may be being charged or discharged. You will learn over time to pretty accurately estimate the state of charge of your battery by watching the voltage. Sure, if you want to get fancy you can install a Trimetric or other device that calculates state of charge by measuring amperage in and amperage out, but these devices are expensive and require careful installation and calibration.
I am attaching a battery state of charge chart for use with AGM batteries (the type of deep cycle lead acid batteries most commonly used in RVs). A new AGM battery when fully charged will measure close to 13 volts. And, again, remember that this is for a battery that is not being charged (because the voltage will appear to be higher) and not under load (a battery supplying electricity to an appliance will test at a lower voltage than it will test after the load is removed).
Nevertheless, it is good to make friends with your battery by regularly checking the voltage. You will learn over time how long you can run your regular appliances without depleting your battery too far. To avoid unduly shortening the life of your battery you should not let the battery discharge more than 50%. Referring to the chart, this means to not let the battery voltage drop below about 12 volts. But remember, this voltage measurement accurate only when nothing is charging or loading the battery because either will give false state-of-charge readings.
Solar panels, shore power and power from a running truck's alternator are all ways to charge the battery. Each have their disadvantages and advantages. Shore power may not be available; the solar panel may not be in direct sunlight; the length and small size of the wire from the engine compartment to your camper battery may prevent a high enough voltage to get to your battery to fully charge it. But we do the best we can, keep an eye on the voltage, and have fun outdoors!
Ed