After 10 years of RV’ing, I have noticed that two of the biggest mistakes campers often make is overloading their tow vehicle and not understanding how 12-volt deep cycle battery power management works. Most manufacturers and dealers either add to the confusion through their own ignorance or even outright lie just to make a sale. So here it is, a very simple way to make your battery last…

Don’t ever discharge your battery below 50% capacity.

It’s really as simple as that, assuming you are on a traditional RV battery (whereas I am assuming that Lithium battery owners already educated themselves on this topic before spending the big $).

If you do discharge your battery below 50%, it will quickly start to degrade / won’t hold a charge well.

This is why 90% of the people I know or meet in the campground own a generator. They just top off their battery each day and ruin the sound of nature in doing so. Keep your battery above 50% and it will last much longer.

Voltage meters are nearly worthless.

You can’t rely on anything that reports battery voltage to determine true state-of-charge. This applies to everything from the nearly worthless low/medium/charged LED lights to the fancier LCD displays showing an exact voltage. Even the fancy solar controller that came with your RV is simply showing the energy those panels are generating and sending to the battery as well as the battery’s current voltage – neither of which tells you the true state of charge for the battery. There are plenty of other resources you can read to learn why this is true. The oversimplified explanation is that the voltage currently being reported by a battery has very little to do with its actual stage of charge unless you are measuring the voltage after the battery has sat for a fair amount of time, often up to 24 hours. No RV’er I know uses their battery every 24 hours. Instead, RV appliances are constantly discharging from the battery to run the lights, furnace fan, water pump, fridge, and more. Every time the battery provides energy, its voltage no longer represents the current state-of-charge. And even if you do wait 24 hours to measure the voltage, many people incorrectly assume that 12.0 volts = fully charged whereas a 12-volt battery is only 50% charged at 12.0 volts and is 12.6+ volts or higher when fully charged.

Shunt-based, amp-hour battery monitors are what you need.

For much less than the price of a generator or regular battery replacement, you can get a true battery monitor like the Victron BMV-712 or SmartShunt (easier install with less wiring required). These devices and many others like them monitor the exact energy being inputted (charged) to and outputted (discharged) from the battery. They do so by reporting a metric called amp-hours, with 1.0 amp-hour meaning that 1.0 amp of energy has been discharged for 1 hour. This in turn tells you the true state of charge in either amp-hours remaining or percentage of charge.

Here’s an example. Let’s say you have two 12-volt batteries, each rated at 60 amp-hours for a total of 120 amp-hours. You don’t want to go below 50% state-of-charge, which is 60 amp-hours. First, the Victron battery monitor will tell you the exact amperage that each 12-volt appliance is using while it is running – an important tool in predicting your energy usage to remain above that 50% mark (see below for my observations). Secondly, the battery monitor will tell you how many amp-hours have been discharged thus far so that you can remain above the 50% mark, or 60 amp-hours in this example.

It really is as simple as that.

100% of the people I have explained this to who do not need the additional wattage that a generator creates, such as to run an air conditioner, have been able to purchase new batteries and stay above that 50% mark while also enjoying the sound of nature instead of an annoying combustion engine. It is so darn simple and so darn cheap…it is a true travesty that RV manufacturers don’t install a true battery monitor device on every unit sold.

Energy usage reference

Below is a list of the actual amperage measurements I have observed when running a solo appliance (everything else turned off/disabled) on my late-model (decently insulated) 25-foot travel trailer. Your mileage may vary, but this should at least give you an estimate of energy usage for planning battery and/or solar panel needs.

LP Detector: 0.15 amps
Dometic Fridge: 0.6 amps when not cooling / 14.4 amps when cooling
Single LCD Light: 0.18 amps
Heater Fan: 3.65 amps when heater is running
Dometic EZ Up Fan: 1.3 amps
Maxx Deluxe Fan: as low as 0.08 amps on lowest fan speed, the main reason why I replaced the Dometic EZ Up

What about solar?

Recharging your battery with solar is easier than many people think. You determine the amperage added to a battery by simply dividing the panel wattage by 12 (volts) and then account for the non-productivity of less-than-perfect sunlight. For example, a 100-watt panel divided by 12 volts equals 8.33 amps generated. Run that thing for an hour and you should recharge your battery by 8.33 amp-hours. Except when the sun isn’t perfectly angled to your panels, the panels/controller is a cheap model and doesn’t perform optimally, etc. As a reference, my Acopower 100-watt panels generate about 4 – 5 amps on a sunny day, so you might want to take 50% off what that simple formula outputs for a more realistic number.

My family uses between 15 and 35 amp-hours every 24 hours that we are out camping. The range largely depends on the weather, such as how often the heater needs to run at night during cooler trips or how much the fridge runs during warmer trips. It’s pretty easy for our portable 100-watt solar panels to generate 50% – 100% of the energy we use. Even if there are a few cloudy days here and there, we’re usually able to stay out 7-days and use 55 of the 110 amp-hours that our battery provides (without dropping below 50%) and then rely on the solar to provide the rest. For example, we might use 20 amp-hours in the first 24-hour period and only see 16 amp-hours generated by solar, thus taking us down from 110 amp-hours to 106 amp-hours. That could continue for almost 2-weeks before our battery hits 50% state-of-charge and becomes a concern.