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“Uh… How much battery do we have left?”
Your battery bank is the beating heart of your RV’s electrical system. It powers things when you’re not plugged in to shore power. It stores up energy when you are plugged in. If your rig has solar, those panels are meant to charge your battery. And if it has an inverter, it will draw on the battery to generate “household” electric current. Your battery is a resource that makes “off the grid” RVing possible, and just like your fresh water supply, it’s a good idea to know exactly how much of it you have on hand.
But measuring battery capacity is more difficult than measuring something like water. After all, you can’t exactly see energy, it has no mass, and it won’t fill up a tank like water does. So, we have to be a little more clever when we want to measure it. Fortunately, there are some basic ways to get a handle on how much of this resource you have left.
Measuring Voltage: You got this!
This method of estimating battery capacity is available to everyone, and it likely won’t require you to modify your rig or get any special equipment. Most RV manufacturers build basic battery voltage monitoring right into their rigs. You’ll typically find a way to monitor battery voltage right on your RV’s control panel.
If your rig doesn’t have a battery monitoring capability on its control panel, don’t worry! If you can find a 12 volt outlet that’s working somewhere in your RV (it will look like a cigarette lighter plug), there are simple devices you can plug in to that port, and they’ll show you the voltage.
Another option is to use a multimeter to check your battery’s voltage right at the terminals.
It may surprise you to learn that your 12 volt battery doesn’t necessarily put out 12 volts. This is true for traditional lead-acid batteries, AGM batteries, and even newer lithium batteries. The voltage of the battery starts off higher, and decreases as the battery becomes discharged. We can use this decrease in voltage to get a handle on how much charge is left in the battery. It’s not measuring the charge directly – it’s measuring an indicator of the charge.
But here’s the catch: While voltage decreases with discharging for all batteries, the way it does so depends on what type of battery (or battery bank) you have. We’re going to cover lead acid (this includes flooded cell, and AGM) batteries, and Lithium batteries (Lithium Iron Phosphate or LiFePO4), since those are the most common. If you don’t know what kind of battery you have, you can assume you have a lead acid battery, since you would certainly remember paying for a lithium upgrade.
Let’s start off with lead acid batteries. In a happy coincidence of electricity and chemistry, the voltage of a lead acid battery decreases consistently and predictably as it discharges. That makes it relatively easy to read the voltage, and get an approximation of capacity from a state of charge chart. I know many people who carry little copies of these charts in their RV travels. As an example, if you check your battery voltage, and it checks in at 12.1 volts – your chart will tell you your batteries are about half full.
I say this is an “approximate” capacity because we’re not measuring charge directly – we’re measuring voltage. It’s kind of like measuring your vehicle’s speed by counting “1 Mississippi, 2 Mississippi” while you roll by mile markers. It’s not exact. But since we’re camping and not in a laboratory, it’s good enough.
This method doesn’t work quite as well with lithium batteries. This is because the voltage of a lithium battery doesn’t change as much as you discharge it. The voltage stays relatively constant until the very end.
The difference between 70% full and 40% full can be as small as 0.1 volts, which could approach the accuracy of your voltage meter.
Nevertheless, charts for lithium battery voltage do exist. And if that’s all you’ve got, it’s better than nothing, and should still suffice to keep you happily camping:
The next level: The Shunt.
It’s possible to get more accurate readings for battery capacity, but this requires more than just checking voltage. It requires some additional equipment to do this, so you’re probably looking at an upgrade to get this capability. These pieces of equipment are called “Shunt-Based battery monitors”, and they’re not usually included in a factory RV.
I’ve actually installed one of these in every RV we’ve had. I didn’t do it because the voltage method to estimate state of charge wasn’t working (it was). I did it because I just wanted to know more about how my batteries were holding up.
Shunt based battery monitors are available from various sources like Trimetric, Victron, and Balmar, and they all work similarly. You install a shunt on the negative lead into your battery bank. The shunt acts as a kind of “flow meter” and measures current flowing from your battery. The shunt is connected to a monitor panel of some kind, where you read the state of charge. This State Of Charge, or SOC, is the “gas gauge” type of output you probably thought was missing.
Shunts are a fairly common piece of technology. So it’s in the monitor panels where each manufacturer builds in their proprietary “smarts” to convert the data from the shunt into useful information for you, the end user. I’ve used several of them, and they typically require you to input some features about your battery so that they can figure out how much charge you have remaining. The programming isn’t difficult, but you will have to follow the instructions closely.
The payoff for the extra equipment and programming is a much more accurate readout of battery capacity. Whereas with simple voltage readings, you’ll know you’re 60% full… ish, with a shunt based battery monitor installed, you’ll know if you’re 63% full or 57% full. Depending on the model you have installed, you might also get information on how long your batteries would last under current conditions, or even some indication of how “healthy” your batteries are.
Shunt based monitors also work better than standard voltage readings for lithium batteries. By measuring the electric flow through the battery, they’re able to overcome the limitations presented by the relatively constant voltages.
Battery Integrated Monitoring
There’s another level still where you can take battery monitoring, but it will require you to upgrade to a lithium battery to get there. Some lithium battery manufacturers are building advanced monitoring directly into their Battery Management Systems (BMS). When you add a monitor panel – or, in some cases, an app on a bluetooth smartphone – you essentially get to “talk” to the battery.
Using an app or monitor like this gives you access to instantaneous battery performance, detailed battery health data, internal temperatures, and even error codes from individual battery cells. The information varies depending on the battery manufacturer.
I’ll be honest – that kind of detailed battery information is a LOT more than you need to enjoy a camping trip. But it is nice to know it’s there if you need it. Not all lithium battery manufacturers build this kind of data interface into their batteries, though many of the better ones do. Typically, they do this through additional connection ports on the battery. So, if you’re looking at a lithium battery, and you see more than just the positive and negative terminals to connect to, it’s reasonable to assume that there will be some kind of advanced monitoring option available with that battery.
There’s a LOT more you can learn about batteries. For example, we haven’t even touched on how a battery’s capacity decreases over time. That can be a big deal, and it’s one of the ways you might determine when it’s time to replace the batteries. And we haven’t gotten into more advanced topics like “voltage sag” either. But the good news is that most of the time, those things don’t matter too much and all you really need to know is how much battery do you have left in the proverbial tank. And answering that question is something you can do with the tools already built in to your RV.
Camping in the Name of Science! How To Figure Out How Much Battery You Need:
Now you know how to check how much battery capacity you have left, but how much do you NEED if you want to do some dry camping? The easiest, most accurate, and most fun way to figure out your electrical needs is to simply go camping… in the name of science! Here’s how to run your camping experiment:
- Start your experiment at nightfall, with full batteries.
- Disconnect all your charging sources. Unplug from shore power. Don’t run the generator. If you have solar panels, unplug them. If you have a motorhome, keep the engine off. The idea here is that you only want to discharge the batteries, not charge them.
- Camp normally. Don’t try to use extra electricity, and don’t conserve either. If your typical RV day involves you getting away from the rig for a while, do that.
- Keep tabs on your battery every few hours. You want to know how long it takes you to run your batteries down to empty (or 50%, if you have lead acid batteries).
If you can’t make it through the night without depleting your batteries, you’ve got a battery capacity shortage that will get in the way of any serious boondocking. But if you do make it through the night, let’s look at a hypothetical camping scenario:
Suppose your RV has 2 Group 31 AGM batteries. That’s about 210 amp hours of capacity. You begin the test at nightfall with full batteries. In the morning, you check your battery voltage and you’re down to 12.4 volts. That’s 80% full according to a state of charge chart, so you keep going.
As the sun sets again, the voltage is down to 12.2 volts. That’s still 60% full on the state of charge chart, but since the overnight might take you below the recommended 50% discharge for lead acid batteries, you call the test off. So, in one day, you used 40% of your capacity. 40% of your 210 amp hour battery bank is 84 amp hours. So in one day of your style of camping, in a disconnected or boondocking scenario, you use 84 amp hours of battery capacity. Now you know!
Hope this helps! Feel free to leave any questions or comments below.