When you're relying on solar power to keep your Airstream charged during a trip, the question that's always in the back of your mind is, "How much power do we have left?"
Modern Airstreams are equipped with a voltmeter that tells you the current voltage in the batteries (you'll find it on the "SeeLevel II" panel in late model Airstreams). But a voltmeter is not very useful in determining how much usable power you have left, or how much power you're using at any given time. You're basically left to guess, based on your experience. Further, the voltage reading on a voltmeter goes up and down depending on a lot of factors (that's a whole separate blog post). It's like trying to weigh a cat that doesn't want to sit still.
So, what's a solar power loving Airstreamer to do?
People who rely on solar frequently–such as those who like to camp without electrical hookups–use an amp-hour meter to monitor their power usage and understand how much power they've got to work with.
What an amp-hour meter does
The amp-hour meter is a totally different animal than a voltmeter. It uses a little device called a shunt which very accurately measures every bit of electricity that goes into or comes out of the battery. It's like the power meter on your house, keeping careful track of exactly what you use.
Because an amp-hour meter counts the power going in and out, it's great for people with solar. You can see the real impact of your solar panels, minus the power you're using. In other words, an amp-meter tells you the real story of how much power is reaching your batteries.
I suggest the use of an amp-meter in the instruction guide for our Portable Solar Kit. Recently, someone asked me about installation and usage of these devices. Here are the answers to four of his questions:
1. Which brand should I buy?
All of the popular amp-hour meters on the market do the same job, so your decision really comes down to features and cost. Some offer Bluetooth connectivity to your phone, for example. Popular models come from Victron, Bogart Engineering, and Xantrex.
2. How hard is it to install an amp-hour meter?
It ranges from "fairly easy" to "a bit of work" depending on the type you buy. There are usually two parts: a shunt that connects to the battery, and a monitor that gets mounted somewhere inside your Airstream. Running wires from the shunt to the monitor can be a tricky job if you pick a difficult-to-reach location. For this type, expect installation to take a few hours.
Victron makes a product called "SmartShunt" which doesn't have a monitor panel. Instead, you connect to it through Bluetooth from an app on your phone. The installation is much simpler, but in our experience the Bluetooth range is not impressive, so you may have to stand right next to the battery box to check the state of your batteries.
With either type, everything that is connected to the batteries must be disconnected and run exclusively through the shunt that comes with the amp-hour meter. You need to be knowledgeable about 12 volt wiring, have the tools to cut and crimp wire connectors, and understand the basics of electrical safety. If you're not comfortable with this sort of job, any competent RV or RV solar installer can do the job for you.
3. How will we know how much power we have?
The meter (or app) will give you the batteries' state of charge in percent, from zero to 100%.
There will be a little setup required first. You'll need to know the capacity of your batteries, in amp-hours.
The amp-hour rating is typically printed on the battery case with a number and the letters “Ah.” If not, search online for the rating, using the make and model of batteries you have. A typical Airstream trailer will have AGM or "wet cell" batteries rated at 60 to 85 amp-hours per battery. Lithium batteries will usually be rated at 75-100 amp-hours per battery.
Keep in mind that discharging lead-acid batteries more than 50% will shorten their overall lifespan. For that reason, it's recommended to set a "floor" level at 50% of the total rated battery capacity. For lithium batteries you can set the floor much lower (10-20%) because they can be discharged more deeply without harm.
4. Can an amp-hour meter also tell us how much power we're using?
Yes. If the Airstream is not plugged into power and there's no sun shining on the panels, the meter will show how much power is coming out of the batteries, in real time. It will also show you the net gain/loss in the batteries over a period of time. This is really helpful to understand what activities and appliances are the energy hogs in your Airstream.
For example you'll find that the furnace and water pump consume a lot of power (7 to 10 amps when running). But because the furnace usually runs a lot more than the water pump, its impact on your batteries is much bigger.
You'll also be able to see the actual stand-by (or "parasitic") drain that happens even when everything is supposedly turned off. With an amp-hour meter installed, you'll learn a lot—which will make you a more confident camper and avoid killing your batteries.