On some of our other Solar Panel posts I’ve mentioned Handy Bob Solar as a good resource for people thinking about adding solar panels to their RVs. I recently was re-reading some of his stuff and realized that it isn’t very beginner friendly and some of his advice is getting out of date. So here are what I think are Handy Bob’s biggest bullet points with updated information for 2017 and some of my own commentary and links to additional resources if you still have questions about a particular concept.
1. Get a Good Battery Meter
Summary: It is important to know how full your batteries are or how much energy they are currently storing. This is known as State of Charge (SoC). A good battery meter will tell you exactly how much energy you are currently using and how much energy you have stored. Without that information, there is no way to know if you’re fully charging your batteries or discharging them too far. A good battery meter is by far and away the most useful piece of an RV electrical system even if you don’t have solar panels, but most RVers don’t bother for some reason.
There are many different ways to measure the state of charge of your batteries (Battery University gives tons of details about five different methods to measure state of charge). Most RVs rely on one of the least accurate and most difficult to use methods, terminal voltage. To measure state of charge accurately with the terminal voltage method, you need to disconnect the batteries from everything and let them sit for 2 hours and keep them at exactly 72 degrees (F) then take a volt meter and measure the voltage across the battery and compare it to a chart like this:
No one actually does all of that. They leave everything hooked up and just check the voltage that their display can measure. Some “battery meters” are really just volt meters with that chart stored in memory and automatically convert voltage to state of charge. So let’s say your batteries are actually almost empty but you’re charging them back up at 14.8V. By just checking the volt meter, you’d think your batteries must be 100% full even if they are actually almost empty.
The problem is that charging or discharging, even recently, can have a huge affect on the actual SoC number. So much so, that it almost makes the voltage measurement meaningless in practice. But unfortunately most RVs only have a volt meter installed because they are cheap.
Another problem with this method is that if you end up upgrading to a lithium battery in the future, you basically can’t use the terminal voltage method at all because the chart above would have the same value of voltage for for almost all states of charge.
The much better solution is to get a “coulomb counting” or “amp-hour counting” “shunt-based” meter. Handy Bob recommends the Bogart Engineering Trimetric TM-2030 and I have one too, but it was basically the only decent option available when Handy Bob got started and there are better options out there now in my opinion. The biggest issue I have with the Trimetric is pretty technical, so I won’t go into it too much here, but it doesn’t account for Peukert losses which can make it significantly less accurate when you have high loads like microwaves, electric heaters, or air conditioners. Eric Udell at JenEric Ramblings has by far and away the best page I’ve ever seen comparing different battery meters. My current recommendation is the Victron BMV-702 which comes with the required shunt (you have to buy one separately for the Trimetric). It reports percentages with a decimal place, it takes Peukert losses into account, and you can use it to monitor a second battery or the midpoint of a two battery bank.
2. Beware Voltage Drop
Summary: If you don’t use thick enough wires, you’ll loose a large percentage of the energy generated by your panels. Use a voltage drop calculator to calculate how thick your wires should be. When in doubt, use thicker wires.
Most RV’s that are advertised as “Solar Ready” and even some systems installed by professional solar installers, use wires that are way too small for serious solar systems. Using a wire that is too thin can cause voltage drop. So if your panels are generating 17V but the wires are really thin, you might only see 15V at the controller. That is energy right out the window!
Ideally, you should use a voltage drop calculator to be sure you don’t lose anymore than about 2% of your voltage. Lets use an example. Say you got two 165W panels like ours and you’re going to wire them in parallel so that you don’t have as many issues with shading (parallel vs serial is a whole long conversation). There is a label on the back of all panels that will give you all the info you need that looks like this:
So now we enter into the voltage calculator 18.10 volts, 9.12 amps * 2 panels = 18.24 amps, DC current and lets say 15 feet from the panels to the charge controller. If you use the standard 10 gauge wire that most installers use, you’ll see a 3.04% loss which is more than you probably want. Thus you should bump up your wires to at least 8 gauge (1.88% loss).
That all said, today, this advice matters WAY less than it used to when Handy Bob wrote his stuff. Even 2 years ago, most charge controllers were PWM type which can’t handle wiring panels in series. Today though, most people use MPPT type charge controllers which have come way down in cost. Using MPPT controllers allows you to wire your panels in series which gives you lower losses per size wire. Wiring the two panels from above in series instead of parallel would only cause a loss of 0.75% with 10 gauge wire (vs 3.04% above in parallel). The only downside of wiring in series is that shading a corner of one panel is a bigger issue but that can be mitigated with good panel placement on the roof.
So basically the takeaway here is know how your system is being wired and check your voltage losses especially if you run your panels in parallel.
3. Charge With Enough Voltage
Summary: Most battery chargers have set voltages that are too low and will either take longer than necessary to charge or won’t charge a battery fully. Every battery manufacturer recommends slightly different parameters and your charger should be adjustable to match the batteries. Lots older RV battery chargers will just set the voltage to 13.7 volts and expect you to leave your RV plugged in forever. That setting might charge your batteries all the way, but it will take many days of being plugged in.
Instead, your charge controller (whether a solar controller or a plug-in battery charger) should be able to be adjusted to charge as recommended by the battery’s manufacturer. Our batteries came with this chart: http://www.eastpennmanufacturing.com/wp-content/uploads/Renewable-Energy-Charging-Parameters-1913.pdf and a note saying ours are AGM Monoblocs.
Lets walk really quickly through the three main stages you should care about and what they mean.
In bulk, your charger is supplying power as fast as it can up to a maximum current. A 30 amp charger should be supplying 30 amps of current in this phase. Looking at the chart above, it says to also limit this to 30% of our C20 which is just a fancy term for our rated capacity. Since our rated capacity is 530 Amp Hours, our maximum charging current is 160 amps. We shouldn’t by a charger that can supply more than 160 amps.
Once your batteries get up to a high enough voltage, your charger should hold them at that voltage. This usually happens when your batteries are about 80% full. As soon as this happens, you aren’t charging as fast as the charger can go, you’re charging as fast as the batteries can accept charge. Therefore, the longer you can stay in the bulk stage, the less time it will take to fully charge your batteries.
On our batteries, that voltage is 14.25V (the midpoint of 14.1 and 14.4 listed in the chart). On some batteries, this number could be as high as 14.8V. This number is one of Handy Bob’s biggest sticking points because the battery manufacturers obviously say 14.8V but many bad charger manufacturers won’t go above 14.4, 14.2 or even 13.8V in some cases. If you come out of the bulk phase earlier than recommended, your charging rate slows way down and you might have trouble getting your batteries fully charged with solar panels in the limited number of daylight hours you have available.
The Float Stage basically just sets the voltage where the batteries won’t get hurt while they get the last of their charge. The batteries accept power very slowly in this stage but it is necessary to be sure you’re fully charging lead-acid batteries. On our batteries this is 13.5V but all of them are different. Again, check with your battery manufacturer.
4. Use Temperature Monitoring
Batteries behave very differently at different temperatures. If batteries are very cold, they can’t produce as much power and are more difficult to charge. If they get hot, they can accept power faster than is safe and overheat (and then do terrible things like release sulfuric acid or hydrogen sulfide gas or even explode in extreme cases). To deal with this problem, all decent battery chargers should be able to monitor the temperature of the batteries and adjust their parameters accordingly. This will allow you to stay in the bulk charging stage longer in cold weather so you charge faster, and also help keep your batteries from exploding. Temperature monitoring is extremely important for all solar charging systems and any installer that says otherwise isn’t worth the money you’re paying them.
Outside of those 4 things, Handy Bob has some other useful information on his site about manufacturers he recommends but definitely take those recommendations with a grain of salt. I know people will go running for their pitchforks when I say this, but he is just one guy on the internet. In some cases his recommendations are spot on. In others, there are new manufacturers who have come on the market since he wrote his recommendations which are fantastic. And in some others, some of his recommendations have held their prices firm and not made improvements while the entire rest of the market has released new versions and come down in price so his recommendations are now much less competitive. My recommendation is to learn about the basics of what is important (see the 4 points above) and then choose a manufacturer who cares enough to listen to that advice that is also in your price range.
Hopefully, that background info is enough to help a newbie understand what Handy Bob is getting at in his work but if you have any questions about any of this or his content, please let me know, I’d love to help.