Nov 11 • 5M

How We Chose the Wrong Charge Controller for Our Solar Panels on Our Aliner Scout

Matching Power Requirements With Capacity Is Challenging

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Devin Thorpe
Gail Thorpe
Follow our journey as we buy a Rivian R1T electric truck to pull a travel trailer equipped to run exclusively on solar power!
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From the moment we installed the solar power system in our Aliner Scout camper, we experienced a problem we didn’t fully understand. On the sunniest days, the system would quit generating power!

The problem seemed to be in the charge controller. We purchased and installed the Victron Energy Smart Solar MPPT 100 volt, 20 amp charge controller. At 100 volts and 20 amps, the little $159 (as of this writing) has a theoretical capacity of 2 kW—quite a bit of power.

That limit is enough to power our air conditioner and refrigerator with power to spare. Recognizing we neither had room for that much solar on our trailer nor needed so much, we installed just 25 percent of that limit or 500 watts of solar. We naively assumed it would be adequate.

Watts = Volts x Amps

A friend, Barry Lindler, who has a solar setup on his RV, warned us that the little charge controller might not be up to the job. We quickly determined that we couldn’t run the panels in parallel because the amperage would exceed the 20 amp limit on the controller, but concluded that for the 12-volt system, we could easily run five panels under the 100-volt limit running them in series.

Connected in parallel, the amperage of the panels piles up, and the voltage remains consistent with one panel. Connected in series, the amperage remains consistent with one panel, and the voltage piles up. So five panels that produce 20 volts and 5 amps could be wired in parallel to produce 20 volts and 25 amps or in series to produce 100 volts and 5 amps—yielding 500 watts in either case.

We were wrong! A YouTube follower with the handle CreepyCharly explained the problem after viewing the solar unboxing video we recorded in May. (If only he’d watched it in May!)

The panels are designed to produce 17 volts (not 12) because charging the battery requires producing at least five more volts than the battery produces. The 17 volts represent a floor. The range tops 20 volts. So, with five panels on a sunny day, the voltage exceeds the 100-volt capacity of the charge controller.

So, as we said up top, on the sunniest days, our charge controller shut down, and the system produced no power. With CreepyCharly’s explanation, we now understand why.

We replaced the little charge controller with a bigger one, the Victron Energy BlueSolar MPPT 150 volt 35 amp one, for nearly twice the price of the smaller one. With 150 volts and 35 amps, the charge controller has a theoretical limit of 5.25 kW! Carefully configuring solar panels in series and parallel (grouping panels in series with the groups in parallel), I think you could come much closer to that limit than we did with our 500-watt (.5 kW) system.

When we say it’s bigger, we mean literally bigger. Although it is still a pretty small device, about six by eight by three inches, it is about four times the size of the little one. That meant we couldn’t put the new charge controller where the old one was.

We had to find a new spot and run longer wires inelegantly back across some of the other components of the system to connect to the solar input and the battery output. Our system wasn’t Instagram-worthy before, but we’ll definitely be keeping our baby out of public view now!

We got the new charge controller just before Hurricane Ian hit Florida. We worked late into the evening before the storm hit to get our trailer ready so we could have a source of emergency power if the power went out.

Luckily, we were unscathed by the storm and never lost power. Devin has spent two weekends volunteering to help with recovery efforts in Southwest Florida since. We’ll have more to share about that in coming weeks.

When you purchase a charge controller for your RV solar power system, you’ll want to shop carefully to ensure you’ve got all the capacity you need for the sunniest days. You don’t want that sunlight going to waste!

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