A solar-powered Teardrop trailer means self-sufficiency. The solar panels generate energy from the sun, which can be stored in batteries for use when the sun is dim or gone for the day. A charge controller regulates the flow of power from panels to batteries in order to safely and efficiently charge them.

As with the other Teardrop trailer electrical system essentials topic, this one began larger and got boiled down to what you most need to know. It may be useful to review the electrical system overview to see how all the components fit together, and Electrical System Essentials: Batteries for some useful terminology definitions.

Types of Solar Panels used with Teardrop Trailers

There are 3 main types of solar panel used is this type of application:

• Rigid panels may be of monocrystalline or polycrystalline construction. Mono is more efficient. Poly is less expensive. As the mono cost-per-watt has continued to fall, poly is becoming less common.
• Folding panels are basically small rigid panels hinged together to make them more portable.
• Flexible panels are a thick film which can be mounted to gently curved surfaces, like a Teardrop roof! While not as efficient as rigid panels, they keep improving, and the lower efficiency is offset by the fact they are generating power while you’re on the road not just when set-up at camp like rigid.

Solar panels used on a Teardrop camper are usually 12 or 24 volts and will have a rated wattage output. Like the amp-Hours shown on a battery, that number is theoretical, under perfect conditions, and will never be seen in real life use. For instance, the panel on top of my Teardrop is 100 watts, but the most I have ever seen displayed on the charge controller has been in the mid-high 80 watt range.

What conditions affect solar panel output?

The closer the sun rays are to hitting the panel at a right angle the better. New types of cover glass on better quality rigid panels are increasing the efficiency of offset sun.

Solar panel efficiency lowers as the temperature rises. This is a dilemma in summer, when the sun is high and closest to a right angle it is also liable to be hot.

I know, it’s obvious, but I’ll say it anyhow: the brighter the sun and clearer the sky the better. Which isn’t to say there’s no benefit on one of those bright overcast days; developments in monocrystalline panels continue to raise efficiency allowing them to squeeze out more power from diffuse light, and the charge controller can help too (see below).

Shadows, even on just part of the panel, can noticeably affect output. Also, the panel should be generally clean, though it’s not worth being fanatical about this, and never clean/wash a panel in direct sunlight.

Best Type of Solar Panel for Teardrop Trailers

There’s no single answer. It depends on, in addition to the solar panel wattage, what panel location would allow you to harvest the most energy from sunshine, for the longest time. In general, get the largest panel that will fit your application; the price difference between, say, 100 and 150 watts is not large and it is always better to have excess than not enough capacity. You also want to be sure your solar panel(s) are likely to output enough output in typical conditions to properly charge your battery(s).

A flexible panel on the Teardrop roof is great since it is always visible to the sun when on the road. Though once you park and set-up camp the trailer stays where it is so your flexible rooftop panel may be in and out of shadow or even in mostly shade. With a rigid or folding panel and a length of good cable, you can position it for best sunshine, plus you will get more efficiency per square inch of panel than flexible.

The ideal scenario is perhaps to have both a flexible panel on the roof and a moveable rigid or foldable panel for when you are set-up at camp. Depending on the panels’ specs, it may be possible to wire them to both output simultaneously. Or you may need to have one or the other connected to your charge controller, depending on which panel you think will be harvesting the most energy.

There may be no best, the worst option is a polycrystalline panel… if you’re going to lug around and set-up a rigid panel, don’t you want to get the most out of it?

Why is Charge Controller needed?

Even though a solar panel is nominally “12 volts”, the voltage (and amperage) it outputs can vary wildly depending on the sun conditions at any given moment. For instance, I commonly see my 12 volt panel output in the 19 volt range in bright, direct sunlight.

A charge controller regulates the power being sent to the battery(s) so that it matches their charging profile as closely as possible. This can happen in 2 different ways, the simplified distinction being:

PWM (pulse wave modulation) charge controllers are essentially turning the flow of power to the battery on and off, sometimes in very rapid succession, as they monitor voltage and try to best follow the charging profile. They can reduce amps but not voltage so PWM controllers can only be used when the nominal voltage of the panel is the same as the battery (i.e., 12 volts). PWM controllers work best in bright sun conditions when panels are maxing their output.

MPPT (maximum power point tracking) charge controllers are more sophisticated and sculpt the power flow by adapting voltage output to the battery’s charging profile. The panel might be sending 19 volts to the controller at 5 amps but the controller can, for instance, send 13.5 volts at 7 amps to the batteries. MPPT charge controllers do a better job than PWM harvesting power in bright-cloudy, changeable and other less-than-optimal conditions. Another benefit is that these controllers can step-down voltage and thus charge a 12 volt battery from a panel of higher nominal voltage.

In addition to regulating power flow for charging, a controller usually does one or more of the following functions: prevents reverse flow from battery to panels at night; provides terminals for 12 volt output and disconnects them from batteries if voltage gets too low; overload protection; meters and/or logs to show input/output performance.

What is a Battery Charging Profile?

It’s useful understand that there are different stages to charging deep-cycle lead-acid batteries:

• Bulk – the charge controller gradually increases the voltage until it gets to a certain set-point (e.g., 14.6V) with as much amp flow as the panel can provide.
• Absorption (aka Boost) – the charge controller holds the voltage set-point used in bulk, while gradually decreasing amp flow over a defined period of time.
• Float – the charge controller holds a voltage set-point (e.g., 13.8V) which is higher than the battery’s regular standing voltage with a trickle charge to maintain it at 100% state-of-charge.

Any charge controller will have preset charging profiles for different lead-acid battery types (e.g., flooded “wet”, gel, AGM, etc), and perhaps lithium batteries too. These presets define the voltage set-points, time periods and other charging variables. Some charge controllers also allow the user to customize these parameters to a battery’s specific charge profile which is on its technical specs. Better controllers have a temperature probe option and can automatically adjust charging voltages based on temperature.

Best Type of Charge Controller for Teardrop Trailers

Even as someone with reasonable solar power experience (my house is off-grid and powered from 3 solar arrays totalling 32 panels), I waffled on this question with my own Teardrop camper. MPPT controllers are, no question, more efficient but they are also several times the cost of PWM, which is older and simpler technology. I figured that, for a system as small as what is in a Teardrop, it’s probably not going to make much of a difference.

However after taking a few short road trips, I came around to thinking that–precisely because the on-board power system of a solar-powered Teardrop trailer is relatively small (in both panel output and battery capacity)–I should want to squeeze all I possibly can from it. And an MPPT controller will do a better job at that, especially in less-than-optimal sun conditions. What’s more, an MPPT controller will more closely follow charging profile and therefore get the longest useable lifetime from the battery.

I still keep the basic PWM controller that came with the trailer as a backup but switched over to an MPPT and, watching the input and output meters when the panel is active, I can see real-time how it makes the most from what the panel is capable of producing.

Another thing to consider with a charge controller is the maximum amperage it is rated to handle. Typically, Teardrop systems are small but if, for instance, you have two 150 watt 12-volt panels wired together that could conceivably produce over 20 amps, especially with sporadic increased levels as sun conditions change. So be sure your controller is sized to deal with input from your solar panels plus perhaps a 25% buffer for safety.

Hints & Hacks

• There is always efficiency loss in the charging and discharging of batteries (as well as conversion to house current with an inverter) so beware of assuming there is a direct linear relationship between the battery state-of-charge and how much power is being created or consumed.
• Depending on sunlight conditions and/or day length, as well as how discharged the batteries are, it may not be possible for them to get a full charge from solar. Keep a watch on the charge controller to see if it is getting to the “float” stage, which would indicated a full charge.
• I also carry a grid-tie battery charger so, if batteries have not had a full charge or there’s been a string of cloudy days, I will get a camp site with a hookup for a night and give them a complete top-up.

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So, that’s how I do things with my Teardrop Trailer but by no means the full story or the best way. What about you? Add your own methods, ideas and experiences using the comment box below for the benefit of all readers.