Finding the right solar panel setup for caravans can be challenging. Marty believes he’s discovered a crucial aspect of solar power that many may overlook.
Have you struggled with your RV batteries meeting your power needs, even with a solid solar panel setup? You’re not alone. Throughout the years, I’ve experimented with various 12V systems that incorporated solar charging, all of which have ultimately disappointed me. What could I possibly be doing wrong?
As I travel across the country and converse with other caravan enthusiasts, I often hear the same concerns. Despite having solar charging systems, they too find that their batteries can’t keep up with their energy consumption. After pondering this dilemma and conducting some tests, I believe I’ve pinpointed a significant mistake we’ve all been making with our caravan solar installations.
First, it’s essential to acknowledge that solar power generation isn’t highly efficient. Modern solar panels convert only around 20 percent of the sunlight they receive into electrical energy. To achieve even this level of efficiency, the panels must operate in optimal conditions, ideally receiving sunlight directly from a 90° angle.
Are your solar panels on the caravan roof?
Many new caravans, motorhomes, and campers come with solar panels installed on the roof or other flat surfaces. While this is convenient for transport and usage, it doesn’t ensure the panels can always face the sun directly. In Australia, depending on the season, the sun often hits our solar panels at less-than-ideal angles, causing efficiency to drop to around 14 percent.
So, how does this affect real-life usage? Our caravan is equipped with 450 watts of solar panels on the roof, which theoretically can generate up to 30 amps in perfect conditions. However, that’s never the case. A while back, we upgraded to a Redarc BMS 1230S Battery Manager, which, though no longer available, has been replaced by the newer Redarc Manager30 S3. This smart device, designed in Australia, provides valuable insights into our 12V system, including the actual wattage produced by our solar panels.
During winter months in southern Australia, our 450W solar panels struggled to produce even 230 watts, roughly half of their rated capacity, translating to just 15 amps of charge.
What happens when we use power while charging?
Consider what happens when we draw power while simultaneously charging our solar panels. Appliances like fridges, TVs, DVD players, and phone chargers drain power from the battery even when it’s being replenished. This results in a charge of less than ten amps per hour. Add some cloud cover or shadows, and that can drop to just five amps.
This isn’t much when trying to charge 200Ah of AGM batteries. In winter, we often have only about six hours of daylight, but only four of those hours present optimal angles for solar power generation between 11 AM and 2 PM. Conservatively, that yields about 20 amp hours of charge for the entire day.
The situation worsens since we rarely park our caravan in direct sunlight, commonly settling under trees or in valleys. Even a slight shade from an air conditioner over part of the solar panel can drastically reduce output.
Considering all these factors, our actual net charge per day is likely only between 10-15 amp hours—far from enough. With nighttime power consumption factored in, it’s no surprise our batteries struggle to last a weekend without supplementary charging.
All is not lost …
However, there’s hope. A simple adjustment can greatly enhance your battery recharging capabilities.
When our caravan was built, we requested the inclusion of a separate Anderson plug connected to the solar panels. This allows us to attach external solar panels that can be positioned to capture direct sunlight, supplementing the roof-mounted panels. Using the Redarc BMS to monitor output, we connected an external 100W panel during full winter sunlight, increasing our solar power generation by an additional 80 watts almost instantly.
This hugely improves our charging capacity. By frequently moving this panel into optimal sunlight throughout the day, we’ve added about 20 amp hours to our total daily charge.
A similar benefit can be achieved by linking an external solar panel to your trailer’s Anderson plug to draw power from the tow vehicle’s alternator. Ensure the panel has a built-in regulator if it’s connecting directly to your battery; otherwise, for a DC-DC charger, an unregulated panel will work fine.
I will admit, moving the panel throughout the day can be a bit tedious and not always feasible. However, this experience shows that supplementing your roof-mounted solar panels with external ones can result in a significant enhancement in battery recharging each day.
Understanding your 12V system
To grasp your 12V electrical system, keep this key equation in mind:
Watts = Volts x Amps or Amps = Watts / Volts.
Understanding this allows us to estimate the amperage from any solar panels. For example, a 100W solar panel divided by 12 volts equals approximately 8.3 amps. However, unregulated 12V solar panels can produce up to 22 volts in bright sun, necessitating a regulator or solar charger to lower this voltage to a level suitable for 12V batteries, thus enhancing the charging amperage. Unfortunately, 12V DC systems can experience voltage drops and other inefficiencies that complicate output calculations. For simplicity, it’s best to assume a 15-volt baseline when estimating solar recharge time.
It’s crucial to remember that battery charging is a gradual process. Batteries are rated in amp hours, allowing us to estimate their lifespan. For instance, a constant one-amp load on a 100Ah battery will fully deplete it in 100 hours. However, most 12V RV batteries shouldn’t be discharged below 50 percent of their state of charge (SOC). Thus, a 100Ah battery running a one-amp load can last about 50 hours before risking damage.
In contrast, lithium batteries can be discharged to much lower levels without incurring long-term damage.
It’s tricky to get caravan solar panels right
Returning to our earlier example, if we deplete a 100Ah battery to 50 percent SOC, it will need 50 amp hours to fully recharge. A 100W panel generating 6.6 amps per hour could theoretically recharge it in about eight hours, assuming no load and the panel is in perfect condition. In reality, it can take around 60 percent longer due to battery inefficiencies and the charging profile of solar controllers, which include an absorptive final stage as batteries struggle to accept current as they reach full charge.
When solar panels are mounted on a caravan’s roof and used during winter in southern Australia, fully recharging could take three days or even longer. If connected loads, such as a 12V compressor fridge, are in use, this time will increase.
So, with your caravan solar setup, hopefully, you’re not making the same mistakes—now you know how to get it right!
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