Adding Solar and a 12-Volt Battery Charger | Project F-250

After lots of bottom-feeding on Marketplace and lots of work, we’d put together our dream tow rig: A 2001 Ford F-250 pickup with a slide-in camper. It might not be the most elegant combination on the road, but we’re thrilled with its ability to comfortably tow our enclosed trailer to the track and then act as our trackside chalet for the weekend.

Of course, there’s still plenty of room for improvement. And the first place we focused was our camper’s electrical system, which left a lot to be desired during long weekends in the paddock. As purchased, the camper’s 12-volt system was patched together with various scraps of household 120-volt wiring and lots of duct tape (thank you, previous owner). It worked, if only barely, but we had no way to tell how much battery charge was left, and no way to charge the camper’s battery without finding an electrical outlet.

What’s so important about having a reliable source of 12-volt power? This RV, like most, uses a 12VDC system for almost everything. Just like your car uses 12 volts to run its lights and HVAC system, our camper does, too. It also uses 12 volts to run its fridge, water pump and safety monitors. When plugged into a 120-volt AC outlet, our RV uses that “shore power” to produce 12 volts as well as to energize the 120-volt outlets and air conditioner.

What’s wrong with charging up from shore power? Simple: It’s too darn expensive! Most tracks charge around $50 for the privilege of plugging in an RV, and that’s if you can even find a free plug in a crowded paddock electrical box. At those rates, we knew some upgrades would pay for themselves after a few races, and then we’d never have to worry about fighting our paddock neighbors for a plug.

This won’t be a story about an expedition-ready power system designed for a self-supported trip through sub-Saharan Africa. We didn’t have the need (or the budget) for a huge battery bank or a roof covered in solar panels. Instead, our goal was to comfortably camp at the track with basic creature comforts without being plugged into a wall outlet. Here’s how we did it.

Let’s Start With the Math

Normally we’d start this process by sitting down with a scratchpad and a calculator, as the right way to size an electrical system is by calculating the reserve capacity of your battery, the amount of power each appliance draws, and the amount of time you’ll be running each appliance. With those numbers, it’s fairly easy to figure out how much power you can store, how much power you want to use, and how much power you’ll need to add to maintain an equilibrium.

Yeah, we didn’t do that. In reality, we don’t actually need much power to camp for a weekend. Because we have a propane water heater, furnace, stove and fridge, we can keep ourselves warm, our food cold, and our coffee and meals hot with almost no electricity and about $5 worth of propane over a race weekend. If we didn’t have all these propane appliances, we’d have gone a different direction and added far more capacity to our camper’s electrical system. But as it was, we only needed to consider the basics: chargers for our cameras and computers, lights so we could see what we were doing, and fans to keep us comfortable. None of these appliances draws much power. What about air conditioning? Unfortunately, it’s just not feasible with our space and budget constraints. We’ll only be able to run the air conditioner when plugged in to shore power.

Since our electrical requirements were low, we started with the other constraints: space and budget. We didn’t have thousands of dollars for lithium batteries or acres of solar panels, so we kept our system simple and upgradeable in case we want to add capacity later.

Storing Electricity

Our camper came configured for a standard group 24, deep-cycle, lead-acid battery, and we kept things simple by replacing it with a fresh one from our local battery distributor. Why not add a second battery? Weight and space, as both are at a premium in this tiny RV. Why not upgrade to lithium batteries to store more power without taking up more weight or space? Cost, and cost alone. Our $75 lead-acid battery should hold plenty of power for what we’re doing. We ripped out the last few feet of the main power lead that the previous owner had butchered and replaced it with properly sized wire and a 60-amp circuit breaker to protect the 10-foot wire running to the factory power distribution center.

What’s a power distribution center? Just like your house has a breaker panel, RVs have a power distribution center. The difference is that your house’s panel only manages AC power, while the RV’s unit manages both the AC and DC distribution. It’s the DC fuse box and the AC breaker panel in one compact unit. It also has an AC/DC converter, meaning it makes 12-volt power for appliances and for battery charging when the RV is plugged into shore power. Since our distribution center was working well and included a modern battery charger that wouldn’t boil our battery when sitting, we left it in place.

Alternator Charging

We had a place to store electricity, but we only had one way to make it: expensive shore power. That meant we were leaving two perfect sources on the table: our truck’s alternator and the sun. Time to hook them up!

We’ll start with the alternator. There’s a cheap way to do this, and then there’s the right way to do this. What’s the cheap way? Simple: Run a long, 12-gauge wire from the truck’s seven-pin trailer connector to the camper’s battery, essentially making a long jumper cable that charges the camper’s battery just as if it were jump-starting it. And that’s actually how our camper was wired when we bought it, though we cut that wire immediately, before even plugging the connector into our truck.

Why cut the wire? Because that solution has a few big flaws, especially for our purposes. When two batteries are connected, electricity will equalize itself, flowing between them until they’re at the same voltage. And the bigger the difference between each battery’s state of charge, the faster that electricity will attempt to flow between them. That’s why jumper cables are made of thick, heavy wire: They’re designed to hook a full battery to a dead battery and endure the subsequent flow of power that’s created as a result.

What’s this mean for our long red wire? It means that if you connect a fully charged truck battery to a camper battery that’s completely dead after a few days in the paddock, a jumper cable’s worth of electricity will immediately try to transfer across the connection. At worst this means a fire, but because our F-250 is designed for trailer towing, it just means a blown 40A fuse under the hood.

For a system where the second battery doesn’t get heavily discharged (think an enclosed trailer with a few interior lights that’s always either hooked up or parked with a trickle charger), the long-red-wire approach will work fine. But it’s not a great idea if you plan on really discharging the deep-cycle battery in your RV or trailer.

Oh, and if your truck has a smart alternator, like our F-250 does, then electricity’s desire to flow faster across different states of charge can hurt you in a second way. Our truck’s alternator doesn’t always make 14.5 volts. Instead, it’s computer-controlled and only charges as much as it needs to in order to save fuel and increase battery life. That means it’s completely normal for our truck to cruise down the highway at 12.5 volts. If you’re electricity, that means you don’t have a good reason to run down that long red wire to the camper’s battery. The result is a lower charge rate—or even an inability to fully charge the camper’s battery.

And if your truck isn’t wired to cut power to the trailer when it’s shut off, that long red wire will keep the batteries connected even when parked. On the upside, this means you’ve doubled the size of your battery bank. On the downside, this also means you might have a dead truck battery after a weekend of running the lights and fans.

In order to solve these drawbacks, we needed a 12-volt-to-12-volt battery charger. Rather than leave electricity free to travel back and forth and do whatever it wants, these devices use a charging strategy to manage things, keeping electricity flowing in the right direction at the right rate to keep our camper charged.

Solar Charging

We had the first item on our to-do list: a 12-volt charger. Next up? Solar charging. After all, paddocks rarely have shade, so it makes sense to put all that sunlight to work. And thanks to the ever-decreasing cost of solar panels, it’s never been cheaper or easier to charge your camper’s battery with the sun.

Our first decision: where to put our solar panel(s). The ideal situation is for the panel to be pointed directly at the sun with zero obstructions. As the panel’s angle strays away from perpendicular to the light, its efficiency drops and it will make less power. Sounds like an easy decision, right? We should buy a portable solar panel, then prop it up in an open area pointed at the sun whenever we go to the track. That’s where the ideal panel placement bumps into the reality: Open space is at a premium in the paddock, and it’s not a great place to leave big pieces of glass lying around. Plus, we didn’t want the hassle of carrying around a portable panel. Instead, we decided to mount a panel on the roof of our camper. It wouldn’t be optimal, but it would mean we’d never forget to set it up or drive over our source of power with the race car.

That led to our next decision: the type of construction for our solar panel. The camper’s roof is big and flat, meaning we could stick to common (and less expensive) rigid solar panels. There are two main types: polycrystalline and monocrystalline. The exact construction differences aren’t worth wading through here, so we’ll cut to the chase: Monocrystalline panels are more efficient and more expensive, while polycrystalline panels are slightly less efficient and slightly cheaper. Polycrystalline panels are also slightly more efficient than monocrystalline panels when partially covered (think a few leaves or some shade on one corner of the panel). After weighing the pros and cons of each, we decided to go with monocrystalline panels. The price difference was negligible thanks to a market full of imported panels, and paddocks rarely have overhanging trees dropping leaves to hurt efficiency.

After measuring our roof and our bank account, we ordered a 100-watt monocrystalline panel from Renology for $110. We spent another $50 or so on mounting brackets and wiring from Amazon, then screwed the panel to an empty area of our camper’s roof and routed the wiring down to our distribution cabinet.

Measuring Electricity

We now had plans for two different methods of putting power into our camper’s battery. But we had no way of tracking its state of charge, meaning we couldn’t easily tell if, for example, charging our laptop overnight would mean waking up to a dead battery and no way to turn the lights on. We also needed a solar charge controller to finish installing our panel, along with one of those 12-volt battery chargers we mentioned earlier.

Fortunately, we’re not the only ones to have had this problem, and there’s a company that has already come up with a very elegant solution. CTEK sent over one of its 20A Off Grid Charger Bundles. For $622, this one-box solution acts as a 12-volt-to-12-volt battery charger, a MPPT solar charge controller and a battery monitor. It also includes a voltage shunt and a handy digital display for tracking power going in and out of the battery and displaying its state of charge and expected lifespan at the current load. It’s compatible with smart alternators, too, making it perfect for our application. Installation took less than an hour: We mounted the unit in our electrical compartment, then connected our truck’s 12-volt feed, our battery and our solar panel. After splicing the voltage shunt into the circuit next to our battery, we screwed the monitoring panel to a free spot on the wall and programmed our battery capacity into the system.

Just like that, we had a smart electrical system that automatically switched between solar and alternator power to keep the starting and camper batteries fully charged. It also keeps the batteries isolated when necessary to ensure we’ll never wake up to a truck that won’t start. And by keeping an eye on the CTEK monitor, we discovered that our basic solar system was more than adequate for a weekend at the track, even if we went nuts with lights and fans.

Are there cheaper ways to accomplish all this? Sure. We could have pieced something together, but it would have taken up extra space we didn’t have and been vulnerable to water damage. It also wouldn’t have intelligently switched between alternator and solar power, or used an advanced charging strategy that even takes into account our battery’s temperature when determining how much power can be squeezed into it. By spending a little more on our charge controller, the reward should be a more reliable system that’s gentler on our batteries than anything we could have cooked up ourselves.

Now that our electrical system was sorted out, we were living like kings. If only we were driving like them, too. We’ll cover some chassis upgrades in the next update. 

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