Now that we have covered some basics, let’s talk power generation… Before vanlife, I (Andrew) was an engineer designing power generation systems so this subject is right up my alley. Of course, the scales were a bit different; it would take roughly 1 million Dolly’s in order to generate enough electricity to match the power plant engines I designed in my previous career. However, the basic principles still hold: you need an energy source, and a method to convert that energy into electricity (a power plant!). Dolly has two power plants: (1) a gasoline-fueled internal combustion engine and (2) solar panels on the roof.

Charging From the Alternator

Let’s start with the engine. The engine’s primary purpose is obviously for driving us around, but it also generates electricity through the alternator. In a normal vehicle (i.e., not a campervan), the alternator is used to charge the automobile’s battery. In Dolly, the van’s battery is wired to the house battery through a smart dual battery isolator which uses a voltage sensitive relay to automatically isolate the batteries when no charging source is present. When the engine is running, the alternator produces ~14V DC, and the relay closes, allowing both the van and house battery to receive charge. When the engine isn’t running, the voltage across the van battery drops below ~13V DC, and the relay opens, isolating the van and house batteries. This is an important safety feature, as you do not want to discharge the van battery while you are sitting in camp running your fridge, fan, lights, etc. Another nifty feature of our battery isolator is that the relay closes if it receives 14V DC on either terminal of the isolator. Therefore, if you are not running the van, but the solar panels are generating electricity, the relay will close and both the house battery and van battery will be charging from the solar panels.

Now, how much electrical power do you get from the alternator? One of my former colleagues was famous in our company for answering questions with “It depends.” It depends on the size of the alternator, the RPM of the engine, and the current state of charge of the two batteries. In order to not get bogged down in all those details, I will just tell you that we have looked at the wattage readings from our Victron Energy battery monitor while the van is running, and the maximum we have seen is ~1000W. This is more than double the max power we could possibly achieve from our solar panels, and it is always available to us as long as we have gas in our tank. The same is not true for solar  panels. If it is cloudy, solar panels produce less power. If you are parked in the shade, less power. Panels get dirty, less power. Main takeaway: when designing your van’s power system, you should strongly consider some method of charging from the engine unless you are very confident in the amount of solar you are producing vs. the amount of power you are consuming. This leads to an obvious follow-on question: how do I know how much power I will consume in my van? Well, that will come in a future post… stay tuned.

Charging from Solar Panels

Dolly’s second source of power is from the sun! Dolly is equipped with 400W of solar panels mounted to our roof rack. Now, you might be wondering why we want to have solar panels if the alternator is capable of producing up to 1000W of power on demand. Well, there are two main reasons. First, charging from the alternator requires that the engine be running, which produces noise, creates exhaust, and costs money in the form of fuel. Solar produces no noise, no exhaust, and the fuel cost is zero because the sun doesn’t charge us for its energy! Isn’t the sun a pretty awesome dude? The second reason that we have solar panels is that we don’t drive every day. Having solar panels on the roof gives us the freedom to stay in one place and not worry about discharging our house battery (as long as we have sun, of course).

Let’s talk now a little bit about the solar setup in Dolly. We have four 100W polycrystalline solar panels wired in series to an MPPT charge controller, which is then used to charge a 200 Ah AGM battery. There were a few key words that you may or may not be familiar with in the previous sentence, and I’m going to talk a little bit about each to try to clear things up. If you want, you can skip any or all of the next sections if you feel comfortable with the topics.

Monocrystalline vs. Polycrystalline Panels

First, you may have heard about polycrystalline vs. monocrystalline solar panels if you have done some research on installing solar panels. A monocrystalline solar panel is more efficient than a polycrystalline solar panel, which means that a 100W monocrystalline solar panel will be smaller than a 100W polycrystalline. However, they will both produce the same amount of power. If your goal is to put a lot of stuff on your roof (vent fans, cargo carriers, a small deck, etc.), I would recommend that you get monocrystalline to save on space. Otherwise, either option will work just fine.

Series vs. Parallel Panels

Another decision that you need to make when installing solar panels is whether to wire them in series, parallel, or some combination of the two. There are a lot of varying opinions on this subject, but my opinion is that either should work just fine and you should choose what seems best for your situation. I chose series connectors mainly because the wiring is simpler. I have four panels. Let’s arbitrarily call them 1 through 4. The positive terminal of panel 1 is connected to the negative terminal of panel 2. The positive terminal of 2 is connected to the negative terminal of 3. The positive terminal of 3 is connected to the negative terminal of 4. The negative terminal of panel 1 and the positive terminal of panel 4 are then connected to the MPPT charge controller. If I had done the panels in parallel, it would have required joiners to connect all of the positive terminals together and all of the negative terminals together. In addition the wire gauge would have to be larger in parallel because the current from each panel adds in parallel, while it is the voltage that adds in series connections. The requirement for wire gauge depends on the current that is traveling through the wire, not on the voltage.

For more in depth discussion on series vs. parallel connections, check out this article by Renogy. https://www.renogy.com/learn-series-and-parallel/

Charge Controllers: MPPT vs. PWM

A charge controller is basically a device that regulates the voltage and current coming from the solar panels into the batteries. One of the main purposes is to keep the batteries from overcharging. There are two commonly used technologies for modern charge controllers: pulse width modulation (PWM) and maximum power point tracking (MPPT). MPPT is a newer and better technology than PWM, but is also more expensive. The advantage of MPPT over PWM is that it can convert excess voltage into current, allowing you to charge your batteries faster and with less power loss through the cables from the panels. With a PWM controller, the voltage across the panels is required to be ~18V. If you are installing multiple panels they have to be in parallel instead of in series.

In Dolly, we have an MPPT charge controller that can deliver up to 40 A of charging current to the batteries. To determine the required charging current capabilities for your charge controller, divide the total power of your solar panels (for Dolly this is 400W) by the voltage of your batteries (for Dolly this is 12V). For Dolly this comes out to 33 A, which means that a 40 A charge controller is sufficient for our solar setup.

Batteries

Because you are not always receiving solar energy and the van’s engine isn’t always running, you need to have a method to store energy to use when the sun’s not out and when your van isn’t running. Hence, your van needs a house battery (or batteries). You really don’t want to use the van’s existing battery; it is not made for that type of application, and will have a much shorter life. You also would risk depleting the van’s battery and getting stuck somewhere.

Two common types of batteries used in campervans and RVs are absorbed glass mat (AGM) and Lithium-Iron-Phosphate (LFP). There’s a lot of discussion on the pros and cons of these two technologies; for an in-depth comparison, Victron Energy has a nicely written article here: https://www.victronenergy.com/blog/2015/03/30/batteries-lithium-ion-vs-agm/. Instead of repeating everything that they have said, I will just say that the main reason we went with AGM over LFP was cost; LFP batteries are 3-4x more expensive than AGM batteries.

I would argue that even more important than the type of battery is the capacity. You do not want your battery to be undersized and to be running out of juice all the time. That is no fun, and you should be having fun in your campervan. Battery capacity is most often given in Amp-hours (Ah) or Watt-hours (Wh). In the next section I will explain how this number is related to the loads (items that consume power) on your electrical system. Before proceeding, I would like to explain a few terms that can come up frequently in discussion on battery sizing:

• Charge Cycles: All batteries have a finite life, expressed as the number of times they can be recharged. This is referred to as charge cycles or number of charges. When manufacturers give this number it is a best estimate, as usage will impact the actual number of charge cycles for a given battery. Typical charge cycle estimates:
  • AGM: 400-1500
  • LFP: 2000-7000
• State of Charge (SOC): This is how much charge is left in your battery expressed as a percentage of the total size. For example, if you have a 200 Ah battery that is down to 150 Ah, your state of charge is (150 Ah) / (200 Ah) × 100% = 75%.
• Depth of Discharge (DOD): This is the opposite of SOC; it measures how much the battery is discharged expressed as a percentage of the total capacity. It is related to SOC by DOD = 100% − SOC.
• Maximum Depth of Discharge: This is the recommended highest DOD to avoid damaging the battery. It is also often referred to as the usable energy of the battery. Going beyond this depth of discharge can reduce the battery’s total number of charge cycles. Maximum DOD based on battery technology:
  • AGM: 50%
  • LFP: 80%