This is the sixth DIY solar post in a series on solar power systems detailing the role of the solar charge controller and why to prefer an MPPT charge controller. While this application was done for an RV solar panel system, the concepts are intended to apply for any do it yourself solar power system. We hope you enjoy this entry (The Solar Series: Choosing a Solar Charge Controller) and links to the rest of the DIY solar panel system series can be found at the bottom.
This post will outline why we need a charge controller in the first place by explaining what it does, then go into a comparison of which technology is best, detail some practical sizing and wiring considerations then finish with an explanation of different models of charge controllers and their customizability. Having personally used the Morningstar Tristar 45 MPPT and found it to be the most efficient, customizable and reliable, I recommend it so much it’s got its own dedicated post.
What is a Charge Controller and Why Do I Need One?
In a perfect world it would seem straightforward that we would take the wires from our solar panels, hook them up to our batteries and be done with it. Unfortunately it’s not that simple. Both the panels and the batteries are “dumb” and so the charge controller needs to act as a brain to keep the charging process running smoothly and efficiently. How “smart” the controller is depends on the technology and manufacturer, but the best models will sense incoming power from the panels, the charge level and temperature of the batteries and adjust the controller’s output accordingly. I refer to it as “smart” because in some fashion or another it needs to be able to sense these outside variables and make decisions based on them. The two main issues it is compensating for are varying input power from the panels and varying charge levels of the battery as explained in the following two paragraphs.
The voltage output of a solar panel is affected by how much sunlight is striking it and at what intensity. In the simplest case of a 12 volt nominal panel this can vary from slightly below 12 V at the crack of dawn to above 17 V in broad daylight, especially if it’s cold outside. If your battery is moderately charged hovering around 12.5 V, it’s going to not get charged at all in the morning, then by midday it will be boiling and destroy itself. The primary function of even the simplest charge controller is to prevent this from happening.
The second concern is the charging profile of a battery. If it’s at 12.5 V, depending on the battery it is perhaps most happy being charged at a constant voltage of 13.8. All the current will flow into the electrolyte ion chemical reactions to store more power. However, eventually the battery will be full and instead of storing more charge it will get hot, start to boil and after an extended period permanently damage itself. Any decent modern charge controller will sense the battery voltage and realize once it’s held it at 13.8 for awhile and the battery is full, it should perhaps back it down to something like 13 V to go easy on the little guy as he can’t store any more charge. A third bonus a good charge controller can take care of here is temperature compensation, that 13.8 V might be perfect if it’s 25 OC, but if it’s -25 OC maybe it should be backed off to 13.2 V to reduce stress.
Controlling Voltage: PWM vs. MPPT
Now that we’ve outlined why the voltage needs to be shaped to suit the needs of the battery, the next question is how? The basic method used in any modern charge controller is something called pulse width modulation (PWM). It might sound like a scary engineering term, but it’s a simple concept. If the panels are outputting at 17 V and I only want to charge the battery at 13.6 V, a simple solution is to have the charge controller only on 80% of the time (17*0.8=13.6). While this is achieved with transistors that turn on and off thousands of times per second, it’s easy to visualize looking at the graph and thinking of it at a high level.
However, pulse width modulation alone has one massive drawback here, you aren’t harvesting energy 20% of the time because the circuit is open. Sure, it’s better than frying your battery, but it’s incredibly wasteful and inefficient. For this reason PWM should only be considered in smaller solar installations where the cost of better technology won’t be recouped over time (explained in the next section), although good affordable PWM controllers certainly exist.
And what is this better technology? It’s something called maximum power point tracking (the abbreviation at the front of MPPT charge controller) and is vastly superior to PWM alone. The basic idea is through more complex and expensive power electronics in the solar charge controller it is able to act as a variable DC power supply. So if you have 17 Volts input from the panel at a current of 10 amps (170 Watts) and still want to output that at 13.6 V, you’ll wind up with just a tad under 12.5 amps from the MPPT controller. How this all actually works you don’t want to know, trust me (for masochist engineers: MPPT theory) but the efficiency winds up being in the 98% range, which means 10 times less loss than PWM only.
The other major benefit of an MPPT charge controller is allowing the use of higher nominal voltage solar panels. My 36 volt panels charging 12 volt batteries would not be possible with PWM (unless you wanted 30% efficiency), but with the variable DC power supply of MPPT I’m able to do it easily with an efficiency around 96%. The greater this step down in voltage, the less efficient the charge controller will be and the hotter it will get, but for the versatility afforded in choosing panels, improved energy recovery in low light and less ohmic loss over a 36 volt wire run, it’s more than worth it.
Expanding on this, a good MPPT charge controller doesn’t just increase efficiency on the output side by monitoring the battery charge state (PWM controllers can do this too), it can also control the input power by modifying the solar array voltage. If you have a 36 Volt nominal array, an MPPT solar charge controller will perform a voltage sweep and see exactly what point produces optimal power. Left to their own devices, the panels might stabilize at 42 Volts, but the MPPT charge controller could find during its voltage sweep that harvested power is highest if the panels are held at 40.5 Volts, so it will force them to stay there.
Choosing an MPPT Charge Controller
The main factors to take into account when selecting a charge controller are whether your solar power system is large enough to warrant the cost of MPPT, choosing an appropriate amp rating based on system size, and what additional charge controller features you need/want.
The most logical way to decide on getting an MPPT charge controller vs a PWM charge controller is payout, i.e. will the increased efficiency pay for the increased cost. Because almost all major solar panel manufacturers guarantee panel output for 25 years, that’s plenty of time to get your money back. Let’s go through a quick example comparing my existing MPPT system versus a hypothetical PWM system. I’ve got 430 Watts of power at 36 volts, my wire run adds a 1% loss and my Tristar MPPT charge controller is 96% efficient stepping down from 36 to 12 volts, which yields an available actual power of 409 Watts. An equivalent 12 volt system with a PWM charge controller would have an 8% loss in the wires (thicker wire would cost more) and just to make it more fair, let’s say 85% efficient at stepping down from the array’s 15-17 volts, yielding an available actual power of 336 Watts, a 73 Watt difference.
The short way of comparing the two is taking the 73 Watts multiplied by the average solar panel value of $4 per Watt, which gives you around $300 extra dollars to spend on the charge controller. However, a far better method of comparison is to analyze how much energy you’re losing via inefficiency over the entire lifetime of the system and what it’s worth monetarily, which tilts the scales far more in the MPPT’s favour. If we make the assumption of 5 hours of peak sunlight a day, the math breaks down like this: 73 Watts of loss * 5 hours peak sunlight per day * 365 days * 25 years = 3.3 megawatt hours. Yes, megawatts. Using the solar power system as a straight up grid tie-in replacement that values the power at about $0.10 per kilowatt hour, that equates to $333, similar to our shorthand estimate. However, in the case of my RV which is never connected to the grid, I value a kilowatt hour at a lot more than 10 cents because I can get it wherever I want, anywhere on the planet. The same goes for an off-grid remote installation where there are no power lines. Lastly, and even more importantly for people in areas with government subsidies on solar power generation, the rate paid by the grid can rise as high as $0.80/kWh, so you’re all of a sudden looking at $2650. Woah.
Choosing an MPPT Charge Controller – Amp Rating
This barely deserves its own section because it’s pretty straightforward, but charge controllers obviously come in a wide variety of sizes, which is measured by their amp rating. My Morningstar Tristar 45 MPPT has the 45 to indicate 45 amps is the most it can handle. 430 watts panel output / 12 volt battery voltage = 36 amps and since the next smallest model is 30 amps, the choice of 45 was dead simple, hopefully you can easily do the same comparison yourself, perhaps leaving a bit of room if you feel like adding more panels in the future.
Choosing an MPPT Charge Controller – Competing Brands
Now that we’ve done all the mathematical heavy lifting, all that remains is to pick an actual controller. Xantrex, Outback and Morningstar are the dominant brands on the market, so it’s just up to you to decide which customizability and extra features you need (temperature compensation, LCD display, customizable charging set points, PC monitoring software, accessories like relay controllers to turn on and off other system components, etc). As I am so pleased with my Morningstar Tristar MPPT, I’ve written an additional post detailing all the features that make it such an enticing MPPT charge controller.
So that’s how to select the best charge controller. Over the coming weeks I will be creating posts for each aspect of the creation of off-grid solar power systems with the hopes that you can follow along and use the the info for your own DIY solar panel system. Of course it gets a little technically involved in spots, so feel free to ask for clarification in the comments or drop me an e-mail.
RV Solar Panels – Goals, Rationale
How Do Solar Panels Work – Power, AC & DC
Solar Power System Sizing – How much power do you need?
Best Solar Panels – Selecting the best solar panels for you
Solar Batteries – Selecting the best solar batteries for you
Choosing a Charge Controller
Installation (coming soon…)
Maintenance (coming soon…)
The Solar Lifestyle (coming soon…)
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