MPPT Charge Controllers


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Blue Sky Energy (
Blue Sky Energy makes a variety of MPPT charge controllers for small PV systems, including the 2512i-HV and 3000i, which are designed for use with 60-cell modules with 12-volt battery systems.
Midnite Solar (
MidNite Solar’s Classic line of controllers (right) includes ground-fault and arc-fault protection, high-voltage input, advanced data logging, battery monitoring, and a computer interface. The KID controller (left) is for use with small PV systems.
Morningstar (
Morningstar offers a range of charge controllers, from its SunSaver MPPT for small systems to its TriStar MPPT 600 V, which can accept a high-voltage PV array or can be used with an existing grid-tied PV array converted to battery backup, without requiring array reconfiguration.
Outback Power (
OutBack Power makes several MPPT charge controllers, including the 80-amp FLEXmax 80 (left) and the outdoor-rated FLEXmax Extreme (right).
Blue Sky Energy charge controller
Many charge controllers have the option for a remote display, such as this one from Blue Sky Energy, enabling users to monitor their systems more easily.
MidNite Solar’s monitoring
Many charge controllers come with computer connection capabilities to help users monitor their systems’ operation. These screen shots show MidNite Solar’s simple computer/smartphone monitoring.
Schneider Electric (
Schneider Electric makes a 150-volt MPPT controller for use with its battery-based inverters. Its high-voltage MPPT controller, which can accept 600 volts, can be used with longer wire runs or for converting existing grid-tied arrays to battery backup without array reconfiguration.
Blue Sky Energy (
Midnite Solar (
Morningstar (
Outback Power (
Blue Sky Energy charge controller
MidNite Solar’s monitoring
Schneider Electric (

In a battery-based PV system, a charge controller is used between the PV array and the battery bank to monitor battery voltage, optimize charging, and keep the array from overcharging the batteries.

There are a few common types of charge controllers: single or two-stage (shunt or relay type); pulse-width modulated (PWM); and maximum power-point tracking (MPPT). While non-MPPT charge controllers are less expensive and still have their place in the battery-based PV market—especially for lighting and small developing-world systems—just about all modern home- and cabin-scale PV systems include an MPPT charge controller, as they offer several advantages.

MPPT Advantages

More watts. Recall the power equation—volts × amps = watts. The more voltage captured from an array, the more power (watts) can be sent to the battery bank. An MPPT charge controller keeps the array operating at the peak of the current-voltage curve, and converts array voltage above battery voltage into extra amperage, thus absorbing more watts from the array. A non-MPPT charge controller chains the array’s voltage to the battery’s voltage, effectively limiting the array’s power output.

Array voltage varies with cell temperature. For example, when the cells are cold during winter, yet receiving full sun, the array voltage is higher. Higher array voltage translates into greater wattage. Here’s an example: Considering average winter and summer temperatures in Boulder, Colorado, there would be about a 12% difference between average winter versus summer array power output, and up to a 25% difference on a cold winter day versus a hot summer day. For off-grid systems that have higher loads in the winter, the extra energy input offered by MPPT-based systems can be a big benefit. At higher temperatures, which usually occur in the summertime or year-round in mild climates, array voltage drops, and an MPPT controller may be less advantageous.

Step-down. Voltage conversion is another benefit that is built into MPPT charge controllers. An MPPT charge controller is a DC-DC converter—with computerized controls. It can take a higher voltage and lower amperage, and convert those to a lower output voltage at higher amperage. For example, instead of an array producing a nominal 24 V and charging a 24 V battery, an MPPT controller can step-down an array producing 60 V to charge that battery. This frees the array from having to be matched to the battery voltage, and mitigates some wire-sizing (and cost) issues.

In that example, pushing 30 A at 24 V a distance of 40 feet would require large-gauge (expensive) cable—2 AWG—to keep voltage drop under 2%. For the same amount of power, pushing 12 A at 60 V that same 40 feet with 10 AWG will keep voltage drop under 2%, with the MPPT charge controller stepping the output voltage down to 24 V for the batteries. THHN #2 wire retails for about $1.24 per foot, and #10 sells for about $0.19 per foot, saving $84.00 on that two-way wire run, even without considering conduit size and the physical difficulties of pulling large wire.

Higher Input Voltages

Until recently, most charge controllers could accept a maximum input voltage of only 150 V. Today, one manufacturer has models that accept 200 or 250 V input, and two have models that accept up to 600 V input. Having these options provides more flexibility in designing module strings for battery-based systems. For example, instead of designing strings of three modules in series, strings of six modules in series are possible. This reduces the number of strings needed by half. At half the amperage and twice the voltage, the same size wire can be used, but at four times the distance—without losing power. A 600 V charge controller may be able to accommodate a single series string of 12 modules, negating combiner boxes completely. This translates into less equipment, wire expense, and labor.

Comments (3)

Zeke Yewdall_2's picture

Looking at some current prices, this is what I'm getting for comparison right now: I'm doing the math for a large 48 volt system, as I would rarely ever do a 12 volt system larger than about 600 watts any more. You get way more bang for your buck on the higher voltage with both charge controllers and inverters generally, not to mention voltage drop is much easier to deal with.

For the MPPT array, I'll pick 5000 watts, this is about 100 amps of charge controller. If we pick a Magnum PT100, it's running about $800 retail right now, so about $0.16/watt. If we can get array at $0.90/watt, total comes to $5300.

For the PWM array, I'll pick 6000 watts, or 20% more. We'll need to use 72 cell modules to be compatible, not the more common 60 cell ones, but probably not too much of a problem. Again, we need 90 to 100 amps of charge controller capacity. Two Tristar 45A controllers should work. I'm seeing about $230 for these -- including the meter (since the MPPT one has a meter). That comes to about $0.076/watt -- about half of the MPPT one. Total cost at $0.90/watt for panel comes to $5860.

If we do a smaller 12 volt system, I'm coming up with these numbers:
500 watt PV array, MPPT -- use two 250 watt 60 cell modules, for $0.90/watt. Need a 40A charge controller, so we pick the Morningstar 40A controller for $465. Total cost is $915. Charge controller comes in around $0.93/watt.

For a PWM system, using 600 watt array, we need 36 cell panels, which generally cost a little more per watt than the 60 cell ones. I'll guess we can get a good deal of $1.10/watt for them. Charge controller can be a 45A tristar for $230 (with meter) -- closer to $0.38/watt. Total cost is $890. Lower, but not by much.

On the voltage drop issue, if we want to run 12 volts at 40 amps about 150 feet, we need 1/0 wire to keep voltage drop below 12% (which is pretty darn high, but given how PWM controllers work, not as big of a deal if we aren't in a really warm climate). For the MPPT controller, we are running about 60 volts at 8 amps -- for 3% voltage drop, I need #8AWG wire. That's a big savings there.

You can see why I rarely do 12 volt systems of much size -- cost of equipment is just so much cheaper per watt when you jump to 48 volts. I still do a lot of 12 volt systems, just nothing very large, and on all but the smallest of them, I usually find that an MPPT controller is a little cheaper overall. For 150 watts and smaller, PWM will still win out.

You could run the numbers with cheaper charge controllers without meters, but I wont do that -- I like knowing whether the array is working, and without the meter on the charge controller, it's much more difficult to tell.

We could also delve into the Chinese made non UL listed charge controllers -- I've seen some amazing deals on both PWM and MPPT controllers there -- half or even a quarter the price of the major brand ones. If you have a situation where you don't need inspections, and don't mind replacing them when they fail, maybe that's okay. I prefer one that I won't have to drive out to a customer's house in a snowstorm on christmas eve to replace the charge controller :) But if it's your own system, just buying two (one as a spare) might be a cheaper way to go, as long as you have enough metering to know if it's working.

Edward-Dijeau's picture

My first instalations were with solar panels costing $5.00 per watt. Today they cost about $1.25 per watt retail and sometimes go on sale for 90 cents per watt. At $5.00 per watt, you wanted to squeeze every last watt out of your panels so you spent an additional $1.00 per watt for an MPPT Solar Charge controler to get that 15% to 30% "Solar Boost". Used 5 MPPT 40 Amp charge controlers. Now with the panels so cheap, you just by another panel or two and use a 5 Cents per watt PWM programable charge controlers to get that 30%. Charge controlers only come with a 5 years or less wattentee and Solar panels come with 10 year wattentees on defects and 25 years on power. if the prices of MPPT Charge controlers ever dropped the same 75% that solar panels did, they could again be a good investment. My current off grid system is 5,000 watts, all at 18 volt to 5000 amp hours of Lead Acid 12 volt batteries.

Ctmccloskey's picture

Having installed a 600 volt grid tied PV system in Washington DC I had to learn new tricks to be safe and not get killed by this lethal voltage range. When designing the system be sure to use components designed and rated for this higher than normal DC voltage range as they weren't commonly available when we did this system. At this voltage the ability of the power jumping or arcing was a real danger and connections had to be kept farther apart. The other thing to watch is when making the PV interconnections be sure to work like a Power Company employee and wear the right equipment designed for working with higher voltages. While interconnecting the strings of modules I used large dark tarps covering the PV modules to reduce the voltages and make the system more manageable. The advantage of the smaller wiring sizes made the system easier to install than one with lower voltages and higher current levels. Higher DC voltages opens a whole new world of safety issues.

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