MPPT Charge Controllers

Intermediate

Inside this Article

Blue Sky Energy (blueskyenergyinc.com)
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 (midnitesolar.com)
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 (morningstarcorp.com)
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 (outbackpower.com)
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 (sesolar.com)
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 (blueskyenergyinc.com)
Midnite Solar (midnitesolar.com)
Morningstar (morningstarcorp.com)
Outback Power (outbackpower.com)
Blue Sky Energy charge controller
MidNite Solar’s monitoring
Schneider Electric (sesolar.com)

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.

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