Z Yewdall
Published In: HP online
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.
The 600 V charge controllers may be used for transforming batteryless grid-tied PV arrays to grid-tied with battery backup. In many cases, rewiring the array is unnecessary.
A disadvantage to using a controller with a higher input voltage is that the disconnects and combiner boxes (if required) are typically more expensive and harder to find. Note that one of the 600 V input charge controllers (Morningstar’s TS-MPPT-60-600) has an optional integrated DC disconnect, which can help mitigate sourcing and finding space on the wall for an external 600 V DC disconnect, though the controller’s additional cost is similar to the cost of a separate DC disconnect.
Single-Module PV Systems
Most module manufacturers have switched to a 60-cell design, resulting in modules in the 200 W to 300 W range with a maximum power point of 25 to 35 V. Nominal 12 V and 24 V modules (having 36 and 72 cells, respectively) are harder to find and more expensive per watt. Several manufacturers have introduced MPPT charge controllers to accommodate a single 60-cell module on a 12 V battery system (which might power, for example, remote lighting or communications, or an off-grid cabin). Blue Sky Energy offers several products for 12 V systems, and MidNite Solar and Morningstar have introduced smaller (30 A) MPPT controllers, which will work for a single module on a 12 V system.
These charge controllers cost more than a simple PWM charge controller that you might use on a system with 36-cell (12 V nominal) modules. However, when you take into account the total system cost—PV module(s) plus charge controller—it can be 10% to 20% less expensive to use the 60-cell module with the MPPT charge controller. Plus, you get the advantage of MPPT. In addition, the wiring of the system often is simpler, since it involves one large module and no combiner boxes.
Matching Controllers to Inverters
For off-grid systems, matching the brand of charge controller to the inverter isn’t usually important, since there is very little coordination between these two. The charge controller routes energy into the battery, and the inverter takes it out—neither of them really cares what the other is doing. However, for a grid-tied system, synchronizing them can matter. While there are thousands of battery-based grid-tied systems that operate without communications between the charge controller and inverter, system programming can be simplified and efficiency can be improved if they are matched. Compatible communications systems enable the inverter to tell the charge controller that the grid is available. At this point, the charge controller’s job is not to regulate battery charge but to track the array’s MPP and get the most energy out of the array that it can. (The inverter will regulate the battery voltage by selling excess energy to the grid.)
Monitoring & Data Logging
All but the most basic charge controllers come with some system monitoring. All of the charge controllers included offer remote display options, enabling you to monitor the system’s operation in the house, for example, rather than at the controller’s location. Most of the MPPT charge controllers include a digital display on the controller as well. If your system has multiple charge controllers (from the same manufacturer), they can communicate with each other to coordinate charging, and can all send data to a single remote monitor.
MidNite Solar offers an amp-hour-counting state-of-charge meter with their Classic charge controllers, and as an option on its smaller KID controllers. Battery state-of-charge (SOC) metering, which shows battery SOC as a percentage, is an important tool that enables users to easily see how full (or empty) their batteries are. But it is often left out of systems because it comes at an extra cost.
Data logging can be another important feature, especially with systems that are not monitored daily. The larger MidNite Solar, Morningstar, OutBack Power, and Schneider Electric charge controllers include data logging, so you can see how many kWh the system produced over a period of time. Having access to this data can be useful for installers when troubleshooting a system.
MidNite Solar, OutBack Power, and Schneider Electric’s charge controllers can be connected to a computer or smartphone (directly for MidNite Solar, and through an extra communications device for OutBack Power and Schneider Electric charge controllers) for monitoring, programming, and accessing historical data.
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