Sunwize Technologies Solar Boost review

RV Power Products makes an exceptional line of Maximum Power Point Tracking (MPPT) charge controllers that provide many potential advantages for medium to large, battery-based PV systems. There are three models of available controllers: the Solar Boost 2000E rated for 12 VDC/25 A, the Solar Boost 50 rated for 12/24 VDC and 50 A, and the Solar Boost 3048, rated for 24/48 VDC and 30 A. The SB 2000E comes standard with a digital display. The SB50 and SB3048 have optional panel mounts and/or remote digital displays. An optional battery temperature sensor is available for all three models.

Why Maximum Power Point Tracking?

Maximum Power Point Tracking (MPPT) extracts the maximum amount of available power from a photovoltaic (PV) array. A battery-based PV system normally only harvests about 65% to 95% of the power available from the PV array. By using an MPPT-type charge controller almost all of this previously unharvested power is made available for charging the battery bank.

Here is the not-so-short story. The advertised rated output of any PV module is determined by multiplying the full voltage potential (Vmp) at standard conditions by the rated current (Imp) at standard conditions. A module rated at 75-watts (at standard conditions) will have a rated voltage of about 17.0 volts and a rated current of about 4.4 amps. Unfortunately, the actual power delivered by a PV module is determined by the actual voltage of the battery bank multiplied by the available current of the PV module. If the battery bank voltage (Vbat) is 12.3 volts (about 50% Depth Of Discharge) the actual power delivered from a 75-watt PV module will be about 54 watts (12.3 volts x 4.4 amps = 54.1 watts). So, what happened to the other 20.9 watts of module power? Some of the power was not available because of elevated module temperatures reducing output voltage. The rest of the power was available but not harvested because the module was operated at battery voltage rather than the maximum power point voltage.

When any PV module gets hot its maximum output voltage decreases; the hotter the module, the more voltage it loses. Module manufactures design their modules to have enough excess voltage potential to insure that there will be enough voltage to fully recharge a battery even at noon during mid-summer in the desert. Typically about 14.5 volts is required to recharge a nominal 12-volt battery, so PV modules are designed to generate up to about 17 volts (this value varies somewhat with specific module design) to insure adequate voltage.

Here is an example of how this all works. At noon on a sunny July day in Phoenix, the maximum output voltage (Vmp) of the 75-watt module will have dropped to about 14.75 volts. During cooler weather, say January in Kansas City, the maximum output voltage (Vmp) would be about 16.75 volts. So, depending upon location and season, there is between 14.75 volts and 16.75 volts of potential voltage from the PV module. In the example of recharging a battery at 50% DOD (12.3 volts) there will be between 2.45 volts and 4.45 volts of unused potential (Vmp minus Vbat). This equals about 10.8 to19.6 watts of available but uncollected power. If the battery is only 10% DOD (13.0 volts) there will be 1.75 to 3.75 volts of used potential; about 7.7 to 16.5 watt of unused power.

A MPPT charge controller operates the PV module at about its maximum available voltage (Vmp)for the temperature conditions. It then “converts” any excess voltage (more than required to recharge the battery) into more available amps of charging current. During cool weather this can translate into 20% to 35% more actual power delivered to your battery bank. During hot weather, the power gains will be much more modest, about 0 to 10%. It is important to note that the most significant increases occur during the cool winter months when most battery-based systems are challenged due to the short solar days. By simply changing to a MPPT controller you can increase the functional winter output of your solar array by 20% to 35%. That is an impressive increase in array output for about the cost of a single 75-watt PV module and you don’t even have to rewire your system.

Why a Solar Boost MPPT Charge Controller?

Why should you consider purchasing a Solar Boost brand of MPPT charge controller? The SB controllers are well-built and highly reliable units that do a good job of extracting the maximum power from a solar array. They have many well thought-out features that can prove to be useful in the design of a battery-based PV system.

The power components in Solar Boost controllers are significantly oversized, thus reducing the stress on the components and greatly increasing reliability. When the controller is at full output power, the FET’s (main power transistors) and other power related components are designed to operate at less than 50% of the maximum design values for the components. In fact, the FET’s in the SB50 are capable of handling up to 74 amps each but RV Power Products chooses to operate them at a maximum of 12.5 amps each!

The SB50 and SB3048 are designed to electronically limit the maximum output current to their rated values; 50 amps for the SB50, 30 amps for the SB3048. These units automatically reduce the amount of power they extract from the solar array to insure current limits are not exceeded. This limiting of the max output power not only protects the controller from overheating and possible damage but should also address the requirement for typical, conventional controllers to be oversized by 130% to 156% to handle ‘edge-of-cloud’ effects. The SB controllers do not require the 130% to 156% derating. RV Power Products recommends limiting the rated output current of the array (Imp) to about 73% of the rated output of the controller. The reason for this recommendation is to allow enough ‘head-room’ so the controller can provide full MPPT power boosting without excessively hitting the max power limiting function.

The SB controllers are thermally protected; they will cycle on and off to maintain safe operating limits during extreme ambient temperatures. They are fully protected against reverse polarity and high voltage transients for both the PV array and the battery. (Note: they are not protected against reverse battery connection to the PV terminals.)

Great Features

RV Power Products designed their MPPT circuit around an interesting characteristic of PV modules. T he voltage differential between the Open Circuit Voltage (Voc) and Maximum Power Point Voltage (Vmp) stays relatively constant over the temperature range PV modules typical experience. As the module gets hotter both the Voc and the Vmp are reduced at the same rate and the voltage differential between them remains relatively constant. This allows the SB controller to track the actual Vmp at any module temperature simply by momentarily testing the Voc of the array and then operating the array at the Voc minus the voltage differential. The factory default setting for a typical 36-cell module is a differential of 4.4 volts, which is field-adjustable for optimal performance with any given PV array. This approach to determining the Vmp of a solar array provides accurate MPPT without the problems that can and have occurred with some other approaches to MPPT. One point to note with the SB controllers: the PV- and the battery- connections cannot be electrically common or the controller will not function properly. For systems that use PVGFP, read RV Power Products technical bulletin #100204 to insure proper integration of the SB controller.

The SB50 and the SB3048 are both dual voltage units; the SB50 can operate at a nominal 12-volts or 24-volts, the SB3048 can operate at 24-volts or 48-volts. Both these units can also be configured to operate in a dual-voltage mode, where the input voltage is at the higher voltage range and the output voltage is at the lower voltage range. This feature permits the solar array to operate at twice the voltage of the battery bank. Thus, a 24-volt solar array can charge a 12-volt battery bank or a 48-volt array can charge a 24-volt battery bank. This feature can be very useful in reducing wire size and line-loss if the solar array is a long distance from the battery bank. Remember the output current of the controller is limited to the same maximum amperage rating at either voltage so if you are using the dual-voltage feature you will need to compensate the max suggested input current to allow for the doubling of the output current when the output voltage is reduced by 50%.

An optional digital display is available that displays Battery Voltage, Solar Panel Current and Output Charge Current. The SB controllers allow for both an optional front panel mounted display and/or remote display. Both displays can be installed on the same controller. These displays provide very accurate readings, unlike displays on some other popular controllers, and are very popular with the end user because they can see actual boost in charging amps moment to moment.

The SB controllers work very well with grid-tied battery-based systems, like the Trace SW inverters. The charge set point for the SB controller is set several volts higher than the sell voltage of the inverter so that Solar Boost is always trying to extract the maximum power from the PV array.

The Solar Boost enclosure provides a good amount of space for wire runs. There are eight 1” conduit knockouts and four ½” knockouts available on the enclosure. The compression terminals for the PV input and Battery output will accept up to 1/0 AWG cable. The Installation and Operation Manual is well written and very complete.

Blue Sky Energy Inc. is known for providing good technical support for odd configurations and the rare, weird problems. They have a collection of good technical bulletins that can be downloaded from their web site, http://www.blueskyenergyinc.com The SB50 and SB3048 have been tested by ETL and are now listed for compliance to UL, CSA and CE requirements.

In conclusion, the SB50 and SB3048 work well and are a good choice for any medium to large sized battery-based PV system. They will provide significantly more power from a PV array, especially during cool weather or when the batteries are discharged. They also provide a good solution for long cable runs from the PV array to the battery bank. I really like my SB3048, which is connected to my grid-tied SW inverter, based system. I have found the unit to work as advertised and real-life performance gains to be consistent with the data presented previously in this article.