How to select a solar charge controller

In its basic forms, solar PV is a very straightforward proposition. Hook a solar panel up to a DC load and it will run until the sun goes down. Connect solar panels to a grid-tied inverter and, as long as the sun is shining, power will be sent to the utility. It’s all fairly easy — until the sun stops shining.

Where it starts to get more complex is with energy storage, for use when the sun isn’t shining or when the grid is down. Storing electricity to do useful work later requires batteries connected to a solar PV system. Once a battery is added, a charge controller becomes one of the most important system components.

Anyone going off-grid or wanting to use a hybrid system that can sell solar-generated electricity during the day and store that power for use at night, during an outage or during peak times will need a solar charge controller.

What a solar charge controller does

Think of a solar charge controller as a regulator. It delivers power from the PV array to system loads and the battery bank. When the battery bank is nearly full, the controller will taper off the charging current to maintain the required voltage to fully charge the battery and keep it topped off. By being able to regulate the voltage, the solar controller protects the battery. The key word is “protects.” Batteries can be the most expensive part of a system, and a solar charge controller protects them from both overcharging and undercharging.

The second role can be more difficult to understand, but running batteries in a “partial state-of-charge” can shorten their life tremendously. Extended periods with a partial state of charge will cause the plates of a lead-acid battery to become sulfated and greatly reduce life expectancy, and lithium battery chemistries are equally vulnerable to chronic undercharging. In fact, running batteries down to zero can kill them quickly. Therefore, load control for the connected DC electrical loads is very important. The low voltage disconnect (LVD) switching included with a charge controller protects batteries from over-discharging.

Overcharging all types of batteries can cause irreparable damage. Overcharging lead-acid batteries may cause excessive gassing that can actually “boil” the water away, damaging a battery’s plates by exposing them. In a worst-case scenario, overheating and high pressure can cause explosive results upon release.

Typically, smaller charge controllers include a load control circuit. On larger controllers such as the Morningstar TriStar, separate load control switches and relays can also be used for load control of DC loads up to 45 or 60 Amps. Alongside a charge controller, a relay driver is also commonly used to switch relays on and off for load control. The relay driver includes four separate channels to prioritize more critical loads to stay on longer than less critical loads. It’s also useful for automatic generator start control and alarm notifications.

More advanced solar charge controllers can also monitor temperature and adjust battery charging to optimize the charging accordingly. This is referred to as temperature compensation, which charges to a higher voltage in cold temperatures and a lower voltage when it is warm.

Many solar charge controllers include on-site and remote data monitoring. Morningstar offers serial communications options so the controllers can be monitored locally or remotely with compatible communications equipment. In addition, Ethernet communication capability is also available for monitoring locally on a LAN or remotely over the internet.

For these and other reasons, the solar controller can be thought of as the system’s heart and brains. It ensures long-term battery health under all sorts of operating conditions as well as provides critical load control and system monitoring functions.

The two main types of charge controller

While charge controllers come in a vast range of prices, power ratings and features, they all fall into one of two basic categories: pulse-width modulation (PWM) and maximum power point tracking (MPPT).

PWM types are relatively simple, using a switch between the PV array and the battery. The switch is able to open and close rapidly, thus being able to pulse or “throttle back” the electricity coming from a solar panel in order to taper off the charge current as the batteries become full. Since PWM controllers operate with a switch only, the array voltage during operation is equal to the battery voltage. This means that you need to use nominal voltage solar panels with a PWM controller (36-cell panels for 12 V nominal and 72-cell panels for 24 V nominal).

Even with a nominal voltage array, a PWM controller will operate below the maximum power voltage (Vmp). When it’s cold outside or when the battery voltage gets low, a PWM controller will operate well below Vmp and the max power (Pmp) rating of the solar array. To take full advantage of a PV array’s maximum power output, you need an MPPT controller.

MPPT controllers are comparatively more sophisticated. They can adjust (or track) the input voltage and current of the PV array to find the optimum operating voltage that will generate the most power at a given moment. Below are graphs of the current vs. voltage (IV) and current vs. power (IP) for a nominal voltage PV array. By continuously tracking and operating at Vmp, an MPPT controller will be able to generate more power than a PWM controller during bulk charging.

MPPT controllers can also be used with higher voltage PV arrays above nominal voltage. This makes it possible to use different solar PV panels which may cost less or be more optimal in size. For example, 60-cell cost less than 36-cell modules and are a more manageable size for mounting than larger 72-cell modules. Higher voltage arrays also allow for fewer strings in parallel resulting in fewer combiner box fuses, lower array current and less voltage drop, so smaller wires can be used, meaning that MPPT controllers can save money by reducing costly copper wiring, especially for longer array wire runs.

Note that while MPPT technology is more expensive, it’s not necessarily better. For a properly-sized system, MPPT and PWM controllers will both do a fine job of keeping the batteries charged. Choosing PWM or MPPT really depends on one’s application and location.

In places with variable sunshine, fluctuating temperatures and shading, in northern or southern latitudes with snowfall in winter, MPPT is by far more desirable since it can maximize output under challenging conditions. It all gets down to the right tool for the job.If there is not a long wire run and nominal voltage solar modules are being used, a PWM controller often is the best choice. The same is true in locations that may also have a lot of constant, dependable sunshine — in deserts or the tropics. In these locations, PWM controllers are the right tool for the job since some wasted solar electricity isn’t critical. Any advantage in using an MPPT controller may be minimal since the array voltage is lower in warm conditions. Another consideration is the size of the system. PWM controllers are often used in smaller, cost-sensitive systems where the added cost with MPPT is not worth it.

Things to look for in a charge controller

It’s important to choose the right charge controller in terms of size and features. For remote systems, reliability and performance are very important considerations. Lower cost solar controllers are often not going to be the most reliable and may not meet vital charging requirements. Poor performance or reliability can end up costing many times over the cost of the solar controller in terms of replacement of the battery bank, site visits and loss of operating time.

Solar charge controllers must be designed to take a beating, since they deal with a lot of heat and have to manage it properly. Small charge controllers have the advantage of being fanless — they get rid of heat by simple passive cooling. By eliminating the fan they achieve three advantages:

1. Higher reliability — Fans have moving parts, usually the only component with moving parts on a charge controller. Eliminate the fan and you eliminate one of the most common points of failure.

2. Longer life — Fans pull in dirt, dust and even insects, which can clog the charge controller’s insides and shorten its life.

3. Greater efficiency — Fans require electricity to run, and that electricity comes from the solar electricity flowing from the panels. Fans are a “parasitic load” in the system, diverting and consuming power that could be used elsewhere.

Some larger controllers (including all Morningstar controllers) also use passive cooling with no fans, incorporating very advanced thermal mechanical design and software. They are preferred in remote, mission-critical installations where maintenance is infrequent and replacements are difficult.

Smaller charge controllers will often only have preset charge settings. If the battery charging requirements are not sufficiently met with these presets, a controller with more settings options can be selected. Custom settings can be simple adjustments to voltage set points, particular applications or environments. For example, a system which does not cycle much can be set up with reduced daily absorption time, which is the amount of time before it puts the battery into float.

Select Morningstar controllers also have custom settings options for daily lighting on/off control. This type of control automatically adjusts the lighting on/off control independent of the time of the year so lights will come on when it gets dark in the evening and/or in the morning before it gets light.

Whatever your application, location or budget, the most important step in controlling a solar + storage investment is spending time and care selecting the right charge controller. Morningstar has sold over 4 million charge controllers in 100 countries, over the past quarter-century — and we’ve yet to have a single customer tell us they wished they skimped on this critical system component.