Solar Charge Controllers

What is a solar charge controller?

A solar charge controller is a solar-powered voltage and current regulator. They are used in off-grid and hybrid off-grid applications to regulate power input from PV arrays to deliver optimal power output to run electrical loads and charge batteries. Solar charge converters are also commonly called solar charge regulators.

Solar charge controllers maintain batteries at their highest state of charge without overcharging them to avoid gassing and battery damage. Learn more by watching our on-demand webinar below, and by reading the additional information on this page.

Reliable charging to maintain battery health and extend battery life is affected by how well, and how fast, the controller’s software controls the electronic component hardware. Battery and PV voltage and current can change in split seconds and the power electronics in the controller must be able to respond fast enough to accommodate these changes.

Most people don’t see or recognize these dramatic voltage and current changes going on within their controllers, but if they did have the scopes and sophisticated monitors that could show what is actually happening, they could tell which controllers are going to have the best effect on battery life. 

You can have the best batteries and best modules in the world, but they’re only as reliable as your controller.  A poor controller can cause battery failure, and complete failure with your entire solar system. The best solar charge controllers extend battery life for many years beyond their normal life expectancy, whereas inferior controllers cause premature battery failure and can render your PV system inoperable.

The charge controller generally represents about 10% of your total off-grid power system costs, whereas batteries can be about 40% of your first-time cost and 80% of your lifetime costs. So, the small amount of money you might save deploying a cheaper charge controller will pale in comparison to the money and time wasted on battery replacement. You can use the best PV modules, batteries, wiring, and loads, but their capabilities will be restricted by the quality of the charge controller.

How does a solar charge controller work?

The first solar charge controller schematic below (Figure 1) illustrates how a solar charge controller is connected to power a direct current (DC) load, and the second one (Figure 2) pertains to an alternating current (AC) load.

Figure1: Off-grid Diagram with DC Load

When installing a solar charge controller, it is recommended that you connect and disconnect in the following order:

  1. Battery to the controller first
  2. PV array to the controller
  3. Electrical load to the controller

When disconnecting, you reverse that order. The battery provides power to the controller so always make sure that solar and loads are disconnected before connecting or disconnecting the battery from the controller. Connections between battery, load, PV array, and the controller should have disconnect switches to enhance safety and facilitate ease of installation and breakdown.

In the wire diagram schematic above with DC load, sunlight contacts the solar modules, which convert solar into DC electrical power that it delivers to a charge controller. The charge controller regulates the amperage and voltage that is delivered to the loads and any excess power is delivered to the battery system so the batteries maintain their state of charge without getting overcharged. During the evening when there is no sunlight, battery power is used to run the load.

You’ll notice that the battery is grounded at the negative battery terminal. This is because all our PWM and MPPT controllers have a common negative ground. Therefore, it is possible to establish a common negative ground for the entire system: the solar array, controller, battery and the load. This meets NEC code requirements for grounding. If you need an equipment ground for any metal parts on a controller enclosure, some of our controllers include an equipment ground terminal lug. Otherwise, for our controllers that don’t have this terminal lug, you can connect an equipment ground directly to the controller enclosure.

The next diagram (Figure 2) depicts the components and connections to power an AC load. This diagram with an AC load looks similar to the previous example with a DC load, except that in this example, we have added an inverter to the system. The purpose of the inverter is to convert the DC power from the battery to AC power that can be used to run an AC load like the TV you see in the schematic.

Figure 2: Off-grid Diagram with AC Load
It’s important to note that the inverter is connected and powered from the battery, not the controller’s load terminals, like we did in the DC load example. That’s because the inverter can have a high energy surge upon start up, and this high current surge might be higher than the rated capacity of the charge controller, whereas the batteries will be able to meet the high energy surge requirement.

What are the solar controller’s battery charging stages?

Solar charge controllers put batteries through 4 charging stages:

  1. Bulk
  2. Absorption
  3. Float
  4. Equalize
Figure 3: Multi-Stage Battery Charging
1. Bulk Charging - charges the battery to 80-90% SOC 2. Absorption Charging - charges teh battery to approximately 98% SOC After Absorption: 3. Float charging counteracts daytime loads to prevent discharge. 4. Periodic Equalization charging for flooded open vent batteries to maintain specific gravity consistency for all the cells.

For lead acid batteries, the initial bulk charging stage delivers maximum allowable current into the battery to bring it up to a state of charge of approximately 80 to 90%. During bulk charging, the battery’s voltage increases to about 14.5 volts for a nominal 12-volt battery.

Then when the battery reaches the Absorption voltage set-point, constant-voltage regulation is applied but the current is reduced as the batteries approach a full state of charge. This prevents heating and excessive battery gassing.

When absorption charging is complete, the battery has about 98% state of charge. Then, the charging current is reduced further so the battery voltage drops down to the Float voltage.

The Float stage keeps the battery at maximum capacity throughout the day. For flooded open vent batteries, an equalization charge is applied once every 2 to 4 weeks to maintain consistent specific gravities among individual battery cells. The more deeply a battery is discharged on a daily basis, the more often equalization charging is required. Equalization is for flooded, not for sealed, GEL or valve-regulated batteries which can be damaged by equalization.

Although lead acid batteries are the most common type of battery regulated by solar charge controllers, lithium batteries are starting to gain traction. Morningstar launched an Energy Storage Partner program that involves work with many lithium iron phosphate battery manufacturers to maintain the highest state of charge for their batteries and to help maximize battery life.

The integration guides you can download provide custom solar charge controller voltage and time settings for absorption and float charging, and other information that you will need to charge your batteries safely and to increase their longevity. In addition to lead acid and lithium, Morningstar solar charge controllers can also charge nickel, aqueous hybrid ion, and flow or redox flow batteries.

What are the different types of solar charge controllers?

The two major types of solar charge controllers are:

  1. Pulse Width Modulation (PWM) controllers
  2. Maximum Power Point Tracking (MPPT) controllers

As shown in the chart below, PWM controllers tend to be smaller and they operate at battery voltage, whereas MPPT controllers use a newer technology to operate at the maximum power voltage. This maximizes the amount of power being produced which becomes more significant in colder conditions when the array voltage gets increasingly higher than the battery voltage. MPPT controllers can also operate with much higher voltages and lower array currents which can mean fewer strings in parallel and smaller wire sizes since there is less voltage drop.

PWM controllers need to be used with arrays that are matched with the battery voltage which limits what modules can be used. There are many 60 cell modules with maximum power voltage (Vmp) equal to about 30V, which can be used with MPPT controllers, but are simply not suitable with PWM controllers.

To answer the question: Which is better, PWM or MPPT? All things being equal, MPPT is a newer technology that harvests more energy. However, the advantages of MPPT over PWM controllers come at a cost, so sometimes a less expensive PWM controller can be the right choice, especially with smaller systems and in warm climates where the MPPT boost is not as significant.

PWM vs. MPPT Solar Charge Controller Comparison

PWM Controllers MPPT Controllers
Array voltage is "pulled down" to battery voltage Convert excess input voltage into amperage
Generally operate below Vmp Operate at Vmp
Suitable for small module configurations Suitable for large module configurations that have a lower cost per watt
Often chosen for very hot climates which will not yield as much MPPT boost Provide more boost than PWM especially during cold days and/or when the battery voltage is low

Every Morningstar PWM and MPPT solar charge controller is listed in the Morningstar Product Series page. Each listed product is hypertext linked to its product page that includes data sheets, operation manuals and other helpful information.

What applications use solar charge controllers

Systems below requiring limited power that have no grid connectivity because they are remote or in developing regions often deploy solar charge controllers with batteries to supply power:

Other applications such as traffic/railroad signaling, agricultural, mining, and environmental monitoring use charge controllers if they are not connected to the electric grid.

What are the most important solar charge controller features?

Depending on your system you may want to consider some or all the following features:

  • Low voltage disconnect (LVD) – sometimes referred to as “load control” is a feature that instructs the solar charge controller to disconnect the battery’s energy flow to electrical loads when the battery voltage decreases to a low voltage setpoint. This protects the battery from over-discharging and compromising the battery life. 
  • Load reconnect voltage (LRV) – the battery voltage setpoint where the controller reconnects loads to receive electricity from batteries. For 12-volt batteries, the LRV is usually at least 1 volt higher than the LVD.
  • Lighting control to turn lights on or off at a specific period(s) during the night and day.
  • Meters to display battery voltage, charging stages, system amperage and temperature. Advanced meters are interactive as opposed to “read only”, allowing you to program and adjust settings associated with charging stage setpoints, low voltage disconnect, and lighting controls. 
  • Self-diagnostics to indicate/alert installation or operational errors and faults such as short circuit, high temperature, and low battery
  • Communications protocols such as MODBUS, HTTP, SMTP and SNMP are used by some controllers to communicate with a PC and other hardware. See the Morningstar Connectivity Manual regarding networking and communications
  • Data monitoring of present or logged system metrics over several months. Some controllers can be configured to facilitate SNMP monitoring
  • Low noise to meet US Federal Communications Commission Class B specifications.

What protections should a solar charge controller have?

Solar charge controllers may include the following environmental and electronic protections:

  • Epoxy encapsulation and conformal coating. This is a best practice for electrical device manufacturing.
  • Wide operating temperature ranges to support your system’s environment during every season of the year.
  • Humidity, 100% non-condensing
  • Marine rated terminals: infers proven suitability in boating and marine applications
  • Automatic recovery without fuses
  • Solar Input: overload, short circuit, high voltage warning, reverse polarity, high temperature, nighttime reverse current
  • Load Output: overload, short-circuit, high temperature, reverse polarity
  • Battery: reverse polarity
  • Low temperature “foldback” (discontinues charging) in cold conditions to protect LI-ion and other batteries

What certifications do solar charge controllers have?

Solar charge controllers may include the following certifications:

  • CE
  • UL1741
  • IEC62109
  • FCC title 47 (CFR), Part 15 Subpart B for Class B Device
  • Hazardous locations – Class I, Div. 2 Groups A-D
  • Manufacture in ISO 9001 facility

What is solar charge controller efficiency?

Efficiency is a ratio of controller output power to input power from the PV array.  Lower efficiency causes greater power losses by the controller in the form of heat.
Efficiency (%) Output Power (Watts)

The Morningstar solar charge controller represented by the orange line on top, provides significantly higher efficiency at both low and high output power levels, whereas the other controller’s efficiency represented by the blue line, drops off dramatically at output power levels away from its peak efficiency.  You can see how the Morningstar controller reaches near peak efficiency very quickly and remains at high efficiency throughout virtually all power levels on the graph.

Other MPPT brands have been shown to have more than 2 times the energy losses at higher output power levels. When a manufacturer quotes you a conversion efficiency for a controller, you need to know the output power at which that efficiency was calculated.  And how much that efficiency will vary at higher output power levels.

So really, you have to appreciate what might otherwise be perceived as a small difference.  “99” and “98” might look pretty similar but when you’re talking about efficiency, it’s a big difference.  Twice as much energy is being lost in the form of heat in the 98% scenario.

What is different about Morningstar solar charge controllers?

Morningstar solar charge controllers are different from other brands, as you can see in this five minute video. The high frequency design allows Morningstar solar charge controllers to react more quickly to rapidly changing conditions. This provides higher efficiency over a wide variety of Power Ranges. Moreover, Morningstar’s TRAKStar technology deployed in its MPPT controllers provides industry-leading sweeping speed of the entire IV curve and leads to a higher energy harvest compared to our competitors.

Regarding our high temperature architecture, Morningstar uses a higher grade of copper pour in our printed circuit boards, higher temperature rated terminals, and 105 degree C-rated capacitors for Reliable off-grid operation in extreme temperatures. This all adds up to very long life and high reliability of the electronic components.

Morningstar controller cases are metal or very durable Lexan polycarbonate which is much more durable and protective than thermoplastics that are used in other controller brands.

If you are using lithium batteries, sometimes temperatures delve into the ranges that are outside the charging window for the battery. For Morningstar’s newer controllers, the battery charging algorithm can be set to account for low temperatures and reduce charging current during those periods in order to maintain long battery life.

Many controller brands have a 2-year warranty, but Morningstar’s Professional Series controllers have a 5 year warranty.

And all Morningstar controllers are passively cooled without fans which create noise and are known to draw in dust and debris across the circuit board. Furthermore, fans can fail, thus causing the controller to retain heat which diminishes the products quality, performance and longevity.

Below is a snapshot of some of the components within the case of a Morningstar ProStar MPPT solar charge controller to give you a better idea about what sets it apart.

Figure 4: ProStar MPPT controller components

This Morningstar controller’s meter provides a clear crisp back light for high visibility even in direct sunlight.

The specific choice of Coilcraft inductors are made for compact design, high-speed operation and surface mount Manufacturing to maximize design longevity.

A three-phase design in our MPPT buck converter allows us to cancel Much of the DC Ripple which is both hard on Power Electronics as well as the battery.

Fast-acting transient voltage suppressors provide a 4500W surge protection to prevent the number one cause of failure: induced surge damage from nearby lightning.   These devices do not degrade and are designed to last as long as the life of the product.

The Wire terminals are high torque and corrosion resistant, and large to help facilitate easy and reliable wire connections. 

The direct ​field-effect transistors (FET’s) package for much of Morningstar’s switching technology is used because of the benefits of cooling, compact design and reliable surface mount technology. By applying the FET’s on the backside of our board in direct contact with the heatsink thermal pad, they stay extremely cool.

The extruded heat sink design contains more pure aluminum than cast designs and ensures very high conductivity.

The data port on this controller supports MODBUS protocol and data logging of battery voltages, absorption charging times, and power inputs and outputs from the controller.

Morningstar utilizes ARM processors for their high speed control ability and extensive IO and memory features.

Self-diagnostics are provided via LED lights that display or flash different colors to alert you if you have made an installation error, or if your controller is experiencing a short circuit, or high temperatures, or high voltage inputs.

Will oversized PV arrays damage solar charge controllers?

Morningstar’s MPPT solar charge controllers support oversized PV arrays. For example, a 240 watt module won’t damage a SunSaver MPPT solar charge controller and won’t cause it to exceed its rated 15 amps of output current as long as you don’t exceed Voc limits and you adhere to the other operating manual guidelines. 

The graph below displays operating power versus time of day for the SunSaver MPPT controller that has a maximum operating power rating of 200W. 

Here we are comparing performance of a 200 watt module represented by a black curved line, to that of a 240 watt module represented by a blue line.  This is an operation on a clear, sunny day at Standard test conditions and maximum power (Pmp).   When using the 240 watt module, even though the power being delivered to the battery is limited to 200 watts, and the red area at the top of the production curve represents lost power, the larger 240 watt module is still harvesting more energy than the 200 watt module, as shown in green.

The larger module will provide better production with no power-shaving early and late in the day as compared to a smaller module.  In this case almost twice as much energy is gained (green area) than lost (red area) and you get 12.5% more energy available to charge your  batteries than you would with the 200W module.

But what happens on days when it isn’t clear and sunny?  The next graph below used real array data to depict what happens on days with intermittent sun and clouds for the 200 watt and 240 watt module.

On these days there’s going to be little or no power-shaving and the extra power will serve the battery well with more energy harvest.  On this day, for the 240 watt module, there is < 1% loss due to power-shaving (energy above the red line), so almost all of the excess power over the 200W module, represented by the blue over black on the graph, can be utilized for charging.  So, systems that experience a lot of intermittent sun and shading during the day are very good candidates for oversized arrays.

Other Solar Charge Controller FAQs:

How do I choose a solar controller?

In short, the solar charge controller you choose must be able to support the power requirements of your loads, your battery voltage, and your PV current and voltage inputs. Additionally, you will need to consider whether you need a controller with low voltage disconnect, lighting control, or other features or certifications. Your climate, choice of modules, and price will also determine whether you choose a PWM or an MPPT controller.  A good solar distributor will be able to help you size and configure your modules, batteries and controllers for an optimum system.

Can solar charge controllers monitor data?

Yes, some controllers are equipped to monitor data including battery voltages, system current, absorption and float charging metrics, and faults and alerts.

What temperature ranges do solar charge controllers support?

Many Morningstar solar charge controllers support operating temperature ranges between -40C and +60C.

Are MPPT controllers worth it?

MPPT controllers provide more power, especially in colder temperatures. They can also be used with less expensive 60 cell modules which are usually unsuitable for PWM controllers. Weighing these benefits versus the lower cost of a PWM controller will determine if an MPPT controller is right for your system.

Can two solar charge controllers charge the same battery?

What solar charge controller accessories should I consider?

Accessories to consider are meters, adapters, remote temperature sensors, relay drivers, and other balance of system components.

How can I purchase a solar charge controller?

You can describe what type of charge controller and system you are interested in deploying on our How To Buy page which includes links to a product catalog

How long do solar charge controllers last?

The longevity of a charge controller depends on the brand and the system environment. The Morningstar Professional Series controllers have a 5-year warranty and many have been in operation for 10 years or more. 

What is lighting control?

The ability to program settings on controllers to power on and off lights during the day and evening.  Some controllers have simple controls to turn lights on as dusk and off at dawn. Other controllers have more sophisticated programability to allow for multi-event on/off light switching during a 24-hour period.

What is diversion control?

As batteries become fully charged a controller will direct excess current from the battery to a dedicated load that is large enough to absorb the excess energy, but not too large to cause a controller overload condition. 

What is a charge controller's rating?

A charge controller is rated by the current it can accept from a PV array to charge batteries and they battery voltages it will support (e.g. 60 amps for 12, 24, and 48 volt batteries).

What is load current rating?

The rated current that the controller will deliver to loads.

Are charge controllers rated for marine environments?

Some controllers, but not all, are built to withstand the harsh environments associated with boats and marinas.

What controllers are best for RV’s caravans, mobile homes, campers and boats?

What controllers are best for telecommunications?

What controllers are best for security/surveillance monitoring?

What controllers are best for lighting?

What controllers are best for rural electrification?

What controllers are best for residential?

What battery system voltages do solar charge controllers support 12V, 24V, and 48V systems

It is important to check the operation manual or datasheet to see what battery voltages your controller supports. Some controllers support voltages from 12, 24, 48 volts and higher, while others might only support 12 volts. 

What is self-consumption?

The small amount of power that the charge controller uses up for its own operational purposes rather than delivering to loads and batteries. 

Why do you need a solar charge controller?

To regulate power from the PV array to prevent batteries from being overcharged or undercharged, and to prevent the battery from reverse-discharging to the array at night or when there is no power from the sun.

What are other names for a solar charge controller?

Other names for a solar charge controller include: solar panel controller, solar panel charge controller, solar power charge controller, solar controller, solar charge regulator, solar regulator, and charge regulator.

What is a shunt charge controller?

A type of charge controller that short circuits the array to reduce current sent to a battery.  PWM and MPPT controller technology is newer and more prevalent than shunt technology. 

What is a controller's Maximum PV Open Circuit Voltage (Voc) limit?

The maximum voltage, listed in the operator manual, that a controller can support. To determine whether your system will operate under this Voc limit you must know the lowest temperature your system will be exposed to, and calculate the voltage produced by your array at that temperature. Exceeding Voc limits will damage your controller.