The Evolution of the Solar Charge Controller

A charge controller is a piece of equipment designed to prevent the batteries in your system from overcharging.

It is similar to the voltage regulator in a car. However, unlike a voltage regulator in an automobile, charge controllers have advanced in ways never even imaginable to the hippies in Northern California, USA in the 1970s…

A typical 12 volt solar panel is capable of producing up to 17 to 22 volts. If the panel was left connected to a 12 volt battery, with no charge controller, the battery voltage would continue to rise until the battery/batteries become permanently damaged.

A charge controller can maintain healthy battery voltage by either:

  • short circuiting the solar panel
  • disconnecting the solar panel from the battery or by
  • diverting the excess electricity to a load such as an air heater, water heater or other electrical load.

Solar electricity can be either disconnected or diverted.

Wind and water turbines must remain connected to the batteries at all times so their electricity must be diverted or dumped in order to protect the batteries.


Shunt type charge controllers- these are the first to be developed and most crude form of charge controller on the market today. They are only used as solar charge controllers (not for water or wind power). They shunt (or short circuit) the energy from a solar panel when the battery is full. These controllers have a set on and off voltage with a hysteresis (difference) sometimes adjustable by the user and sometimes factory preset. When the batteries hit the high voltage setting the entire electricity source is shunted (short circuited) until the voltage hits the low voltage setting. This keeps the batteries regulated but the voltage can vary between the on and off settings. They are the least accurate type of controller.

Relay type charge controller – these were the second type of charge controller to be developed. They are similar in function to the shunt type except instead of short circuiting the panel output they open circuit between the solar module and the batteries. These controllers have a set on and off voltage with a hysteresis (difference) usually adjustable by the user. When the batteries hit the high voltage setting the entire electricity source is disconnected until the voltage hits the low voltage setting. This keeps the batteries regulated but the voltage can vary between the on and off settings and is not that consistent. These also can only be used as a solar charge controller (not for use with water power or wind power).

PWM (Pulse Width Modulated) charge controller – This style of charge controller keeps the batteries regulated by disconnecting and reconnecting portions (or part) of the electricity available from the solar module several times per second keeping the battery voltage more constant.

With the development of these charge controllers came anew and improved way of charging batteries using a bulk, absorption, float and equalization charge.

These are a great improvement over relay charge controllers as they are able to keep the battery voltage much more stable. They do not increase the output of the solar array but are more efficient than the previous controllers. Aside from being a charge controller, these units can have many features including digital displays, remote displays, load control, lighting control, and dump/diversion control.

MPPT (Maximum Power Point Tracking) charge controller – This style of charge controller uses pulse width modulated technology to keep the batteries regulated but is able to also increase the output of the solar array by finding the maximum power point of the array (which is a higher voltage than the battery bank) and reducing the voltage to charge the batteries. This can result in up to a 30% increase in output of the solar array. It also allows for a higher voltage transmission from the solar array to the controller keeping wire losses to a minimum.

Outback Power’ s FM60 MPPT charge controller is our favorite as it is useful over a wide range of voltages and super efficient. Aside from being an MPPT charge controller, these units can have many features including lighting control, diversion control and an extra programmable relay. Some of these charge controllers can accept voltages up to 150 volts DC to make long distance transmission possible and efficient. There are MPPT controllers being designed now that are capable of input voltages up to 200 volts DC.

MPPT (maximum power point tracking) technology is constantly being improved and is very exciting for our industry. Learn more about MPPT technology.

Today’s charge controllers do more than just maintain correct battery voltage…


Digital Displays – Some charge controllers have an optional or included (built in) digital display on the front of the unit.

This display shows the user how many volts, amps and/or watts the system is generating as well as keeps track of total power production and other pertinent info as available.

To the right is an photo of an installed front mount display for a Morningstar Prostar PS-15 PWM charge controller.

Remote Displays – Some charge controllers include, or have as an option, a remote meter that can be installed in another room or building. They usually display the same info as the installed digital displays but can be more conveniently located for easy access.

Load Controls – To prevent a battery from becoming deeply discharged by a load (such as a light or motor), a load controller is used. The load controller monitors battery voltage and disconnects the load from the battery at the disconnect voltage and does not reconnect the load until the battery reaches the reconnect voltage. These on and off settings are usually user adjustable. Common settings would be to disconnect at 10.5 volts and reconnect at 12.5 volts. Many charge controllers have a load control feature but can only be used as a charge controller or a load controller, not both. Two controllers would be required to perform both functions. Two exceptions to this are Morningstar’s SunSaver SS-6 and SunSaver Duo dual function charge and load controllers.

Lighting Controls – A lighting controller is a charge controller used to control lighting. They have several lighting functions such as dusk to dawn lighting or other on and off settings. They are used for security lighting, home lighting and billboard lighting. They are microprocessor controlled, fully automatic and also act as a load controller to protect the battery from deep discharge. Morningstar’s SunLight charge controller is an example of a good quality lighting controller.

Diversion (Dump) Controls – When charging a battery with a solar panel the energy source (solar panel) can simply be disconnected to regulate the voltage. When charging the battery with a water turbine or wind turbine the energy source cannot be simply disconnected. This would over-speed the water turbine or wind turbine and destroy it. Instead…the turbine is connected directly to the battery and the excess power is now dumped (or diverted) from the battery to a water or air heater using a diversion controller. This type of controller monitors the battery voltage and diverts all or a portion of the produced energy as heat to the dump load. The dump load can either be a useful load (like a hot water heater) or not useful and be dumped into an outside air heater. Many charge controllers have a diversion control feature but can only be used as a charge controller or a diversion controller, not both. Two controllers would be required to perform both functions. The Morningstar TS-45 is an example of a good quality diversion controller.

Extra Programmable Relays – Outback Power System’s FM60, OutBack Power System’s FM80, Apollo Solar’s T80 and Xantrex’s MPPT60-150 charge controllers have a user programmable relay that can be adjusted to perform many functions. The relay can be used manually as a switch for pretty much anything you want to control remotely or the relay can be used, a lighting controller, a load controller (with an optional solid state relay). Another option is to use it to control an exhaust fan for your battery enclosure or an alarm to tell the user the batteries have not been charged for a set amount of time.

Remote Temperature Sensors – An RTS or remote battery temperature sensor is used for temperature compensated battery charging. As a battery gets warmer gassing increases. As a battery gets colder it becomes more resistant to charging (requires a higher voltage to charge). The more the temperature changes, the more important a battery temperature sensor is. The image on the right is an Xantrex RTS used for the C40, and C60 charge controllers.


Bulk Charge – The bulk charge is the beginning of the three step process of most advanced charge controllers on the market today. During the bulk stage all current available from the solar panel is applied to the battery until the battery reaches the preset absorb voltage as specified by the charge controller or user. At this point the charge controller enters absorbing mode. For a 12 volt nominal lead acid battery this voltage might be anywhere from 14.0 – 15.0 volts.

Absorption Charge – During the absorb stage just enough current is applied to the battery to hold a preset absorb voltage for a set period of time. This stage is designed to prevent overheating and over-gassing of the battery. The current is tapered down to maintain battery voltage. The voltage for this mode in a 12 volt system is 14.0 – 15.0 volts. If the voltage is able to be maintained for the preset period of time the charge controller will enter float mode.

Float Charge – During float mode, a maintenance charge is applied to the batteries until there is no more excess energy available (the end of a sunny day) The voltage for this mode in a 12 volt nominal lead acid battery will be approximately 13.4 volts.

Equalization Charge – An equalization charge is a periodic boost charge applied to stir the electrolyte, level the cell voltages and complete the chemical reactions within the battery. It is usually done at a set period of time from every month to every three months depending on battery manufacturer recommendations. The higher quality charge controllers will perform this stage automatically. The voltage for this mode in a 12 volt nominal lead acid battery will be above 15.0 volts and up to 16.0 volts. For more information on battery equalization read our equalization page.


Most systems require a charge controller. However, very tiny systems do not.

As a general rule you need a charge controller if your solar modules are making more than 2 WATTS PER 50 AMP HOURS (AHs) of battery/batteries (at the same voltage).


For example: if you have a 12 volt, 120 amp hour battery (a typical large RV battery), any module 5 watts or less will not require a charge controller.


(120 amp hours divided by 50 amp hours) X (2 watts)=4.8 watts.

If your solar module is larger than 5 watts …you need a charge controller.


First you need to decide what type of controller is suitable for your application.

Things to consider include:

  • the distance from your solar array to the batteries. The more distance, the higher you might want the voltage to keep wire losses to a minimum. In this case consider an MPPT charge controller.
  • the open circuit voltage of your panels
  • is the voltage of your solar panel the same as the battery voltage? If not you are going to need an MPPT charge controller.
  • do you need to get every watt possible out of your array? If yes then you are going to need an MPPT charge controller.
  • is cost the most important factor to you? If yes, you might consider a PWM or relay type charge controller.
  • is this just a very small one panel system? If so the Morningstar SunKeeper SK-12 might be a good solution.
  • is your single panel system 40 watts or less? If so the Morningstar SunGuard SG-4 would be a good solution.

Battery Voltage – Charge controllers come in various sizes (current and voltage). To choose the correct controller you need to know your system’s battery voltage (12, 24, 36, 48 or 60) Most controllers are designed for either 12, 24, and 48 volt systems or a combination of two or three of these voltages. Some are designed for only one voltage. Morningstar’s Tristar TS-45 and Tristar TS-60 are user programmable using a serial cable, to any battery voltage between 12 and 48, making them suitable for even 32 and 36 volts systems.

Amperage – All controllers have a maximum current limit. They can be as small as 4.5 amps and as large as 100 amps. Please note this is the amperage between the charge controller and the battery and not the input amperage of the solar panel(s). Generally you should size your charge controller at least 25% larger than what is required. This allows the charge controller to operate cool and can also increase the lifetime of the unit.

Options – Many charge controllers contain different options as listed above. Be sure to check to check the specification sheet of your controller before making a purchase. It should give you a good idea of the options available for the specific charge controller and whether on not it will work for you.