Beginners Guide to Solar Charge Controllers | MPPT vs PWM | Off Grid Permaculture
Solar Charge Controller

Beginners Guide to Solar Charge Controllers | MPPT vs PWM

Daniel Mark Schwartz Profile Picture

When I talk to people about designing an off grid solar system, the part people tend to find most confusing is the charge controller. So, today I decided to sit down and describe as simply as possible what a charge controller is, why you need one, and how to choose between all the different options.

How to choose a solar charge controller. Choose a charge controller with a maximum current rating (amps) and voltage rating (V) as big or greater than the current rating of your solar array. PWM controllers are best for small arrays (200 W or less) with array voltages that match the battery bank voltage, and MPPT controllers are best for all other cases. Controllers with temperature sensing or recommended if your batteries will operate in cold conditions.

While the above is probably enough to get you started with choosing an off grid charge controller, there is a lot more to know. Read on where I’ll discuss, in simple terms, what a charge controller does, and what the options are.

A Very Basic Introduction to Off Grid Charge Controllers

A charge controller is an essential part of any off grid, renewable energy system such as solar, wind, or micro-hydro power. They sit between the power generating source (solar panels, etc) and the battery bank.

The most important job of a charge controller is to protect your expensive battery bank from over charging. Every battery has a limit to how much energy they can store. In order to protect your batteries from damage, the charge controller will slow down or stop the batteries from charging when they are full.

All but the most basic charge controllers also come with additional safety features. Many will sense potentially hazardous conditions such as shorts, battery under-voltage, or excessive current draw and disable the power system to prevent damage.

Additionally, MPPT charge controllers have built in DC-DC converters. This allows you to run your solar array, wind generator, or water turbine at a different (usually higher) voltage than the battery bank. They also have internal computer chips that intelligently adjust the load on your solar panels and generators to ensure they are operating at maximum efficiency.

Charge Controller Buyer’s Checklist

When choosing a charge controller for your system, make sure that the following aspects are suitable for what you need.

  • Amps / Power rating is sufficient
  • Maximum array voltage is greater than or equal my array
  • Battery bank voltage matches my battery bank
  • Controller has battery charge profile that matches my battery bank type (most important for lithium or other cell types. Lead acid batteries are widely supported)
  • Wire connection terminals are large enough for my wire
  • Charge controller has temperature compensation, if batteries are in unheated room/compartment

How Big of a Charge Controller Do I Need?

The size of the charge controller puts an upper limit on how much power your off grid energy system can produce. You need to size your charge controller to meet you needs and to accommodate how much power your solar array or generators can produce.

Charge controllers are typically rated in either maximum current (A, amps) or maximum power (W, watts). Most of the time, you will want to get a charge controller that meets or exceeds the capacity of your solar array.

For instance, if you have a 1kW array, then you will need a 1kW or greater charge controller.

Much of the time charge controllers are rated in maximum current, which is amps. If you know the short circuit current of your solar array, then you can use that as the basis for choosing a charge controller.

If you don’t know how much current your array produces, then you can easily figure it out from the array wattage and array voltage by dividing the array power by the array voltage. So, if you have a 1 kW array that runs at 18V (typical for a “12v” panel), then you would need an 83 amp charge controller:

Power/Volts = Amps 1000W/18V = 83A

Calculating Solar Array Amps from Power and Voltage

Is It Better to Get a Larger Charge Controller?

There is no harm in getting a larger charge controller than you absolutely need, except for the possible extra expense, since larger charge controllers are usually less expensive.

Running a charge controller well below its maximum rating may increase the lifespan of the charge controller. Electronic components are more likely to go bad when they run at high temperatures. Using a larger charge controller than necessary to charge your batteries will allow the controller to operate at lower temperatures.

This is especially true for lower cost charge controllers, which are likely to have very less safety margin and poorer thermal design than more respected brands.

What Happen If My Charge Controller Is Not Big Enough?

The consequences of using an undersized controller depends on the design of the controller itself.

Much of the time, and especially when using MPPT charge controllers, the controller will simply limit the amount of current that passes through it. Essentially, this means that you are possibly not getting all of the power that your panels are producing.

If you live in a climate that is frequently cloudy, or very far north, then this might be acceptable, since your panels will not usually produce their maximum possible output. You could choose to save money by under sizing the charge controller, since you will rarely produce that much energy anyway. And, your system will have been sized to account for this, so you may not be able to use the extra electricity anyway.

However some charge controllers will not work or will be destroyed when they are driven with too much current. This is most common with smaller capacity charge controllers, especially PWM charge controllers. Always check with the manufacture’s documentation before connecting an undersized charge controller.

What is the Difference Between a PWM and MPPT Charge Controller?

PWM and MPPT charge controllers are both “smart” electronics that control your solar system, but they employ different strategies to do so.

PWM (pulse width modulation) charge controllers are the cheapest and least capable. They simply use an electronic switch to limit the power flowing through to the battery bank when the batteries are full.

When solar panels are producing more energy than can be safely used, this switch is rapidly opened and closed to create pulses of energy. The width of these pulses in time (eg duration) is controls how much energy gets through, and gives this controller it’s name.

MPPT (maximum power point tracking) charge controllers, on the other hand, use a DC to DC converter to control the energy flow from solar panel to battery bank. They use this converter to adjust voltage and current coming from the solar panels in order to maximize efficiency, since solar panels have a “sweet spot”, called the maximum power point, where they produce the most possible power. This is what makes MPPT charge controllers more efficient that PWM charge controllers.

A nice side effect of the DC to DC converter inside of MPPT controllers is that you can run the solar array at any voltage independently of the battery bank voltage. This gives you more options and flexibility with how you wire your solar array.

Should I Get A MPPT or PWM Charge Controller?

PWM charge controllers are generally cheaper, but MPPT charge controllers are usually more efficient.

If your solar system is larger than about 600 W, you are probably better of with a MPPT charge controller, due to the increased efficiency.

Also, if you have a long run of wire between your battery bank and your solar panels, then you should consider using and MPPT charge controller. An MPPT charge controller allows you to increase your array voltage, by wiring panels in series, without affecting your battery voltage. Higher array voltages reduces the cost of wire and improves the efficiency of your system.

For more information on how to choose your array voltage and wire your solar array, check out my solar power guide:

Can I Connect More Than One Charge Controller to My Battery Bank?

I you have two are more charge controllers, you can use them to charge a single battery bank, so long as they are configured to follow the same charge profile. The easiest way to ensure that this is the case, is to use multiple of the same brand / model of charge controller, and make sure all the settings are set to the same option.

If your charge profiles don’t match, then you will probably not damage the charge controllers or battery bank, so long as they have a profile that is safe for your batteries, but you will end up using one charge controller more than the other.

Other Considerations When Choosing a Charge Controller

Besides the main features, there are a number of smaller considerations to make when choosing a charge controller for your solar system.

Voltage Rating

Be sure that both the max input and max output voltage of your charge controller is equal to or greater than than your array voltage and battery bank voltage. Note that most “12V” panels actually produce up to 21V and typically operate at 18V. You will need ensure that your charge controller can handle the full range, and not just the 12V.

Most can at 12V, but this is more important when you are wiring panels in series to increase the voltage.

Safety Features

Charge controllers are essential to the safety of your off grid system. While all charge controllers have battery over-voltage protection (disconnecting the solar panels when the batteries are fully charged), there is also a wide range of optional safety features.

Larger capacity charge controllers tend to have an optional third set of terminals for the load, which allows the charge controller to protect against:

  • Under voltage – disconnects the load before batteries are damaged by over discharge
  • Over current protection – cuts power in case of a short
  • Low temperature cutoff — cuts power when batteries are too cold to operate safely

Temperature Compensation

If your batteries are in an unheated compartment or partially heated to where they can drop below about 60F, it is best to have a charge controller that support temperature compensation.

Temperature compensated controllers have an included digital temperature sensor which it uses to adjust the charge voltage of the battery bank depending on how cold they are. This can significantly increase the lifespan of batteries subjected to colder conditions.

Also, most temperature sensing charge controllers have a low temperature cut off mode, which disconnects the batteries if they get too cold to operate safely. Lithium batteries are dangerous to operate in freezing cold conditions.

Wind and Hydro Support

Some charge controllers are designed to be used with other forms of renewable energy including small scale wind and micro hydro, which are fitted with DC alternators or have been rectified.

The biggest difference between solar and wind / hydro, is that generator based systems must have a dump load to operate safely.

Unlike solar, completely disconnecting a wind turbine or water power turbine from the battery bank would allow the turbine to spine completely freely. In some conditions this would lead to the damage or destruction of your generator.

Charge controllers that are made for wind/hydro power have a second “dump load” output, where excess energy can be sent when the batteries are full. This can be a backup system, like a pump, or simply a heater that dissipates excess energy. The dump load must be able to accept the full output of your generator, and must be able to be run continuously if necessary.

Monitoring and Logging

One of the areas which charge controllers seek to differentiate themselves from the competition is through their fancy displays and data logging systems. Higher end charge controllers will show you graphs and log energy production day by day for years at a time. Some even offer bluetooth / Wifi interfaces and apps to control them with.

In my opinion, most of the time these extra features are not necessary or helpful. Once you have a functioning solar system, you generally just want to leave it alone. All I want from a charge controller is a display of the current battery capacity and has a way of signaling a problem.

Custom Settings

If you are planning on running a special battery, or trying to cobble together a discount lithium battery system from electric vehicle parts, then you might need the ability set up a custom charging profile to match your battery bank’s unique composition. This is the domain of only a few, fairly high end charge controllers.

Otherwise, if you are using lead acid or pre-manufactured “solar battery” banks, then you probably won’t need or want this level of customization.

How big of a solar charge controller do I need?

Choose a solar charge controller who’s maximum power (or current) is the same or greater than the maximum power or output current of your solar panels. Choosing a controller that is too small will limit the amount of power your panels produce. An controller that is too large will not negatively impact operation, but may be more expensive than necessary.

What does a charge controller do for a solar panel?

Charge controllers protect solar battery banks from over charging and any damage that can result. MPPT charge controllers also increase the efficiency of your solar panels by constantly adjusting the operating point, and provide conversion between array voltage and

Is solar charge controller necessary?

Any solar system with a battery bank will need a solar charge controller in order to operate safely. Additionally, MPPT charge controllers offer smart tracking to increase the efficiency of your solar array, so you may want to have one even if you don’t have a battery bank.

How do you connect a solar panel to a charge controller?

Connect the positive (red +) wire to the positive “solar” terminal, and the negative (black -) to the negative. Connect the positive battery wire (red +) to the positive battery terminal, and the negative (black -) to the negative. If your charge controller has a load connection, + to + and - to - for the fuse box or AC inverter. If your controller has a temperature sensor, connect according to manufacture’s directions and place the sensitive end close to the batteries.