A solar charge controller is a component of a solar panel system that controls the charging of a battery bank. Armed with that knowledge, you’ll be one step closer to building an off-grid solar system!

Modern solar charge controller is charging the battery system precisely and efficiently, plus functions like DC load terminals for lighting. Generally, most smaller 12V-24V charge controllers up to 40A in size. They are widely for caravans, RVs and camping, and have DC load output terminals built-in. While most larger, more advanced 60A+ MPPT solar charge controllers do not have load output terminals. They are specifically for larger off-grid system with solar arrays and powerful off-grid solar inverter.

Solar charge controllers value according to the maximum input voltage (V) and maximum charge current (A).

  • Current Amp (A) rating = Maximum charging current.
  • Voltage (V) rating = Maximum voltage (Voc) of the solar panels.

MPPT and PWM solar charge controller

There are two main types of solar charge controllers, PWM and MPPT.

PWM Solar Charge Controllers

PWM solar charge controller

PWM solar charge controllers have a direct connection from the solar array to the battery. And it can use a basic ‘rapid switch’ to modulate or control the battery charging. The switch (transistor) is open until the battery reaches the absorption charge voltage. Then the switch starts to open and close rapidly. In order to reduce the current and maintain a constant battery voltage. This in turn pulls the panel voltage away from its optimum operating voltage (Vmp) and reduces power and efficiency.

Here’s an image to illustrate how the pulses work:

PWM solar charge controller work

For example, if the charge controller accepts 18 volts from the solar panel, it might adjust the pulses so they’re on 82% of the time, and off 18% of the time. This would reduce the average voltage by 18%, down to about 14.8 volts, which can use to charge a half-full AGM lead acid battery. As the battery gets close to a full charge, a PWM charge controller shortens the pulses even further, down to around 77% of the time, or 13.8 volts, to prevent the battery from overcharging.

Unfortunately, the excess energy produced by solar panels is wasting to reduce the output voltage. In our example, the charge controller would average around 80% efficiency. This means it’s very important to make sure the output voltage of the solar panels. It is not too much higher than the voltage of your gel battery bank. When connecting a PWM charge controller to minimize wasted energy. If your solar array outputs a much higher voltage, the PWM charge controller will cut that voltage down to what the battery can accept, and waste the rest.

MPPT Solar Charge Controllers

MPPT Solar Charge Controllers

MPPT or ‘maximum power point tracking’ controllers are far more advanced than PWM controllers and enable the solar panel to operate at its maximum power point, or to be more precise, the optimum voltage for maximum power output. Using this clever technology, MPPT solar charge controllers can be up to 30% more efficient, depending on the battery voltage and voltage of the solar panel connected.

The major advantages of MPPT charge controllers are greater efficiency and compatibility with higher voltage solar arrays. This means that you can charge a 12V battery bank with a larger solar array wired in series, as long as you stay within the limits of the controller’s amperage rating. You can calculate this limit by taking the total wattage of the solar array and dividing it by the voltage of the battery bank to get the maximum possible output in amps.

Let’s use the same example numbers as before. The solar panel is putting out 100 watts, or about 5.5 amps into 18 volts. The MPPT charge controller converts the output to 14.8 volts, but loses about 5% power in the conversion process. So the MPPT controller’s output current is about 6.4 amps, times the 14.8 volts, or 95 watts.

Do I always need a charge controller?

Not always, but usually. Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5-watt panels. A rough rule is that if the panel puts out about 2 watts or less for each 50 battery amp-hours, then you don’t need one.

For example, a standard flooded golf car battery is around 210 amp-hours. So to keep up a series pair of them (12 volts) just for maintenance or storage, you would want a panel that is around 4.2 watts. The popular 5-watt panels are close enough, and will not need a controller. If you are maintaining AGM deep cycle batteries, such as the Concorde Sun Xtender then you can use a smaller 2 to 2-watt panel.

Why 12 volt solar panels are 17 volt output?

The obvious question then comes up – “why aren’t solar panels just made to put out 12 volts”. The reason is that if you do that, the solar panels will provide power only when cool, under perfect conditions, and full sun. This is not something you can count on in most places. The PV panels need to provide some extra voltage, so that when the sun is low in the sky, or you have heavy haze, cloud cover, or high temperatures. A fully charged “12-volt” battery is around 12.7 volts at rest (around 13.6 to 14.4 under charge), so the solar module has to put out at least that much under worst-case conditions.

Which is better PWM or MPPT?

In the example above, a common 60 cell (24V) solar panel with an operating voltage of 32V (Vmp) is connected to a 12V battery bank using both a PWM and a MPPT charge controller. Using the PWM controller, the panel voltage must drop to match the battery voltage and so the power output is reduced dramatically. With an MPPT charge controller, the panel can operate at its maximum power point and in turn can generate much more power.

Battery Voltage VS Solar Charge Controller Voltage

Unlike battery inverters, most solar charge controllers can be used with a range of different battery voltages. For example, 10A to 60A charge controllers can be used to charge either a 12V or 24V battery. But most larger capacity or higher input voltage charge controllers will be used on 24V or 48V battery systems.

The maximum solar array size is generally limited by the battery voltage. Higher 24V battery enables more solar power to be connected to a solar charge controller with 20A charging.

Based on Ohm’s law and the power equation, higher battery voltages enable more solar panels connecting. This is due to the simple formula – Power = Voltage x Current (P=V*I). For example 20A x 12.5V = 250W, while 20A x 25V = 500W.

  • 20A MPPT with a 12V battery = 260W max Solar recommended
  • MPPT 20A with a 24V battery = 520W max Solar recommended
  • 20A MPPT with a 48V battery = 1040W max Solar recommended

Choosing the right solar controller/regulator

The PWM is a good low-cost option:

  • for smaller systems.
  • where the efficiency of the system is not critical, e.g trickle charging.
  • solar panels with a maximum power voltage (Vmp) of up to 18V for charging a 12V battery (36V for 24V battery, etc)  

The MPPT controller is best:

  • For larger systems where the additional 20%* or more energy harvesting is worthwhile
  • When the solar array voltage is substantially higher than the battery voltage e.g. using house panels, for charging 12V batteries

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