Solar Charge Controllers

What is a Solar Charge Controller?

The Solar Charge Controller is an essential part of an off-grid solar power system. The main purpose of a charge controller is to ensure that the battery doesn’t overcharge during the day, and to prevent energy in the battery from trickling back to the solar array (solar panels) at night.

There are two types of solar charge controllers widely used today that I’ll cover in this post, but for information purposes, I’ll briefly cover a third kind of charge controller called a Shunt Controller. A shunt controller is merely an ON/OFF switch. When the battery is low the switch turns ON to charge, when the battery is charged the switch turns OFF. Shunt controllers are the least efficient method of solar battery charging and are not commonly used in modern day solar power systems.

Two types of solar charge controllers used in off-grid solar systems

PWM – Pulse Width Modulation
MPPT – Maximum Power Point Tracking

PWM & MPPT charge controllers both regulate charging rates to allow charging closer to the battery’s maximum capacity, they also monitor battery temperature to prevent overheating.

PWM vs. MPPT Charge Controllers. What’s the Difference?

PWM Charge Controllers work by dropping the voltage coming from the panel. The voltage generated by a 12V solar panel is roughly between 16 and 20 volts, which is too much for a 12V battery. Charging with too many volts can reduce lifespan and even damage the battery. PWM controllers, as stated in the name, use Pulse Width Modulation to charge the battery, which means they send a series of short charging pulses to the battery opposed to a steady output of energy. For a nearly fully charged battery, the pulses are short and sporadic, for a discharged battery the pulses are long and frequent.

PWM Charge Controllers are better suited for smaller off-grid systems, often used in 12V RV solar power systems. They make for a cost-effective RV solar battery maintainer as well.

  • Operate best in warmer sunny weather
  • Performs best when the battery is near the full state of charge
  • Operates efficiently with a solar array of 150W or less
  • PWM controllers are unable to capture excess voltage because the pulse width modulation technology charges at the same voltage as the battery
  • PWM controllers are less expensive than MPPT charge controllers
  • It’s important to note that PWM controllers can create interference to radio and TV signals due to the electrical pulses they generate

MPPT Charge Controllers work by enabling the input power of the controller to be equal to its output power. Like the PWM controller, if the output voltage of the solar array is higher than the battery bank voltage (i.e. 12 volts) the MPPT charge controller brings it down to 12 volts, but the MPPT controller compensates the drop in volts by increasing the current, so the power remains the same. Because there’s no loss in solar-generated power, the MPPT controller provides the flexibility of connecting many solar panels in series which increases the voltage of the array without losing any of the solar-generated power.

MPPT controllers maximize power from the solar panel by converting generated voltage into optimal voltage for maximum charging current to the battery. It operates at the most efficient combination of voltage and current, also known as ‘Maximum Power Point’.

The MPPT charge controller prevents the solar power system from losing solar-generated power and forces the solar panel to operate at a voltage close to its maximum power point.

An MPPT controller is better suited for colder conditions because the voltage of a solar panel operating at peak power is tested at 25c – 77f. The efficiency of a solar panel drops as the temperature rises, whereas they perform closer to peak power in cool or colder weather.

The MPPT controller is able to capture the excess voltage to charge the batteries. As a result, an MPPT controller can produce 20–25% more charging in cooler temperature than a PWM controller.

  • Performs best when the battery is in a low state of charge. When battery voltage is low the MPPT controller provides a higher charging current to the battery so that it fully charges in a shorter time
  • Opportunity for system expansion
  • Better for long wire runs between the solar panels and the battery. Long wires mean higher voltage drop and loss of power
  • MPPT controller is less efficient in low power applications (works best in systems with 150W solar array or higher)
  • MPPT charge controllers are more expensive than PWM controllers

Regardless of which Solar Charge controller best suites your needs there are a couple of things to look for when making a purchase.

The charge controller you select should include indicator lights that display charging status, battery status, and errors in the system. A control display panel should also display the battery voltage, state of charge, and current coming from the solar array.

Note: You don’t need a solar charge controller if you have a low power solar panel (below 10W) and a battery of 100 amp-hours of capacity or more. A low watt solar panel is not capable of overcharging a high-capacity battery.

The idea is to build a solar power system that works for your needs without spending unnecessary money on unneeded components.

I hope this post helps in determining which solar charge controller will work best for your system.

To figure out the amp rating for your solar charge controller please see this post.

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