How does a solar charge controller work?

A solar charge controller is an essential element to any solar electric panel system. At a most basic level, the solar charge controller prevents batteries from overcharging. It also prevents the batteries from discharging through the solar panel array at night.

If you have a storage battery installed with your grid-tie home system, you do not need a charge controller. Typically, your solar inverter will do the job of maintaining your battery health. Charge controllers are best for off-grid use such as for an RV or a small, off-grid cabin.

Solar charge controller types

There are two main types of solar charge controllers, Pulse Width Modulated (PWM) and Maximum Power Point Tracking (MPPT). PWM controllers are better suited for small solar+storage systems with low voltage panels and small batteries. MPPT controllers are more expensive, but have more features and advantages.

PWM Controllers

The PWM controller can make sure solar panels have the same voltage as the battery. The battery voltage must match the solar panel “nominal voltage”. The voltage the panel is marketed as having, even if the actual voltage can vary and often be slightly higher.

The PWM controller controls the flow by pulsing the power, it can slow down or speed up the charging. Some PWM controllers can only handle one voltage level, while others can handle different levels. Either way, the voltage of both battery and panel must still be the same.

PWM controllers are uncomplicated devices, though some can have additional features added to their essentially basic systems.

MPPT Controllers

MCR MPPT Solar Charge Controller 10A - 60A 12V 24V 48V

MPPT controllers can charge a lower voltage battery from a higher voltage solar array. In some cases, a higher voltage battery from a lower voltage solar array.

In electrical systems, voltage and amperage are inversely related. The higher the voltage, the lower the current (amperage), and vice versa. Since an MPPT controller controls the rate and current flowing from solar panels to a battery. Off-grid solar+storage systems can have panels of different voltage than their batteries.

So while a 24 volts solar panel is insufficient to power a 48-volt battery, an MPPT controller allows it to work by halving the amperage, thereby doubling the voltage flowing into the battery.

How does the solar charge controller work?

One key thing to know is that electricity flows from high voltage to low voltage. So for instance, solar panels produce the most energy in the middle of the day so their voltage is high while the battery bank’s voltage is low.

But at night, the battery has a higher voltage than the solar panels because they are all charged up from the day. To stop this high voltage from flowing to the solar panels, the solar charge controller steps in to keep the electricity in place.

Battery charger working

When the battery charger is empty or close to it, the charge controller directs a great deal of power to the battery and charges it quickly. Once the battery is close to being full or is in use, the charge controller sends a smaller amount of power to the battery via a trickle charge to keep the battery always ready to supply power.

Additionally, charge controllers automatically disconnect non-critical loads from the battery bank when the voltage falls below a certain threshold.

Work as DC load

The first solar charge controller schematic below (Figure 1) illustrates, how a solar charge controller connects to power a direct current (DC) load.

Solar charge controller with DC 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:

Battery to the controller first
PV array to the controller
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 the 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. Any excess power is delivered to the battery system so the batteries maintain their state of charge without getting overcharged.

Work with AC load

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.