Panels photovoltaic solar energy

Solar charge controllers: description, function and types

Solar charge controllers: description, function and types

A solar charge controller is a piece of equipment that manages the power during a battery charging process. It controls the voltage and electrical current that solar panels supply to a battery. Charge controllers check the state of charge of the battery to optimize the charging process and the life of the device

A solar battery charger controller is specially designed for a photovoltaic system for your deep cycle battery.

The charge controller can be supplied as a separate device (for example, an electronic unit in a wind turbine or solar PV system) or as a microcircuit for integration into a battery or charger.

Solar panels are designed to give a higher voltage than the final charging voltage of the batteries. They ensure that the solar panels can always charge the battery, even when the temperature of the battery cells is high, and the generated voltage decreases.

Uses of a solar charge controller

Charge controllers perform the following functions:

  • First, it checks the state of charge of the battery.

  • It optimizes the loading process by limiting the speed of loading and unloading.

  • It extends the useful life of the device.

  • It protects the battery bank from possible overloads.

  • It supplies an electric current greater than the self-discharge current, avoiding draining the batteries accidentally but less than the maximum charge current to prevent destroying the battery.

  • It compensates for the differences in the amount of power flowing when power is supplied to the consumer simultaneously as the battery is charged.

  • It measures the temperature (using a temperature sensor) for emergency shutdown of cold load or during overheating.

  • It measures the pressure for emergency load shutdown in case of gas leak.

Why should battery overvoltage be controlled?

This overvoltage has two drawbacks:

  • On the one hand, a small part of the maximum theoretical energy that the PV panel can provide (10%) is lost, which would be obtained if it worked at voltages slightly higher than those imposed by the battery.

  • On the other hand, when the battery reaches its full charge state, it will not reach its maximum potential that the solar panel can theoretically give. As a result, solar panels will continue trying to inject energy into the battery terminals, which will produce an overload that will harm the battery that can damage it.

Solar charge controllers: description, function and typesThis last inconvenience can be solved manually: disconnecting the battery when full charge is detected, but obviously, it is not the most reliable or practical method.

Parameters of a solar charge controller

The parameters that define a controller are:

  • Maximum admitted voltage or maximum regulation voltage: it is the value of the maximum nominal voltage that the controller allows applying to the battery.

  • Upper hysteresis interval is the difference between the maximum regulation voltage and the voltage at which the controller allows the passage of all the current produced by the solar panels. For an intermediate voltage value, the controller enables a fraction of the current produced by the photovoltaic panels to pass, which is smaller the closer the voltage of the battery terminals is to the maximum regulation value.

  • Cut-off voltage: voltage at which the consumer loads are automatically disconnected in order to avoid overcharging the battery.

  • Lower hysteresis interval: it is the difference between the disconnection voltage and the voltage at which the loads are allowed to connect back to the battery.

Usual features of charge controllers

The following parameters define the most common features of charge controllers used in autonomous solar plants:

  • Battery overload protection (high cut-off): this is the essential function of the controller. It prevents the battery from heating up, losing water from the electrolyte and the plates from oxidizing.

  • Low battery alarm: sound / light indicators indicate that the battery is sufficiently discharged. From this moment on, the user can moderate the consumption, which will prevent a harmful and excessive discharge of the battery.

  • Low battery disconnection (low battery cut-off): this function causes the controller to cut off the current supply to the consumers if the battery charge level is too low and, therefore, runs the risk of a deep discharge, a fact that would cause sulfation problems.

  • Protection against short circuits: this function allows using a fuse to protect the controller and the battery output from suffering high currents in the event of a brief electrical circuit in any of the consumption circuits of the installation.

  • Visualization of functions: most controllers have some visual system that allows obtaining information about the status of the installation, simply with some indicators saying that the panels are giving power if the battery is charged or discharged, or more carefully through indicators of current charge levels, battery voltage...

Types of solar charge controllers

There are two types of solar charge controllers: 

PWM Solar Charge Controller

PWM controllers modulate the current by pulses (PW stands for Pulse Width Modulation). It only stops the current flow between the photovoltaic modules and the batteries when they are fully charged.

This battery controller must work with the same nominal voltage between the solar panels and the batteries. To do this, the solar panels do not always work at maximum power, so the performance decreases since part of the energy generated are lost.

There are PWM controllers designed to work with voltages of 12V, 24V, and 48V.

MPPT Solar Charge Controller

Maximum power point tracking (MPPT) controllers are designed to take advantage of the maximum production of the photovoltaic panel.

This type of solar controller adjusts that voltage to the one efficiently required by the battery, gaining intensity and conserving the total power of production.

This option is more expensive than PWM controllers, but the better capacity to take advantage of the electrical production of the solar cells compensates for the extra cost.

The MPPT solar controller includes a DC-DC transformer and a maximum power point controller. The DC-DC transformer converts high voltage direct current to lower voltage direct current during battery charging.

Security systems

The charge controller aims to regulate the current absorbed by the battery so that it never becomes dangerously overcharged. For this reason, it constantly detects and measures the battery voltage and state of charge.

If these parameters reach a certain value, it acts in two possible ways:

  • Cutting off the flow of current to the battery

  • Leaving only a part to pass to keep it in a fully charged state without exceeding it.

This minimum current is called float current and occurs when the battery is fully charged and receiving just enough power to keep it that state.

What is a blocking diode?

An especially important element that many controllers incorporate is a blocking diode.

The blocking diode allows current to flow in one direction only from the battery panels and not the other way. This diode is necessary when the solar radiation is low and the battery voltage is higher than that of the photovoltaic panels, thus preventing the battery from being discharged by the photovoltaic solar panels.

Publication Date: April 8, 2016
Last Revision: July 16, 2022