Panels photovoltaic solar energy

Charge Regulator

Charge regulator

The charge regulator is the element that ensures that the accumulator charge and discharge processes are carried out so that they are always within the correct operating conditions.

Although in principle it seems that in a photovoltaic solar energy installation we only need the solar modules and the batteries, the charge regulator is a key element.

Solar panels are designed so that they can give a higher voltage than the end-of-charge voltage of the batteries. This ensures that the solar panels are always in a position to charge the battery, even when the temperature of the battery cells is high and there is a decrease in the generated voltage.

This overvoltage has two drawbacks:

  • On the one hand, a small part of the theoretical maximum energy that the photovoltaic panel can give (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, and the solar panel will continue trying to inject energy into the battery terminals, which will produce an overload that will harm the battery that can damage it.

The latter can be solved inconveniently, manually: disconnecting the battery when full charge is detected, but obviously it is not the most reliable or practical method.

What is the function of the charge controller?

The charge regulator has the mission of regulating the current absorbed by the battery so that it is never dangerously overcharged. For this reason, it constantly detects and measures the battery voltage and charging status. If these parameters reach a certain previously established value that corresponds to the value of the maximum allowed voltage, it acts in two possible ways:

  • Cutting off the current flow to the battery
  • Letting only a part pass to keep it in a fully loaded state, without overshooting.

This minimum current is called float current and occurs when the battery is fully charged and receives only enough energy to keep it in that state (which, over long periods, will compensate for self-discharge).

What are the parameters of a charge controller?

The parameters that define a regulator are:

  • Maximum allowed voltage or maximum regulation voltage: it is the maximum voltage value that the regulator allows to apply to the battery.
  • Upper hysteresis interval: it is the difference between the maximum regulation voltage and the voltage at which the regulator allows the passage of all the current produced by the solar panels. For an intermediate voltage value, the regulator lets a fraction of the current produced by the photovoltaic panels through, which is smaller the closer the voltage of the battery terminals gets to the maximum regulation value.
  • Disconnect Voltage - Voltage at which consumer loads are automatically disconnected to prevent over-discharge of the battery.
  • Lower Hysteresis Interval: This is the difference between the disconnect voltage and the voltage at which the consumptions are allowed to reconnect to the battery.

The following parameters define the most common performance of charge regulators used in autonomous solar photovoltaic installations:

  • Accumulator overload protection (shutdown): this is the basic function of the regulator. It prevents the battery from heating up, water from leaking from the electrolyte, and plates from rusting.
  • Low battery alarm: audible indicators / lights that indicate that the accumulator is quite discharged. From this moment, the user can moderate the consumption, which will avoid a harmful and excessive discharge of the accumulator.
  • Low battery disconnection (low cut): this function causes the regulator to cut off the current supply to the consumptions if the charge level of the accumulator is too low and, therefore, there is a risk of a deep discharge, which would cause sulfation problems.
  • Short-circuit protection: this function allows, by means of a fuse, to protect the regulator, as well as the output of the accumulator from suffering high currents in the event of a short-circuit in any of the installation's consumption circuits.
  • Function visualization: most regulators have a visual system that allows obtaining information on the state of the installation, simply with some indicators saying that the panels are giving current, if the battery is charged or discharged, or more carefully by means of indicators of current charge levels, battery voltage ...

What are the types of charge regulators?

    Depending on the operating principle, there are the following types of charge regulators:

    • Parallel type shunt regulators.
    • Series type charge regulators.

    Parallel type load regulators (shunt)

    Parallel type charge controllerShunt charge regulators base their operation on a transistor that derives the current from the modules to a dissipative resistive load. It allows to establish battery voltage values ​​for which this deviation is made intermittently in order to keep the accumulator at the maximum charge level (float).

    This system causes heating of the regulator itself, which causes wear and losses and, therefore, regulators of this type have a limited working current of a few amps and, therefore, will be valid for small photovoltaic installations.

    Series type charge regulators

    This type of regulators base their operation on the interruption of the current to the battery, depending on its voltage. Thanks to current technologies, this switch is progressive, so it can be controlled to have different load levels.

    Charge controller

    The float current can be done by maintaining a low level of charge intensity or by switching charging moments and non-charging moments to favor non-gasification of the battery.

    These types of regulators are connected in series between the panels and the battery and, since they do not dissipate heat, they can be rather small and can be mounted indoors if necessary.

    Other models of regulators of this same type, used in large installations, divert the current from the panels in other circuits when the batteries are charged to use this energy for other uses.

    Other models automatically disconnecting solar panels or groups of photovoltaic panels as the battery voltage increases, allowing only the necessary current to pass through and never in excess.

    Many charge regulators have other functions incorporated for the visualization and control of the operation of the photovoltaic solar installation such as: 

    • volt millimeters and ammeters;
    • low battery voltage alarms;
    • temperature sensor that automatically regulates the value of the maximum charge voltage;
    • automatic disconnectors of the low voltage consumption circuit; amp hour meters; digital displays;
    • data acquisition module; regulation module with maximum power point follower, etc.

    An especially important element that many regulators incorporate is a blocking diode, which allows the passage of current in one direction from the panels in the battery and not in the opposite direction. 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.

    It is important not to confuse this blocking diode with the bypass diode (variant) of photovoltaic modules, since the functions they perform are very different.

    If by accident or insulation defect there is an error in the grounding protection system, the current can flow in the opposite direction to normal and pass through a solar panel or group of solar panels before fleeing through the outlet. land.

    In these cases, the presence of the blocking diode is very important to avoid damage to the photovoltaic modules.

    Very good insulation and good safe grounding could avoid the need to install the blocking diode. Since the blocking diode produces an additional voltage drop of 0.5 to 1 V, there is one more reason to design the panel voltage higher than that needed to charge the batteries.

    Charge regulators with maximum power monitoring

    This is the most sophisticated version of the regulators on the market, since it incorporates a continuous current to alternating current converter at the output of the solar modules, which allows isolating the working voltage of the photovoltaic modules from the voltage of the batteries. In this way, the modules can work at their maximum power point and, therefore, the maximum possible performance.

    If we take as an example a photovoltaic module in which the data provided by the manufacturer are: 53 Wp at 17.4 V and 3.05 A.

    When we connect the module directly to an accumulator with a voltage between terminals that, at this moment, is 12 V, the module is really delivering a power of:

    P real = 12 V · 3,05 A = 36,6 W

    In other words, of the 53 W available, when a 12 V battery is directly charged to terminals, we only take advantage of 36.6 W, which implies a 30% loss of power.

    The question is: where are the rest of the Watts missing?

    The answer is: nowhere, since the module generation is current and not power.

    The solution: get the maximum power from the photovoltaic module using charge regulators with a maximum power finder. In this search engine, the subsequent compensation of voltage by intensity is developed.

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    Last review: March 30, 2020