Basic principles and functions of the controller

Basic principles and functions of the controller

A complete independent photovoltaic power generation application system, if there is a battery, the photovoltaic charge and discharge controller is indispensable. Batteries, especially lead-acid batteries, frequently overcharge and overdischarge during use will affect the service life of the battery. The service life of the battery pack has a great influence on the life of the solar photovoltaic power generation system and prolongs the service life of the battery pack. The key is to control its charging and discharging conditions. The photovoltaic power generation system uses a set of control system to control the charge and discharge of the battery pack, so that the battery pack can be used in the best state, so as to extend the service life of the battery. This system is called the charge and discharge controller. The photovoltaic charge and discharge controller monitors the state of the battery, regulates and controls the charging voltage and current of the battery, and controls the output of the photovoltaic cell and the battery to the load electric energy according to the demand. It is the core part of the entire photovoltaic system, and its control performance directly affects the battery life and system efficiency.

The basic principle of the controller

The control circuit of the controller differs in complexity according to different distributed power sources, but its basic principles are the same. Figure 1 is a basic diagram of the working principle of a charge and discharge controller. The system consists of solar cells, control circuits, storage batteries and loads. The switches S1 and S2 are respectively a charging switch and a discharging switch, and they all belong to a part of the controller circuit. The opening and closing of S1 and S2 is determined by the control circuit according to the charging and discharging state of the system. When the battery is full, the charging switch S1 is turned off to stop the photovoltaic battery from supplying power to the battery. When the battery is over-discharged, the discharge switch S2 is turned off, and the battery stops supplying power to the load. Switches S1.S2 are switches in a broad sense, which include various switching elements, such as various electronic switches, mechanical switches, and so on.

Basic principles and functions of the controller
Figure 1 – The working principle diagram of the controller

In the distributed power supply, the basic function of the charge and discharge controller is to provide the most suitable charging voltage and current for the battery, while protecting the battery. It has the function of disconnecting and restoring the charge and discharge when the input is full and the capacity is insufficient, to avoid the occurrence of overcharge and overdischarge.

Controller function

In addition to providing the function of the photovoltaic cell to the DC load, it also needs to charge the battery through the controller. That is, the controller must firstly protect the battery from charging and discharging, and secondly, it must provide a stable DC voltage to the DC load or inverter. Generally speaking, the controller should have the following functions:

(1) Disconnect and restore function. The controller shall have the function of disconnecting and restoring the connection of the input high voltage.

(2) Undervoltage alarm and recovery function. When the battery voltage drops to the undervoltage alarm point, the controller should automatically send out an audible and visual alarm signal.

(3) Low-voltage disconnection and recovery function. This function can prevent the battery from over-discharging. The load is connected through a relay or electronic switch, which can automatically cut off the load at a given low voltage point. When the voltage rises to the safe operating range, the load will automatically reconnect or require manual reconnect. Sometimes a low-pressure alarm is used instead of automatic cut-off.

(4) Protection function. The controller has a load short-circuit protection circuit; an internal short-circuit protection circuit in the controller; a protection circuit for the reverse discharge of the battery at night through the solar cell assembly; a protection circuit for the load, solar cell assembly or battery polarity reverse connection; prevent the breakdown of the protection circuit caused by lightning strikes in the mine area.

(5) Temperature compensation function. When the battery temperature is lower than 25°C, the battery should require a higher charging voltage in order to complete the charging process. On the contrary, higher than this temperature battery requires a lower charging voltage. Generally, the temperature compensation coefficient of lead-acid batteries is -3~-5mV/℃.

(6) Various working status display functions of photovoltaic power generation system. This function mainly displays battery voltage, load status, battery matrix working status, auxiliary power supply status, ambient temperature status, fault alarm, etc.