Battery charge and discharge control

The charge and discharge control of the battery such as lithium ion battery ups is an important function of the whole system, which affects the operating efficiency of the entire solar street light system, and can also prevent the overcharge and overdischarge of the battery pack. (Click to understand the knowledge of Charging and discharging of storage batteries.) The overcharge or overdischarge of the battery has a serious impact on its performance and life. The charge and discharge control function can be divided into switch control (including single and multiple switch control) type and pulse width modulation (PWM) control (including maximum power tracking control) type according to the control mode. The switching device in the switch control type can be a relay or a MOS (semiconductor metal oxide) transistor. Pulse width modulation (PWM) control type can only use MOS transistors as its switching devices. In the case of sunny days during the day, according to the remaining capacity of the battery, select the corresponding duty cycle method to charge the battery, and strive for efficient charging; at night, according to the remaining capacity of the battery and future weather conditions, adjust the brightness of the lamp by adjusting the duty cycle to ensure a balanced and reasonable use of the battery. In addition, the system also has a protection function for battery overcharging, that is, when the charging voltage is higher than the protection voltage, the charging voltage of the battery is automatically lowered; after that, when the voltage drops to the maintenance voltage, the battery enters the floating charge state. When the voltage is lower than the maintenance voltage, the float charge is turned off and enters the per capita charge state. When the battery voltage is lower than the protection voltage, the controller automatically turns off the load switch to protect the battery from damage. Charging through the PWM method can not only maximize the efficiency of the solar panel, but also improve the charging efficiency of the system. (How to use and maintain batteries?)

Any independent photovoltaic system must have a method to prevent reverse current from flowing from the battery to the array. If the controller does not have this function, a blocking diode will be used. The blocking diode can be on each parallel branch or on the main circuit between the array and the controller. But when multiple branches are connected in parallel to form a large system, blocking diodes should be used on each branch to prevent the current from flowing from the strong current branch to the weak current branch due to branch failure or shadowing. In addition, if several batteries are shaded, they will not generate current and will become reverse biased, which means that the shaded battery consumes power and generates heat. Over time, a fault is formed, so a bypass diode is added for protection.

In most photovoltaic systems, a controller is used to protect the battery from overcharging or overdischarging. Overcharging may vaporize the electrolyte in the battery and cause malfunctions, while overdischarging may cause premature battery failure. Both overcharge and overdischarge may damage the load, so the controller is an important part of the photovoltaic system. The function of the controller is to control the system depending on the state of charge (SOC) of the battery. When the battery is almost full, the controller will disconnect part or all of the array; when the battery discharge is lower than the preset level, all or part of the load will be disconnected (the controller includes a low-voltage circuit breaker function at this time).

The controller has two action set points to protect the battery. Each control point has a motion compensation set point. For example, for a 12V battery, the array circuit breaker voltage of the controller is usually set at 14V, so that when the battery voltage reaches this value, the controller will disconnect the array. Generally, the battery voltage will quickly drop to 13V at this time; the reconnect voltage of the controller array is usually set at 12.8V, so when the battery voltage drops to 12.8V, the controller will act and connect the array to the battery to continue charging the battery. Similarly, when the voltage reaches 11.5V, the load is disconnected and cannot be switched on again until the voltage reaches 12.4V. These on/off voltages of some controllers are adjustable within a certain range. This feature is very useful for monitoring battery usage. When in use, the controller voltage must be consistent with the system’s nominal voltage, and must be able to control the maximum current generated by the photovoltaic array.

Other characteristic parameters of the controller are: efficiency, temperature compensation, reverse current protection, display meter or status light, adjustable set point (high voltage disconnection, high voltage connection, low voltage disconnection, low voltage connection), low voltage alarm, maximum power tracking, etc.

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