The control principle of the photovoltaic controller is usually expressed in the form of the charge and discharge protection mode of the controller. The photovoltaic controller must have the following charge and discharge protection modes:
(1) Direct charge protection point voltage
Direct charging is also called rapid charging, which belongs to fast charging. Generally, the battery is charged with high current and relatively high voltage when the battery voltage is low. However, the charging has a control point, also called a protection point. When charging, when the battery terminal voltage is higher than the protection value, the direct charging should be stopped. The voltage of the direct charge protection point is generally the “overcharge protection point” voltage, and the battery terminal voltage cannot be higher than this protection point voltage during charging, otherwise it will cause overcharging and damage the battery.
(2) Voltage of equalization control point
After the direct charge is over, the battery will generally be allowed to stand still for a period of time by the charge and discharge controller to allow its voltage to drop naturally. When it drops to the “recovery voltage” value, it will enter an equal charge state. Why design equalization? That is, after the direct charge is completed, there may be individual batteries “lag behind” (the terminal voltage is relatively low). In order to pull these individual molecules back and make all the battery terminal voltages uniform, it is necessary to match the high voltage with a moderate When the current is recharged for a short while, it can be seen that the so-called “equal charge”, that is, “equal charge”. The equalization time should not be too long, generally a few minutes to ten minutes, setting the time too long is harmful. For a small system equipped with one or two batteries, equal charging is of little significance. Therefore, street light controllers generally do not have equal charging, and only have two stages.
(3) Floating charge control point voltage
Generally, after the equalization is completed, the battery is also left to stand for a period of time, so that its terminal voltage naturally drops. When it drops to the “maintenance voltage” point, it enters the floating charge state. Currently, the PWM method is adopted, which is similar to “trickle charging”. “(I.e. low-current charging), when the battery voltage is low, charge a little, and when the battery voltage is low, charge a little, one by one, so as to prevent the battery temperature from rising continuously. This is very good for the battery, because The internal temperature of the battery has a great influence on charging and discharging. In fact, the PWM method is mainly designed to stabilize the battery terminal voltage and reduce the battery charging current by adjusting the pulse width. This is a very scientific charging management system. Specifically, in the later stage of charging, when the remaining capacity (SOC) of the battery is greater than 80%, the charging current must be reduced to prevent excessive outgassing (oxygen, hydrogen and acid gas) due to overcharging.
(4) Over-discharge protection point voltage
The discharge voltage of the battery cannot be lower than this value. This is the national standard. Although each battery manufacturer has its own protection parameters (enterprise standard or industry standard), it must eventually rely on the national standard. It should be noted that for safety reasons, the over-discharge protection point voltage of the 12V battery is generally added to 0.3V as temperature compensation or the zero-point drift correction of the control circuit, so that the over-discharge protection point voltage of the 12V battery is 11.10V, that is 24V The over-discharge protection point voltage of the system is 22.20V. At present, many manufacturers of charge and discharge controllers adopt the 22.2V (24V system) standard.