The following takes lead-acid battery as an example to introduce the over-discharge protection principle of the controller.

**1. Discharge characteristics of lead-acid batteries**

The discharge characteristics of lead-acid batteries are shown in Figure 1. It can be seen from the discharge curve that the battery discharge process has three stages. In the beginning (OE) stage, the voltage dropped rapidly, and in the middle stage (EG), the voltage dropped slowly, and continued for a long time. After the G point, the discharge voltage dropped sharply. The main reason for the continuous decrease of the voltage with the discharge process is that the acid concentration decreases with the discharge of the battery, causing the electromotive force to decrease, and the second is the continuous consumption of the active material, the reduction of the reaction area, and the continuous increase of polarization; the third is that due to the continuous generation of lead sulfate, the internal resistance of the battery continues to increase, and the internal resistance pressure drop increases. If the battery is close to the end of discharge, stop the discharge immediately, otherwise it will cause irreversible damage to the lead-acid battery.

**2.**** The principle of conventional battery over-discharge protection**

Through the above analysis of the discharge characteristics of the battery, it can be known that during the discharge process of the battery, when the discharge reaches a voltage equivalent to point G, it marks the end of the discharge of the battery. According to this principle, a voltage measurement and voltage comparison circuit is set up in the controller, and by monitoring the voltage value at point G, it can be judged whether the battery should end discharge. For open-type fixed lead-acid batteries, the end-of-discharge voltage (G point voltage) under standard conditions (25°C, 0.1C discharge rate) is about 1.75~1.8V. For VRLA batteries, the end-of-discharge voltage under standard conditions (25°C, 0.1C discharge rate) is about 1.78~1.82V. The voltage at point G set by the comparator in the controller is called “threshold voltage” or “voltage threshold”.

**3. Battery remaining capacity control method**

In many fields, lead-acid batteries are used as starting power or backup power, such as car starting batteries and UPS power systems. In this case, the battery is in a floating state or a fully charged state most of the time, and its remaining capacity or state of charge (SOC) is always at a high state (80%-90%) during operation. And there is a reliable charging power supply that can quickly recharge the battery once the battery is over-discharged. The battery is not easy to be overdischarged under such conditions of use, so the service life is long. In photovoltaic and wind power generation systems, the charging power of the battery comes from the solar cell array and the wind turbine, and its guarantee rate is much lower than that in the case of alternating current. The change of the climate and the excessive power consumption of the user are easy to cause the over-discharge of the battery. If the lead-acid battery is frequently deeply discharged (SOC is lower than 20%) during use, the service life of the battery will be greatly shortened. Conversely, if the battery is in a state of shallow discharge (SOC is always greater than 50%) during use, the service life of the battery will be greatly extended.

As can be seen from Figure 2, when the depth of discharge DOD (SOC=1-DOD) is equal to 100%, the cycle life is only 350 times. If the depth of discharge is controlled at 50%, the cycle life can reach 1000 times, and when the depth of discharge is controlled at 20%, the cycle life can even reach 3000 times. The remaining capacity control method refers to the battery in the process of use (when the battery is in a state of discharge), the system detects the remaining capacity (SOC) of the battery at any time, and automatically adjusts the size of the load cut or the working time of the load according to the SOC of the battery, so that the load matches the remaining capacity of the battery, to ensure that the remaining capacity of the battery is not lower than the set value (such as 50%), so as to protect the battery from being over-discharged.

In order to control the discharge process of the battery according to the remaining capacity of the battery, it is required to accurately measure the remaining capacity of the battery. For the detection of the remaining capacity of the battery, there are usually several methods, such as the electro-hydraulic specific gravity (relative density) method, the open circuit voltage method, the discharge method and the internal resistance method. The electro-hydraulic specific gravity method is not suitable for VRLA batteries, and the open-circuit voltage method is based on the principle that the electro-hydraulic density has a definite relationship with the open-circuit voltage based on the Nernst thermodynamic equation, and can still be used for new batteries. When the capacity of the battery drops in the later period of use, the change of the open circuit voltage can no longer reflect the real remaining capacity; in addition, the open circuit voltage method cannot be used for online testing. The internal resistance method is based on the more certain relationship between the internal resistance of the battery and the capacity of the battery, but usually the internal resistance-capacity curve of a certain specification and type of battery must be measured first, and then use the comparison method to obtain the remaining capacity of the battery of the same type and specification by measuring the internal resistance. The versatility is relatively poor, and the measurement process is quite complicated.

It can also interact with various physical and chemical parameters such as the remaining capacity of the lead-acid battery and its charging and discharging rate, terminal voltage during charging and discharging, electro-hydraulic density, and internal resistance, to establish a mathematical model of the remaining capacity of the battery, which requires that the mathematical model can more accurately reflect the impact of changes in various physical and chemical parameters on the remaining capacity of the battery. With a mathematical model with strong versatility that can reflect the influence of the continuous changes of various physical and chemical parameters on the state of charge of the battery, the remaining capacity of the battery can be easily measured online, so as to further control the discharge process of the battery according to the remaining capacity of the battery.

**4. Battery remaining capacity (SOC) discharge process control**

The controller designed by the battery residual capacity control method can control the whole process of the battery discharge. It is mainly used in photovoltaic power generation systems that are unattended and allow proper adjustment of working hours, the most typical of which is solar street lights.

The load can also be divided into different levels, and the controller can adjust the power of the load according to the remaining capacity of the battery, which can also achieve the same purpose. For loads that do not allow automatic adjustment of load time and power, the remaining capacity of the battery can be displayed on the controller, so that the user can know the state of charge of the battery at any time and take necessary adjustment measures manually.