At present, the lead-acid battery is still the most used in the photovoltaic power generation system. Therefore, only the lead-acid battery is used as an example to introduce the basic principle of charging control of the controller.
The lead-acid battery charging characteristic curve is shown in Figure 1. It can be seen from the charging characteristic curve that the battery charging process has three stages. The initial (OA) voltage rises rapidly, and the intermediate (AC) voltage rises slowly for a long time. Point C is the end of charging, the electrochemical reaction is nearing the end, and the voltage begins to rise rapidly. When approaching point D, hydrogen gas is released from the negative electrode, oxygen gas is released from the positive electrode, and water is decomposed. All the above signs indicate that the voltage at point D indicates that the battery is fully charged and should be stopped, otherwise it will cause damage to the lead-acid battery.
Through the analysis of the charging characteristics of lead-acid batteries, it can be known that during the charging process of the battery, when the voltage equivalent to point D appears, it means that the battery is fully charged. 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 D, it can be judged whether the battery should end charging. For open-type fixed lead-acid batteries, the end-of-charge voltage (voltage at point D) under the standard state (25°C, 0.1C charging rate) is about 2.5V; for VRLA batteries, the end-of-charge voltage under standard conditions (25°C, 0.1C charge rate) is about 2.35V. The voltage at point D set by the comparator in the controller is called “threshold voltage” or “voltage threshold”. Since the charging rate of the photovoltaic power generation system is generally less than 0.1C, the charging point of the battery is generally set at 2.45~2.5V (fixed lead-acid battery) and 2.3-2.35V (Min-controlled sealed lead-acid battery).
The purpose of battery charging control is to avoid electrolyzed water as much as possible on the premise of ensuring that the battery is fully charged. Both the redox reaction and the electrolysis reaction of water in the charging process of the battery are related to the temperature. When the temperature increases, the redox reaction and the decomposition of water become easier, and the electrochemical potential decreases. At this time, the full threshold voltage of the battery should be lowered to prevent the decomposition of water. When the temperature decreases, the oxidation-reduction reaction and the decomposition of water become difficult, and the electrochemical reaction potential increases. At this time, the charging threshold voltage of the battery should be increased to ensure that the battery is fully charged, and at the same time, a large amount of water decomposition will not occur. In the photovoltaic power generation system and the wind-solar hybrid power generation system, the electrolyte temperature of the battery has seasonal long-term changes, and also fluctuates due to the influence of the local environment. Therefore, the controller is required to have the function of automatic temperature compensation for the full threshold voltage of the battery.
The temperature coefficient is generally 3-5mV/℃ for a single battery (standard condition is 25℃), that is, when the electrolyte temperature (or ambient temperature) deviates from the standard conditions, for every 1°C increase, the battery full threshold voltage is adjusted downward by 3~5mV for each battery; for every 1°C drop, the battery full threshold voltage is adjusted upward by 3~5mV for each battery. The temperature compensation coefficient of the battery can also be referred to the battery technical manual or the manufacturer. Generally, no temperature compensation is made for the over-discharge protection threshold voltage of the battery.