Battery charge and discharge characteristics

The battery has a self-discharge effect. From the manufacturing workshop to the user's use, it takes about a few months. Taking PA-NASONIC battery as an example, it can be stored for 8 months at an ambient temperature of 30 °C. The residual capacity of the battery is only half of that at the factory. Therefore, it is generally necessary to carry out a long time for the newly purchased battery that is compatible with the UPS. Charging, this is called initial charging. The initial charging current of the battery should be charged at 0.1C. The battery can be recharged after the discharge is finished. This is called normal charging. At present, two charging methods are commonly used in UPS: floating charge and pulse charging. The so-called floating charge means that the output of the rectifier works in parallel with the battery and supplies power to the load at the same time. In fact, the current supplied by the rectifier is divided into two paths, one for the load and the other for the battery to supplement the internal loss of the battery. The charging and charging mode is simple and easy to improve the UPS output transient response characteristics. The characteristic of pulse charging is that the charging current varies with the capacity of the battery. Charging in this way can shorten the charging time.

Charging voltage

Since the UPS battery is in the standby mode of operation, the utility power is in a charging state under normal conditions, and will only be discharged when the power is off. In order to prolong the service life of the battery, the UPS charger is generally controlled by constant voltage current limiting, and the battery is turned into a floating state after the battery is fully charged.

For a battery with a terminal voltage of 12V, the normal float voltage is between 13.5 and 13.8V. The float voltage is too low, the battery is not full, and the float voltage is too high, which will cause overvoltage charging. When the float voltage exceeds 14V, it is considered to be overvoltage charging. It is strictly forbidden to charge the battery pack overvoltage, because overvoltage charging will cause the water contained in the electrolyte in the battery to be electrolyzed into hydrogen and oxygen to escape, causing the electrolyte concentration to increase, resulting in shortened battery life or even damage.

2. Charge current

The battery charging current is generally expressed in C, and the actual value of C is related to the battery capacity. For example, if it is a 100Ah battery: C is 100A. The optimal charging current of Panasonic lead-acid maintenance-free battery is about 0.1C, and the charging current must never be greater than 0.3C. Excessive or too small charging current will affect the battery life.

The ideal charging current should adopt the staged constant current charging mode, that is, use a large current in the initial stage of charging. After charging for a certain period of time, change to a smaller current, and switch to a smaller current at the end of charging. The design of the charging current is generally 0.1C, which can be considered as overcurrent charging when the charging current exceeds 0.3C. Avoid charging with a fast charger, otherwise the battery will be in the state of “instantaneous overcurrent charging” and “instantaneous overvoltage charging”, causing the battery to be used down or even damaging the battery. Over-current charging can cause the battery plate to bend and the active material to fall off, causing the battery power supply capacity to drop, which can damage the battery in severe cases.

3. Charging method

The discharge product of the lead-acid battery is lead sulfate. If it is not converted in time, the battery will be undercharged, thereby reducing the battery discharge capacity and shortening the battery life. Therefore, the battery pack must be in a fully charged state. For different situations, it can be divided into floating charge and equal charge.

(1) Floating charge. The online battery pack is a long-term parallel connection between the charger and the load line as a backup power supply. Under normal circumstances, the use of floating charge, the battery voltage is controlled at 2.25V (relative to 2V battery), and periodically observe and record the change in float voltage. If the voltage of the single battery is low, it means that the battery is not fully charged and the capacity is not enough.

(2) Balanced charging. The so-called equalization charging is to connect each battery unit in parallel and charge it with a uniform charging voltage. If the battery pack has a backward battery during the floating charge process (the cell voltage is lower than 2.20V, compared to the 2V battery), or after 3 months of floating charge, the equalization process should be performed, and the single battery is controlled at 2.35V. 6~8h (note that the average charge time should not be too long), then adjust back to the float voltage value, and then observe the backward battery voltage change, if the voltage is still not in place, then charge again after two weeks. Under normal circumstances, after a new battery pack has been floated for 6 months and fully charged, its voltage will tend to be the same. The equalization charging current is generally selected to be 0.3 C or slightly less than 0.3 C. For batteries with a rated voltage of 12V, the equalization charging voltage is generally selected to be 14.5V.

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