Charging and discharging characteristics of battery

The battery has a self-discharge effect. From the manufacturing workshop to the user's use, there will be a delay of several months.

Take the PA-NASONIC battery as an example. When stored for 8 months at an ambient temperature of 30°C, the remaining capacity of the battery is only half of that at the time of leaving the factory. Therefore, for the newly purchased battery that is matched with the UPS, a longer period of time is generally required. Charging, this is called initial charging. The initial charging current of the battery should be charged at 0.1C, and the battery can be recharged after the end of discharge, which is called normal charging. Currently, two charging methods are commonly used in UPS: floating charging and pulse charging. The so-called floating charging 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 provided by the rectifier is divided into two ways, one is sent to the load and the other is sent to the battery to supplement the internal loss of the battery. The charging mode is simple to connect, which is good for improving the transient response characteristics of UPS output. The characteristic of pulse charging is that the charging current changes with the capacity of the battery. Charging in this way can shorten the charging time.

1. Charging voltage

Because the UPs battery is a standby mode of operation, the mains is in a charging state under normal conditions and will only be discharged when the power fails. In order to prolong the service life of the battery, UPS chargers are generally controlled by means of constant voltage and current limiting. After the battery is fully charged, it will turn to a floating state.

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

2. Charging current

The battery charging current is generally represented by 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 batteries is about 0.1C, and the charging current must not be greater than 0.3C. Excessive or too small charging current will affect the service life of the battery.

The ideal charging current should adopt a staged constant current charging method, that is, a larger current is used in the initial stage of charging, and after a certain period of charging, it is changed to a smaller current, and at the end of charging, a smaller current is used. The charging current is generally designed to be 0.1C. When the charging current exceeds 0.3C, it can be considered as overcurrent charging. Avoid using fast chargers to charge, otherwise the battery will be in the state of "instantaneous overcurrent charging" and "instantaneous overvoltage charging", resulting in a decrease in the available power of the battery or even damage to the battery. Overcurrent charging will cause the battery plate to bend and the active material to fall off, causing the battery's power supply capacity to decrease, and in severe cases, it will damage the battery.

3. Charging method

The discharge product of lead-acid batteries is lead sulfate. If it is not converted in time, the battery will be under-charged, thereby reducing the battery discharge capacity and shortening the battery life. Therefore, the battery pack must be fully charged. 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 circuit as a backup power source. Under normal circumstances, float charging is used, and the voltage of the single battery is controlled at 2.25V (relative to the 2V battery), and the changes in the float charging voltage are regularly observed and recorded. If the voltage of the single battery is low, it means that the battery is not charged enough and the capacity is not enough, so you should pay attention to tracking.

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

When the UpS battery encounters one of the following situations in use, in order to restore the charge and discharge characteristics of the battery, a balanced charging method should be adopted to solve the problem.

1) Over-discharge makes the terminal voltage of the battery lower than the discharge end voltage allowed by the battery. For a 12V M-type lead-acid battery, the end-of-discharge voltage is about 10.5V.

2) In the UPS battery pack, the terminal voltage difference between the battery cells exceeds about 1V.

3) Storage batteries that have been left unused for a long time and exceed the static storage time. Normal temperature environment, the static storage time of UPS battery is generally 9 months. When the temperature is 31-40℃, the static storage time is 5 months (including newly purchased batteries).

4) Replace the battery with electrolyte.

5) A battery that cannot be charged in time after discharge.

6) Long-term work in the floating state (that is, the UPS works in the mains state for a long time) and exceeds the static storage time.

7) Discharge accidentally and put the battery terminal voltage lower than the termination voltage.

For the NP6-12 sealed lead-acid battery, the equalizing charging voltage is about 14V, and the maximum allowable equalizing charging current is less than 0.28C; for the LCL12V24P sealed lead-acid battery, the equalizing charging voltage is about 14V and the maximum allowable equalizing charging The current is less than 8A.

(8) Temperature compensation. Although the operating temperature range of the battery is very wide, it can operate in the range of -15~+45℃, but the best ambient temperature for battery operation is about 25℃. If the ambient temperature changes greatly, the temperature coefficient needs to be compensated (-3mV/ ℃).

(9 Charging operation. The initial charging current of the battery is generally carried out according to the specified value in the manual, or according to the current of 1/10 of the rated capacity. During normal charging in use, it is best to use the hierarchical constant current charging method, that is, at the initial stage of charging Use a larger current, after charging for a certain period of time, switch to a smaller current. As for the later stage of charging, switch to a smaller current. This charging method has higher charging efficiency, it requires shorter charging time, and the charging effect is good. It is beneficial to extend the battery life. Some new smart UPSs use regular automatic monitoring and cyclic charging to charge the battery to extend the battery life.

(10) Therapeutic charge and discharge. For the battery therapeutic charging and discharging process, the "health" of each battery is judged from the discharge capacity and battery voltage value, because the voltage change of each battery in the process of different discharge capacity represents the "health" of the battery. Remedial measures should be taken for qualified batteries.

The undervoltage of some UPS batteries is caused by the damage of the final drive circuit of the UPS inverter, which causes the battery to discharge. If the circuit failure is repaired, the battery should be connected to the original circuit for charging in time, and the battery will still be as good as before. The problem is that the undervoltage battery cannot make the UPS start successfully. At this time, the following solutions can be used:

1) First use a good battery to start the UPS to the mains state, then remove the good battery and replace it with an undervoltage battery to be charged. When replacing the battery, the UpS is required to run without load. Generally, after UpS enters the mains state, as long as the input mains power is maintained normally, removing the battery will not affect the mains power supply state. When charging the under-voltage battery, pay attention to the charging current of the battery.

2) Charge the under-voltage battery to 10.5V (relative to the 12V battery) first, and the UPS can start successfully.

4. Discharge requirements

The actual capacity of the battery is related to the discharge current. The greater the discharge current, the lower the efficiency of the battery. E.g,

When the 12V/24Ah battery has a discharge current of 0.4C, the discharge time to the final voltage is 1 hour and 50 minutes, the actual output capacity is 17.6Ah, and the efficiency is 73.3[%]. When the discharge current is 7C, the discharge time to the final voltage is only 20s, the actual output capacity is 0.93Ah, and the efficiency is 3.9[%]. Therefore, high current discharge should be avoided to improve the efficiency of the battery. In general circuit design and user selection of loads, it is necessary to protect the UPS battery inverter discharge current not to exceed 2C.

The depth of discharge also has a great impact on the service life of the battery. The deeper the depth of discharge of the battery, the fewer cycles it uses. Although the UPS county has a battery low-voltage protection function, generally when a single battery is discharged to about 10.5V (compared to a 12V battery), the UpS will automatically shut down, but if the UPS is in a light-load discharge or no-load discharge, the Current discharge can improve the efficiency of the battery, but when it is discharged with a very small current (less than 0.05C) for a long time, it will cause the actual discharge capacity of the battery to exceed its rated capacity, resulting in serious deep discharge of the battery. When the battery discharge depth is 100[%], the actual service life of the battery is about 200~250 charge and discharge cycles; the discharge depth is 50[/%]

When, about 500~600 charge and discharge cycles. Therefore, when using UPS, it is necessary to avoid heavy load overcurrent discharge, and

Avoid prolonged light-load discharge and deep discharge of the battery. It is also necessary to avoid short-circuit discharge of the battery, otherwise it will seriously damage the recharging capacity and storage capacity of the battery and shorten the service life. In the actual application of batteries, it is not the first to pursue the percentage of the discharge capacity, but to find and deal with the backward batteries, and then do a check discharge experiment after dealing with the backward batteries. This can prevent accidents and prevent the backward battery from deteriorating into a reverse battery during discharge.