一、Necessity of Storage Battery Capacity Verification

Storage batteries operate in a floating charge state for long periods. Over time, their electrodes become covered with a thick oxide film, leading to passivation of the anode plates and a gradual increase in internal resistance. As a result, the actual capacity of the battery will be significantly lower than its nominal capacity. The maintenance-free feature of valve-regulated batteries only means that there is no need for water refilling, acid adding or electrolyte replacement; daily inspection and maintenance work remain indispensable.

Undervoltage charging, overcharging, over-discharging, excessive ambient temperature and other factors during battery operation can degrade battery performance. Conducting capacity verification discharge can objectively and accurately measure the true capacity of the battery. Meanwhile, proper charging and discharging can extend its service life and ensure the operational reliability of the DC system.

According to the Operation and Maintenance Management Regulations for DC Systems of China Southern Power Grid Co., Ltd., newly installed valve-regulated batteries shall undergo a full-capacity verification charge-discharge test upon acceptance. After that, a full-capacity verification charge-discharge test shall be conducted once every 2–3 years. For valve-regulated batteries in operation for more than 6 years, a full-capacity verification charge-discharge test shall be performed annually.

1. Storage Battery Capacity: The amount of electricity a battery can deliver under specific discharge conditions.Capacity (Ah) = Discharge Current (A) × Discharge Time (h). The rated capacity refers to the discharge capacity calculated when discharging for 10 hours with a discharge cut-off voltage of 1.8V. For a storage battery rated at 2V 300Ah, the discharge current is generally set at 10% of the capacity, i.e., 30A. If the discharge is terminated after 8 hours, the actual capacity will only be 80% of the rated capacity.
2. Qualification Standard for Storage Batteries: Battery capacity decreases over time. A new battery is qualified only if it reaches 100% of the rated capacity. For batteries in operation for 6–8 years, reaching 80% of the rated capacity is deemed qualified.

二、Capacity Verification Test Method

1. Single-charger Single-battery System: The Ten Key Anti-accident Measures of China Southern Power Grid Co., Ltd. stipulate that the system shall not operate without storage batteries under any circumstances (including batteries using silicon rectifier charging equipment). When the battery bank has to be taken out of operation, a standby (temporary) battery bank shall be put into use.
2. Dual-charger Dual-battery System: During capacity verification, the second set of DC system shall carry the full substation load, while the first set of DC system and its charging device shall be taken out of operation. After completing the capacity verification test, the first battery bank shall be recharged until fully charged. For capacity verification of a dual-battery dual-charger system, isolation measures are required to avoid mutual interference between operating and maintenance equipment.
3. During on-site discharge tests, it shall be ensured that the rated voltage of the battery bank is consistent with the voltage class of the discharge tester. The rated voltages of substation DC systems are usually 48V, 110V and 220V. There are various types of discharge testers available, including single voltage class (applicable only to 220V or 110V), adaptive type (applicable to three different voltage classes), and charge-discharge integrated machine series, etc. The FDT-220/110 DC System Comprehensive Tester independently developed and produced by our company is compatible with discharge tests of battery banks of two voltage classes, as well as ripple voltage stabilization and current stabilization tests of chargers.

Functional Features

1. Charger Characteristic Testing: It can fully automatically test the voltage stabilization accuracy, current stabilization accuracy, ripple coefficient and ripple waveform parameters of 220VDC and 110VDC DC charging devices under various permissible AC power supply and DC load conditions.
2. Battery Bank Discharging: It can be used as a battery bank discharge tester to automatically test the capacity and discharge curves of battery banks, including the discharge curves of the entire battery bank and each individual battery cell.
3. Battery Bank Charging Monitoring: It can monitor the charging process during battery bank charging and measure the charging curves of the entire battery bank and each individual battery cell.
4. Wireless Individual Battery Cell Monitoring: It can monitor the voltage and discharge curve of each battery cell through wireless communication during discharging and charging processes. It can automatically alarm and pause or resume discharging when the voltage of any battery cell drops below the lower limit. The wireless monitoring system can also be used independently for online monitoring of the charging and floating charge operation status of each storage battery.

For discharge tests of 48V battery banks used in communication power supplies, the FD-48 DC System Comprehensive Tester can be selected.

Within a battery bank, a battery cell with an internal resistance that has a sudden change of 50% compared with historical records is deemed unqualified, which can be verified through further capacity verification discharge. An increase in internal resistance is a sign of battery degradation. Judging battery capacity status through internal resistance measurement is a simple method, but the accuracy of the internal resistance tester largely depends on the clamping force of the test clips. Due to differences in craftsmanship or improper on-site operation methods, the measured data may vary significantly. Conducting verification capacity discharge can accurately determine the battery capacity, which is currently a relatively reliable method for testing battery performance.

4. Precautions for Discharge Tests: Continuing to discharge the battery after it has reached the cut-off voltage is considered over-discharging, which will severely damage the battery. Different voltage classes have different discharge cut-off voltages: 1.8V for 2V batteries, 5.4V for 6V batteries, and 10.8V for 12V batteries.

After discharging, the battery needs to undergo a recovery equalizing charge. The charger must switch to equalizing charge mode, which will increase the voltage of the entire  system (including the control busbar) by 10V for a relatively long time—a practice not permitted by regulations. Therefore, batteries that have been equalizing charged by the charger can only be put into system operation after being fully charged.

After completing the test, fill in the test records, including measuring the internal resistance and voltage of each battery once before and after discharge, measuring the voltage of each battery cell every hour, recording the battery temperature and ambient temperature, and documenting the battery data obtained every hour. During this process, continuously monitor the data displayed on the discharge tester.