Facilities such as data centers, hospitals, airports, public utilities, oil and gas facilities, and railroads cannot operate without 100 percent backup power reliability. Even standard commercial and manufacturing facilities have backup power batteries for their emergency systems, alarms and controls, emergency lighting, and smoke and fire control systems.
Most backup power batteries use an uninterruptible power supply (UPS) and a set of batteries. The UPS serves as a backup to the digital control system (DCS) to maintain control of plant operations until it can be safely shut down or until the auxiliary generator starts.
Although most batteries used in modern UPS systems are maintenance-free, they are still susceptible to deterioration from corrosion, internal shorts, sulfation, drying out, and sealing failures. This article sets out best practices for maintaining optimal performance of these “battery backups” so that the backup is ready in the event of a blackout.
The two main indicators of battery status
One: battery internal resistance
Internal resistance is a test of service life, not capacity. The battery’s resistance remains considerably uniform until it nears the end of its useful life. At that point, the internal resistance increases and the battery capacity decreases. Measuring and keeping track of this value helps identify when the battery should be replaced.
Use only a special battery tester designed to measure battery resistance while the Tadiran batteries are in use. Read the voltage drop across the load current (conductance) or AC impedance. Both results are expressed in ohmic values.
An isolated ohmic measurement has little value without context. Recommended practice requires measuring ohmic values over months and years. Each measurement must be compared to previous recorded values to generate a baseline.
Two: the download test
Discharge testing is the best way to discover the actual available capacity of a battery, but it can be difficult to perform. During this test, the battery is connected to a load and discharged over a specific period of time. Additionally, the current is regulated and a constant known current is established while the voltage is measured periodically. Details of the discharge current, the specified time period for the discharge test, and the battery capacity in ampere-hours can be calculated and compared to the manufacturer’s specifications. For example, a 12V 100Ah battery may require a discharge current of 12A for eight hours. A 12 V battery is considered discharged when the terminal voltage is 10.5 V.
Batteries do not support critical loads during a discharge test, nor immediately after completion. Transfer critical loads to another battery backup until a considerable time has elapsed after completing the test and reconnect a temporary load, comparable in size, to the tested batteries. Also, before testing, prepare a cooling system to compensate for the increase in ambient temperature. When large batteries discharge, they emit a large amount of energy as heat.
Good batteries should maintain a capacity greater than 90% of factory ratings. Most manufacturers recommend replacing the battery if its capacity drops below 80%. When testing the battery, look for the following failure indicators:
- Drop in capacity of more than 10% compared to baseline or previous measurements
- 20% minimum impedance increase compared to baseline or previous measurements
- Continuous high temperatures compared to manufacturer’s reference and specifications
- Degradation in the condition of the plates
How to perform standard battery tests
float voltage
- Isolate batteries from the charging system and load.
- Measure the voltage of an individual cell or string monthly with a digital multimeter or battery analyzer.
charger output
- Measure the output voltage at the charger output terminals monthly with a digital multimeter or battery analyzer, such as the BT500 series.
- Observe the output current displayed on the charger’s current meter or use a clamp meter. Measure monthly.
Float DC Current
- Consult the manufacturer’s specifications for approximate and expected values of float currents.
- Use an appropriate DC clamp meter to measure the expected float current once a month.
Internal Ohmic Values
- Use a battery analyzer, such as the Fluke 500 Series, to measure individual battery ohmic values quarterly.
- Set reference values and enter them into the battery database.
Top 5 Causes of Battery Failure
- Loose terminal and cell-to-cell connections
- Aging
- Overcharge and overdischarge
- Thermal runaway¹
- Undulation
The worst case
A battery with a high impedance level can overheat and catch fire or explode during discharge. Measuring voltage alone will not indicate this risk.
¹ The main cause of battery failure is heat. For every average temperature increase of 8°C (15°F), battery life is cut in half.
² A bad battery increases the charging voltage of adjacent batteries due to the charger configuration, which affects the life of the entire chain.
Common Battery Terms
- Capacity test: discharge of a battery at a constant current or supply, at a given voltage.
- Float current: current that flows while the battery is maintained at float voltage.
- Internal ohmic values: internal resistance of the battery (characteristic of each battery).
- Download test: The battery is connected to a load until its voltage drops below a defined and preset limit.
- Ripple AC Current: Residual AC in rectified voltage in DC discharge and inverted circuits.
