Battery backup systems are essential to keep basic operations running in the event of a power outage.
Facilities such as data centers, hospitals, public companies, airports, oil and gas facilities, and railways cannot function without 100% backup power reliability. Even standard commercial and manufacturing facilities have backup power systems for their emergency systems, alarms and controls, emergency lighting, and smoke and fire control systems.
Most standby power systems use an uninterruptible power supply (UPS) and a set of batteries. The UPS serves as a means of backup to the Digital Control System (DCS) in order to maintain control of the plant operations until it can safely shut down or until the standby generator starts.
Although most of the batteries used in modern UPS systems are maintenance free, they are still prone from corrosion, internal shorts, sulfation, drying, and seal failure. This article establishes the recommended practices in order to maintain the optimal operation of the "battery banks", so that the reserve will be ready in the event of blackout.
The two main indicators of battery status
One: internal resistance of battery
Internal resistance is a test of service life, not capacity. The resistance of the battery remains considerably uniform until it approaches the end of its useful life. At that point the internal resistance increases and the battery capacity decreases. Measuring and recording this value helps to identify when the battery should be replaced.
Use only a special battery tester designed to measure the resistance of the battery while it is 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 the measurement of ohmic values over months and years. Each measurement must be compared to previous recorded values to generate a baseline.
Two: the discharge test
The discharge test is the best way to find out the actual available capacity of a battery, but it can be difficult to carry out. During this test, the battery is connected to a load and discharges over a specified period of time. In addition, the current is regulated and a constant known current is established while the voltage is measured periodically. Details of discharge current, specified period of time for discharge test, and battery capacity in amp-hours can be calculated and compared to manufacturer's specifications. For example, a 12 V 100 Ah battery may require a discharge current of 12 A for eight hours. A 12V battery is considered discharged when the terminal voltage is 10.5V.
Batteries cannot withstand critical loads during a discharge test, or immediately after completion. Transfer critical loads to another battery bank until considerable time elapses after testing is complete, and reconnect a temporary load, comparable in size, to the tested batteries. Also, before testing, prepare a cooling system to compensate for the rise in ambient temperature. When large batteries discharge, they emit a large amount of energy in the form of heat.
Good batteries should maintain a capacity greater than 90% of the factory ratings. Most manufacturers recommend replacing the battery if its capacity drops below 80%. When performing battery tests, look for the following fault indicators:
- More than 10% drop in capacity compared to baseline or previous measurements
- 20% minimum increase in impedance compared to reference 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
- Isolate the batteries from the charging and charging system.
- Measure the individual cell or string voltage monthly with a digital multimeter or battery analyzer.
- Measure the output voltage at the charger's 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. Take the measurement monthly.
DC float current
- Consult 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 on a quarterly basis.
- Set the baseline values and enter them into the battery database.
Top 5 causes of battery failure
- Loose terminal and intercell connections
- Overcharge and over discharge
- Thermal leak
When a battery fails within a chain, the entire chain
- is out of service
- reduces service life ²
The worst case
A battery with a high impedance level can overheat and catch fire or explode during discharge. Measurement of voltage alone will not indicate this risk.
¹ The main cause of batteries failure is heat. For every increase in average temperature of 8 ° C (15 ° F), the battery life decreases by half.
² A bad battery increases the charging voltage of adjacent batteries due to the configuration of the charger, which affects the life of the entire chain.
Common Battery Terms
- Capacity test: discharging a battery at a constant current or supply, at a given voltage.
- Floating current: current that flows while the battery is held 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 pre-set limit.
- AC ripple current: Residual AC in rectified voltage in DC discharge and inverted circuits.