Battery maintenance and testing is crucial to the continued performance of a UPS system. There are a variety of common battery tests including impedance testing and discharge testing, more commonly known as load bank testing.
Most uninterruptible power supplies have built-in functionality that automatically tests their batteries regularly, typically every 24 hours, and will alarm if it detects a battery fault.
Such tests place a load onto the battery set and monitor the discharge performance. However, this only offers a general indication of the overall set. It doesn’t provide an individual cell level report.
Similarly, just measuring the float voltage across a battery set doesn’t provide a true indication of its condition.
The best way to assess the true state of a large battery set is through external battery testing. For large battery sets, individual block testing can be more reliable.
External battery testing should form part of a planned preventive UPS maintenance regime, although it can also be provided as a standalone service.
This is a non-intrusive test designed to build up a performance history of each battery cell. It is typically undertaken annually as this enables performance to be tracked over time. This makes it easier to identify any signs of deterioration or any cells with high internal impedance that might require replacing.
Impedance testing involves applying an AC current to each battery via probes attached to the block terminals. Impedance is measured and recorded in milliohms.
It gives a general indication of the batteries’ status without placing them under too much stress or needing to take them offline.
Another non-invasive check that compares the data from batteries to algorithms for common battery conditions, such as sulphation and electrolyte dry-out.
Traditionally, these tests were carried out in laboratories to predict battery failure on satellites and space vehicles. Now tests can be conducted using portable hand-held units.
This process uses probes on the terminals to measure the frequency response to voltage and current signals passed into the battery. These results are cross-referenced to performance data for “healthy” batteries.
Because electro-chemical testing measures sulphation and electrolyte dry-out rather than just impedance, it is said to provide a more detailed summary of battery condition. Failing blocks can either be recharged at a higher rate to reduce sulphation or replaced completely.
Load Bank Testing (Discharge Testing)
This is the most comprehensive battery test and the only true examination that determines the actual capacity of the battery string.
Load bank testing audits the batteries under both normal and peak load conditions. This demonstrates which cells hold the charge and highlights which might be approaching their end of service life.
The Institute of Electrical and Electronics Engineers (IEEE) recommends performing discharge testing at the time of installation, then repeating the test every year.
The main drawback of load bank testing is that the UPS batteries must be taken out of service during the test. Usually the batteries are available again within 24 hours, although in the worst-case scenario this can last for several days.
Partial Discharge Testing
As the name suggests, this offers something of a middle ground. Partial discharge testing involves discharging the batteries up to a maximum of 80%. While this takes them out of action similar to load bank testing, they should be available again inside 8 hours.
If there’s a fault condition that requires the UPS to run off its batteries it can do so, although they will only have 20% of their full capacity.
UPS Battery Monitoring
In addition to battery testing, dedicated battery monitoring systems also measure UPS battery performance. It is advised all monitoring systems incorporate the parameters outlined by the globally-recognised IEEE 1491 standard, including:
- String and cell float voltage
- String and cell change and discharge voltage
- AC ripple voltage
- AC ripple current
- String charge current
- String discharge current
- Ambient and cell temperature
- Cell internal resistance