The equipment used in a capacity test should be a computer-based battery discharge test system with automatic data logging.

It should be capable of conducting single or multiple steps, constant current or constant power (kilowatt) discharge tests. It should include a video display of all pertinent data relating to a discharge test such as individual cell voltages, load current, overall battery voltage, average cell voltage, minimum cell voltage, maximum cell voltage, elapsed test time, program step number and program step elapsed time, battery location and battery identification number.

Individual cell voltages should be displayed in a bar graph format, allowing the test engineer/technician to spot at a glance failing cells or abnormal conditions. All individual cell voltage changes, overall battery voltage and test current should be automatically recorded for later retrieval.

The load banks should be DC voltage rated, single phase, air-cooled load banks. The load current should be adjustable in one amp steps throughout the range of the load unit.


Station battery testing should be performed during a scheduled outage and after the generating units have sufficiently cooled and are off turning gear. The station battery will be isolated from the DC system during testing so a temporary battery will be required to support the switchgear and any operating control circuits.

Contact a member of our Technical Services Group for a consultation on the specific test procedure used by our crews.


How often should plant personnel perform a Type 1 “Modified Performance Test”? The answer depends on the type of battery in use, the schedule of major outages and whether or not the battery room is temperature controlled.

Annual tests are recommended for Valve Regulated Lead Acid (VRLA) batteries and also for batteries in high temperature environments. Battery life can drop dramatically in high temperature environments; therefore, additional testing may be required to insure reliability.

Flooded lead-acid batteries in reasonable temperature environments should be tested upon installation, after two years in service and at intervals not exceeding 25% of the expected service life. The battery will require annual tests after it reaches 85% of its expected service life or exhibits signs of degradation. Degradation is a drop in capacity of 10% or more between capacity tests or when the battery has less than 90% of rated capacity.


No substitute exists for properly designed, instrumented and conducted battery capacity tests. These tests are the only scientific method of proving that a station battery will support the connected load and the only scientific method of determining when to replace a station battery. Tests go a long way toward helping determine if your generation plant can survive an unplanned or emergency trip without sustaining major damage.

This is the second post of a two-part series on the role of capacity testing in proving a battery will support worst-case emergency loads and measuring battery capacity. View Part One.

Authors: Michael P. O’Brien is Technical Services Manager and Bryan Dardar is Vice President Stationary Service for Nolan Power Group.