Healthy batteries should maintain a capacity above 90 % of the manufacturer’s rating; most manufacturers recommend replacing the battery if it falls below 80 %. A series of regular tests are recommended for ensuring that batteries are maintaining capacity. When conducting battery tests, look for these indicators of failure:
- Drop in capacity of more than 10 % compared to the baseline or previous measurement
- 20 % or more increase in resistance compared to baseline or previous
- Sustained high temperatures, compared to baseline and manufacturer’s specs
- Degradation in plate condition
The Institute of Electronic and Electrical Engineers (IEEE) is the primary source of standard practices for battery maintenance. Over the life of the battery The IEEE recommends performing a combination of tests on a periodic basis as seen in the chart below.
| Voltage and current | Monthly | Quarterly | Yearly and Initial |
| Overall float voltage measured at the battery terminals | • | • | • |
| Charger output current and voltage | • | • | • |
| DC float current(per string) | • | • | • |
| Temperature | |||
| Ambient Temperature | • | • | • |
| Temperature of the negative terminal of each cell | • | • | |
| Ohmic | |||
| Cell/unit internal ohmic values | • | • | |
| Cell-to-cell and terminal connection detail resistance of entire battery | • | ||
| Ripple | |||
| AC ripple current and/or voltage imposed on the battery | • | ||
| Inspections recommended by IEEE 1188 standard ‘Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications’ | |||
The IEEE also recommends the following schedule for discharge testing:
- An acceptance test made at the manufacturer’s factory or upon initial installation
- Periodic discharge testing—at an interval not greater than 25 % of the expected service life, or two years, whichever is less
- Annual discharge testing—when any battery has reached 85 % of the expected service life or dropped >10 % from capacity
Since scheduling full scale discharge testing can be difficult, good regular maintenance is extremely important. By operating the battery according to manufacturer charging requirements and following the IEEE recommendations for battery testing, it should be possible to maximize the life of the battery system.