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Battery lifespan and when to replace
Industrial batteries reach end of life when delivered capacity falls below 80 percent of the rated value. Design life is 5 to 20 years depending on type, but real life is set mainly by temperature, depth and frequency of discharge, and charge voltage accuracy. A lead acid battery loses roughly half its life for every 10 degrees Celsius above 20 degrees.
Batteries rarely fail loudly. They lose capacity quietly for years and then fail on the one night the grid goes down.
The four things that shorten life
Temperature, first and by a wide margin. Depth of discharge, since a lead acid string taken to 80 percent repeatedly will not see a fraction of the cycles it would at 30 percent. Incorrect float voltage, which either dries the cells or leaves them sulphated. And ripple current from a failing rectifier or charger, which heats cells from the inside.
How to know where you stand
Float voltage tells you nothing about capacity. Internal impedance or conductance trending, taken every 6 to 12 months and compared against the commissioning baseline, identifies weak cells early. A load bank capacity discharge test is the only measurement that proves how many minutes you actually have.
Planning the replacement
Replace the string as a set, never cell by cell, because a new cell in an aged string is dragged down to the weakest member. Budget the replacement from the commissioning date rather than waiting for a failure, and schedule it in a maintenance window with temporary backup in place. Old lead acid must go to a licensed recycler with documentation.
Frequently asked
- How often should batteries be replaced?
- Plan on 4 to 6 years for economy VRLA, 8 to 12 for high grade VRLA, 15 plus for OPzS, and 10 to 15 for lithium. Testing, not the calendar, should confirm the exact moment.
- Can a single bad cell be replaced?
- Only within the first year or two, and only with a matched cell. After that the mismatch in internal resistance causes uneven charging and the new cell degrades quickly.
- Does keeping a battery on float damage it?
- Correct float at the right voltage and temperature does not. Float that is too high dries VRLA cells, and float that is too low leaves them sulphated. Compensation for room temperature is essential.