A battery's age is printed on it, and it's one of the quickest checks on a survey. What the date means depends on the battery and how it's been treated, so pair it with a voltage reading or a test.
Reading the date code
U.S. Battery stamps its flooded batteries on the positive terminal: two letters and a number, with the first letter A to L for January to December. Its AGM batteries carry a laser-etched date on the cover. Other makers use their own codes, so look up the maker's chart before you write the age down.
How long they last
Lifeline puts its AGM batteries at 5 to 8 years for serious users. Its red flags for replacement:
- voltage drops under a light load
- it won't hold a charge after a full cycle
- it fails a load test
- it's over 5 to 7 years old
Lifeline also says frequent deep discharges to 80% will cut cycle life by more than half, and recommends recharging at 12.2 V, about 50%. A healthy AGM rests at 13.0 V or more after charging.
Testing on the survey
Crown Battery gives the load test: charge first, never load test below 75% state of charge, then apply 50% of the CCA rating for 15 seconds and compare the voltage with the minimum. On flooded batteries it uses a hydrometer, and says to replace a battery whose cells differ by more than 50 points. Lifeline adds that a full discharge test with a constant load and an amp-hour meter is the reliable way to measure capacity.
On most surveys there's no time for a load test on a house bank. Record the date codes and the resting voltage, and recommend a capacity test where the age or readings call for it.