When the energy it actually delivers between a full charge and the cut-off has fallen too far for the job — a figure a shunt monitor reports directly. Voltage readings, age and cycle counts are all weaker evidence than a measured discharge.
A battery rarely announces its end. It just stops covering the night, and by then the question is which measurement to trust.
The measurement that answers it
Charge fully, run a known load, count the energy out until the system stops. That number against the original usable capacity is the state of health, and it is the only evidence that is not circumstantial. A shunt monitor collects it in normal use.
Why the usual symptoms mislead
A bank that reads low in the morning may be under-charged rather than worn. A pack that cuts out under load may be reporting a cable problem. Both look exactly like a battery at the end of its life and neither is.
What the rating implies and does not
A cycle rating describes a test at a stated depth to a stated remaining capacity. Real service rarely matches those assumptions, and calendar ageing runs in parallel with cycling rather than being included in it.
The decision that follows
Replacing one battery in a mixed-age series bank is generally discouraged by makers, because the bank follows its weakest member. Whether that applies to a specific product is stated in its documentation, and it changes the cost of the decision substantially.
What people ask before they order
What is a capacity test?
A controlled discharge from full to the pack’s stated limit at a known load, with the energy counted. It answers the question directly and it is the only measurement that does.
Does one bad cell condemn the pack?
In a series bank, a weak cell governs the whole pack, so the symptom is a bank that falls over early. Whether the pack can be rebalanced or serviced is a question for its maker.
Last reviewed 16 September 2026