What Capacity Fade Means
Capacity fade describes how much measured capacity has been lost relative to an earlier reference test. If a battery delivered 100 Ah during a controlled baseline test and later delivers 83 Ah under comparable conditions, the lost capacity is 17 Ah. The capacity fade is therefore 17%, while the battery retains 83% of the baseline capacity.
The calculator uses the baseline test rather than the printed nameplate value because a measured starting point is usually more useful for ageing analysis. A new battery can test slightly above or below its nominal rating depending on chemistry, manufacturing tolerance, temperature, discharge rate, cutoff settings and measurement equipment.
Capacity fade is usually discussed as a percentage because that makes batteries of different sizes easier to compare. The absolute amp-hour loss still matters: a 10% loss is 10 Ah on a 100 Ah battery but 20 Ah on a 200 Ah battery.
Worked Example: 100 Ah Baseline To 83 Ah
Suppose a lithium battery was capacity-tested after commissioning and delivered 100 Ah. After a period of service, the same battery is tested again and delivers 83 Ah under a similar discharge procedure.
| Measurement | Value | Meaning |
|---|---|---|
| Baseline tested capacity | 100 Ah | Reference used for the comparison |
| Latest tested capacity | 83 Ah | Capacity delivered by the later test |
| Absolute loss | 17 Ah | 100 Ah − 83 Ah |
| Capacity fade | 17% | 17 Ah ÷ 100 Ah × 100 |
| Capacity retained | 83% | 83 Ah ÷ 100 Ah × 100 |
This result says the battery delivered 17% less capacity than it did during the baseline test. It does not by itself explain why the reduction occurred, whether the change is permanent, or whether the battery has reached the manufacturer’s service limit.
How To Make Two Capacity Tests Comparable
The calculation is only as good as the measurements entered. Capacity tests can shift for reasons that are not long-term ageing, so document the test conditions each time.
| Test variable | Why it matters | Better practice |
|---|---|---|
| Starting state of charge | An incomplete charge reduces the energy available to the test. | Start from a repeatable, manufacturer-approved fully charged condition. |
| Discharge cutoff | A higher cutoff ends the test earlier and reports less capacity. | Use the same cutoff voltage or BMS endpoint for baseline and retest. |
| Discharge current | Higher current can change usable capacity and voltage sag. | Use the same test current or a closely comparable load profile. |
| Temperature | Cold or hot conditions can change measured performance. | Compare tests at similar battery temperature where practical. |
| Measurement equipment | Meter offset or shunt calibration can create apparent change. | Use the same calibrated equipment and connection point. |
| Rest and balancing state | Uneven cells or different rest periods can affect the endpoint. | Use a consistent preparation procedure and note cell imbalance. |
If those conditions changed substantially, treat the calculated fade as an observed difference between tests rather than a clean measurement of permanent capacity loss.
Why Lithium Batteries Lose Capacity
Lithium batteries age through a combination of calendar ageing and cycling. Even a battery that is rarely used can lose some capacity with time, while repeated charging and discharging adds cycling-related wear. The exact mechanisms depend on chemistry and design, but the practical result is that less active lithium and electrode capacity may remain available for reversible energy storage.
Temperature
High temperature generally accelerates unwanted chemical reactions and can increase long-term degradation. Low temperature can temporarily reduce measured performance and may also impose charging restrictions, so a cold-weather capacity test should not automatically be compared with a warm baseline as though the difference were permanent fade.
State of charge during storage
Long periods at very high state of charge can be harder on many lithium-ion chemistries than moderate storage states. The battery manufacturer’s storage instructions should take priority because recommended storage voltage and state of charge vary by chemistry and product.
Depth of discharge and cycle conditions
Frequent deep cycling, high charge or discharge rates and demanding thermal conditions can increase stress. There is no universal fade-per-cycle percentage that applies to every lithium battery, so avoid projecting future capacity from a generic number without product-specific data.
Capacity Fade Versus State Of Health, Cycle Count And Resistance
Capacity fade is one health metric. It answers a narrow question: how much discharge capacity has been lost relative to a baseline? A capacity-based state of health calculation often expresses the same comparison from the opposite direction—83% retained capacity corresponds to 17% capacity fade in the simple example above.
Cycle count answers a different question. Two batteries with the same cycle count can have different capacity because cycle depth, temperature, charge rate, storage conditions and calendar age may differ. Likewise, a battery can develop higher internal resistance and weaker peak-power performance before a capacity test shows equally large loss.
How Much Capacity Fade Is Too Much?
There is no single percentage that defines end of life for every lithium battery. Some applications and manufacturers use a retained-capacity threshold such as 80% as an engineering reference, while others specify different criteria or combine capacity with resistance, power capability, error history or safety diagnostics.
Use the battery manufacturer’s warranty, cycle-life specification and service documentation as the deciding reference. A battery used for backup power may remain useful at a lower retained capacity if its reduced runtime is still acceptable, while a propulsion or high-power application may have additional performance limits that become important earlier.
Do not continue using a battery solely because the capacity percentage looks acceptable if there are safety concerns such as swelling, leakage, unusual heating, damaged terminals, repeated protection trips or other abnormal behavior.
Interpreting Unexpected Results
If the latest test suddenly shows much more fade than previous tests, first check whether the test setup changed. A different cutoff, colder battery, higher current, unbalanced cells, a partially charged starting point or a measurement error can all reduce the reported capacity.
If a repeated test under controlled conditions confirms the reduction, record the result rather than forcing it to match a predicted ageing curve. Real battery degradation is not always perfectly linear. Trending several comparable measurements can show whether the battery is degrading gradually, stabilizing, or changing faster than expected.
If the latest measured capacity is higher than the baseline, this calculator treats the inputs as inconsistent for capacity fade. A higher result can occur when an early baseline was conservative, the battery was not fully conditioned, temperature changed, or the test method improved. In that situation, establish a more reliable reference rather than reporting a negative fade percentage as though the battery gained permanent capacity.
Common Mistakes When Tracking Capacity Fade
- Using the nameplate rating as if it were a measured baseline. The rated value is useful, but a controlled baseline test gives a cleaner ageing comparison.
- Changing the discharge cutoff between tests. Ending one test earlier can look like degradation even when the battery has not changed by the same amount.
- Comparing amp-hours at different system voltages. For packs whose voltage configuration changes, watt-hours may be the more meaningful energy comparison.
- Ignoring temperature. A cold test can show less available capacity without representing the same amount of permanent ageing.
- Equating capacity with power capability. A battery may retain acceptable amp-hours yet have enough resistance growth to perform poorly under high load.
- Overreacting to one reading. Repeat an unexpected test before making replacement decisions, provided the battery is safe to test.
Build A Useful Capacity-Fade Record
For each test, save the date, tested amp-hours or watt-hours, starting charge condition, discharge current or load, cutoff point, battery temperature and measurement equipment. If available, also record cycle count, cell-voltage spread and any BMS warnings.
A simple history such as 100 Ah, 97 Ah, 94 Ah, 90 Ah and 86 Ah under similar conditions is more informative than a single “14% fade” result because the trend shows how capacity changed over time. The same record also helps distinguish gradual ageing from a sudden change caused by a test setup or a developing battery problem.
Use this calculator to convert those test pairs into fade percentages, then make service decisions with the manufacturer’s limits and the requirements of the actual application.