Why Battery Voltage Can Look Normal Even When Capacity Has Degraded

A lithium-ion battery can show a normal open-circuit voltage (OCV) after charging and still deliver far less runtime than it did when new. A device may report 100% state of charge (SOC), then shut down early under load or return less charge across a discharge cycle. This is the key difference between battery voltage and capacity: voltage describes electrical potential, while capacity measures how much charge a battery can deliver under specified conditions. The two are related, but one cannot be used as a substitute for the other, and the gap is not a measurement error.

Resting voltage is not a capacity gauge. The relationship between OCV and SOC depends on cell chemistry and electrode characteristics, and it can also change with aging, so a single reading taken at rest is insufficient on its own to establish remaining capacity or state of health (SOH). A resting voltage within the expected range does not rule out capacity fade, increased internal resistance or other forms of degradation. It provides only limited information about the battery’s condition and cannot establish how much charge or energy the pack can still deliver.

That gap leaves two questions a voltage reading cannot answer. How much capacity can the battery still deliver? And why does the terminal voltage sag or cut off early under load? The rest of this article answers each with a test and the conditions that make the answer meaningful.

OCV, SOC and SOH: What Each Measurement Tells You

Before the definitions, it helps to keep three near-synonyms apart. Capacity is how much charge a pack holds, measured in amp-hours (Ah). Energy is that charge delivered across a voltage, measured in watt-hours (Wh). Runtime is how long a specific load actually runs on it. They are related, but they are not interchangeable, and this article keeps them separate.

The three core terms get used interchangeably in datasheets and dashboards, yet they answer three different questions. OCV describes the cell at rest. SOC describes how much charge remains right now. SOH describes how much the cell has changed since it was new.

Open-circuit voltage (OCV)

Open-circuit voltage (OCV) is the terminal voltage measured when no current is flowing. Its relationship with SOC can be used for estimation only when the cell has had sufficient time to relax and an appropriate OCV–SOC relationship is available for that cell chemistry and condition. The OCV–SOC relationship depends on the electrode pairing and may change with aging. When measured under controlled conditions and interpreted using an appropriate model, changes in the OCV curve can help identify degradation modes such as loss of lithium inventory and loss of active material, as shown in Birkl and colleagues’ degradation-diagnostics work.

The relationship is not uniform along the curve, either. A 2026 open-access study found that a weak local gradient of the OCV–SOC curve makes SOC weakly observable and can distort the resistance estimate. This limitation is particularly relevant to lithium iron phosphate (LFP) cells, which have a relatively flat OCV–SOC relationship across much of their operating range. Small voltage differences may therefore correspond to substantial SOC differences, limiting the accuracy of voltage-only estimation. Charge and discharge history, voltage relaxation, temperature and cell-specific characteristics can further affect the relationship.

State of charge (SOC)

SOC represents the estimated remaining charge relative to a defined capacity reference. That reference may be the rated capacity or an updated estimate of the battery’s available capacity, and the distinction matters because the reference is not fixed. A 100% SOC reading indicates that the system considers the battery fully charged under its estimation model; it does not, by itself, confirm that the battery retains its original capacity. For example, if a new pack can deliver 10 Ah under a defined test protocol while an aged pack can deliver only 8 Ah under the same protocol, both may still be reported as 100% SOC by their respective battery management systems. The aged pack, however, delivers 20% less charge under those test conditions. SOC is a relative estimate, not a direct measurement of absolute remaining capacity.

State of health (SOH)

Capacity-based SOH measures capacity retention against a defined reference. Energy-based SOH can provide a complementary view when the amount of energy delivered under application-relevant conditions matters more than amp-hours alone. The capacity form is written as SOH_Q = Q available,test / Q reference × 100%, where Q available,test is the charge delivered in a defined controlled discharge from a specified full-charge state to a specified cutoff voltage, and Q reference is the new, rated or individually characterised capacity. Both terms in that ratio are protocol-dependent. The same battery therefore returns different apparent capacities when the protocol changes, which is why a supplier’s capacity number is only meaningful alongside the protocol that produced it.

• OCV, SOC and SOH answer three different questions, and none of them alone establishes remaining capacity.

• Voltage tracks state of charge, not state of health, and the mapping itself shifts as the cell ages.

• Capacity-based SOH is a ratio of two protocol-dependent measurements, so the protocol must travel with the number, and different standards apply to different products.

• DCIR can be used for screening and trend monitoring, but it cannot replace a capacity test.

Why Capacity Can Decline While Voltage Still Looks Normal

Capacity fade and resistance growth can occur together, but they are distinct degradation outcomes. Loss of active material or cyclable lithium can reduce available capacity, while increased internal resistance and polarization can cause greater voltage sag under load. The extent to which these changes occur together depends on the cell design and aging conditions. Self-discharge is a separate failure path: it shows up as an abnormal voltage drop at rest rather than a normal voltage with reduced capacity, which is why a normal voltage drop should be separated from abnormal cell leakage before attributing a shortfall to ageing.

1. Loss of active material

Birkl and colleagues’ degradation-diagnostics work groups ageing into three modes: loss of lithium inventory, loss of active positive electrode material and loss of active negative electrode material. In loss of active material, the active mass of the electrode is no longer available for lithium insertion because of particle cracking, loss of electrical contact, or blocking of active sites by resistive surface layers, and the processes lead to both capacity and power fade. The cell may retain a broadly recognizable voltage profile, but aging can also change the curve’s shape and the relationship between voltage and SOC. The extent of these changes depends on the degradation mechanism and cell chemistry.

2. Loss of cyclable lithium

Loss of lithium inventory is the mode most directly tied to capacity fade. Lithium ions are consumed by parasitic reactions such as surface film formation (SEI growth), decomposition reactions and lithium plating, and are no longer available for cycling between the positive and negative electrode. Loss of cyclable lithium can then reduce the charge transferred between the electrodes during a full discharge. Its effect on the measured OCV depends on the cell design and the resulting changes in electrode stoichiometry. A resting voltage that appears plausible therefore does not rule out lithium-inventory loss or capacity fade.

3. Increased internal resistance and earlier voltage cutoff

The load-voltage relation is V load ≈ V OCV − I·R internal − polarization, and the loaded terminal voltage crosses the cutoff while chemical charge remains. The mechanism is well described in Battery University BU-303: a cell with low internal resistance shows little voltage change under load or charge, while a high-resistance cell swings more sharply for the same current. A pack with normal resting voltage and elevated resistance therefore reaches its cutoff earlier under the same current, and the device reports an empty battery while charge remains.

That is also how one durability-testing programme draws the failure line. In the electric-vehicle durability context, the USABC criteria define end of life for a battery electric vehicle (BEV) battery through two tests: net delivered capacity below 80% of rated capacity under the specified DST Reference Performance Test, or peak power capability below 80% of rated power (FEV North America report for the US EPA). These are application-specific durability criteria for that programme, not retirement thresholds for lithium-ion batteries in general. Depending on the application, a pack may reach its practical end of life because of capacity loss or insufficient power capability.

A higher internal resistance does not mean capacity has fallen by the same proportion. Capacity and resistance are separate measurements with separate protocols, and a pack can fail the power criterion while still meeting the capacity criterion.

How to Evaluate Battery Health Beyond OCV

No single measurement separates the two failure paths. The evaluation has to pair a capacity test with a resistance test, then read both against the load behaviour the application actually imposes. If you are deciding which tests a pack or a supplier claim actually needs, the table below maps each diagnostic question to the test that answers it, what the result can establish, and what it cannot.

Diagnostic question	Test	What it establishes	What it cannot establish alone
How much capacity can the battery deliver?	Controlled discharge capacity test	Delivered capacity under defined conditions	The underlying degradation mechanism, or performance under other loads
Why does voltage sag under load?	DCIR or pulse-response test	Resistance-related voltage response at a defined operating point	Remaining capacity, or behavior at all SOC and temperature levels
Is one cell limiting the pack?	Cell-level voltage monitoring under representative load	Whether an individual cell may be limiting pack output	Whether the cause is capacity loss, resistance growth or both
Is the battery degrading over time?	Repeated capacity and resistance measurements	Performance trends against a baseline	Future behavior beyond the measured conditions

Record four conditions before comparing any two capacity numbers: C-rate, temperature, cutoff voltage and rest period. Without all four, the comparison is between protocols, not between batteries.

1. Measure discharge capacity under controlled conditions

Capacity is not a fixed property of a cell. The rest period, the constant-voltage termination current and the temperature tolerance all depend on the test you are running. They should follow the standard, manufacturer specification or test plan you are working to. The goal is not one universal recipe, but comparability: two capacity tests can be compared directly only when their conditions are consistent. Run the same sequence against a reference capacity from the same protocol, and the ratio is the capacity-based SOH figure.

Published standards define test conditions that do not transfer between products. IEC 61960-3, for example, sets capacity tests for portable sealed lithium cells and batteries, and its sequence covers discharge performance at 20 °C (rated capacity), discharge at −20 °C, high-rate discharge at 20 °C, charge retention and recovery, and endurance in cycles. A different product standard will define its own sequence, so the protocol should always travel with the number. IEC 61960-3 does not mandate 80% as a pass threshold, so that figure stays a common reference for end of life rather than a universal limit.

Pack-level results need a second step. Pack-level capacity can be limited by cell-to-cell variation, so a pack-level result does not necessarily reveal the condition of every cell. A pack-level capacity figure describes the pack as a system, not the cells inside it. For module-level comparisons, reading a discharge curve for cell selection shows how discharge-curve shape and voltage behavior can help compare cells beyond their final capacity readings.

2. Measure DC internal resistance under defined conditions

DCIR is the most common second measurement, but its ability to stand in for capacity is limited. Gasper and co-authors’ 79-cell pulse-diagnostic study correlated DCIR with C/10 discharge capacity across four commercial cell types, with a maximum correlation of about 90% for the LMO pouch module (type B) and a minimum of about 40% for the NMC pouch (type A). These results come from four specific cell types under the study’s test conditions. They illustrate how much the predictive value of DCIR can vary, rather than providing a universal conversion from resistance to remaining capacity, which is why DCIR is best used for screening and trend monitoring alongside direct capacity measurements.

The measurement itself is defined by its conditions. DCIR is commonly calculated from the voltage response to a defined current pulse using R = ΔV/ΔI. The result depends on pulse duration, current, SOC, temperature and measurement method. Four-wire sensing can reduce the influence of lead and contact resistance, but the test configuration should follow the applicable procedure. The immediate voltage response is mainly associated with ohmic resistance, while the subsequent response also reflects polarization and diffusion effects. Because all of these factors shape the result, DCIR values from different suppliers are meaningful to compare only when the relevant test conditions are sufficiently consistent.

3. Observe voltage under load

Terminal voltage under load is the measurement that connects the two tests to the application. In a series-connected pack, the cell that first reaches a voltage or protection limit can constrain usable pack discharge under that operating condition, so cell-to-cell divergence under load is where the limitation appears first. Cell-level voltage monitoring under representative loads can help identify it, although additional testing may be needed to distinguish capacity loss from resistance growth. A fixed millivolt gap means little on its own, though, because the same voltage difference has a different meaning across cell types, states of charge, temperatures and loads. Judge imbalance against the spread of the other cells measured under the same conditions and against the manufacturer’s own limit, rather than against a generic number, and treat a growing gap under load as a screening flag, not a specification limit.

4. Track capacity retention over time

A single capacity measurement shows performance at one point in time. Repeated measurements on a consistent schedule reveal the degradation trend and show when the rate of performance loss begins to accelerate. For many applications, tracking delivered capacity or energy retention alongside DCIR is more informative than resting voltage alone.

What Battery Buyers and Integrators Should Ask Suppliers

A capacity-retention claim is only as strong as the conditions attached to it. Ask for the following, ideally as a capacity-retention curve rather than a single number.

• Charge and discharge rate

• Depth-of-discharge or SOC window

• Test temperature

• Upper and lower voltage limits

• End-of-life threshold

• Sample size and batch

• The DCIR trend across the same interval

An 80% capacity-retention threshold is a common reference for defining end of life, but the applicable criterion depends on the application and its test specification. Ask which threshold the supplier uses and whether the pack was balanced before measurement. Two further questions help assess the reliability of a supplier’s claim. Was the measurement performed on individual cells or on the complete pack? If it was a pack-level test, what was the cell-level voltage spread under the test conditions? Also ask for capacity (Ah) and, where power matters, energy (Wh) results separately from resistance data, since these can deteriorate at different rates and may lead to different end-of-life decisions.

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A normal voltage and a degraded capacity are not in conflict. A resting voltage within the expected range does not prove that a battery retains its original capacity or can meet its power requirements under load. Capacity tests, DCIR measurements and load-response observations answer different questions, so they should be interpreted together rather than used as substitutes for one another. When comparing battery suppliers, request capacity-retention data (in Ah, or Wh where delivered energy is the real constraint), DCIR test conditions and the applicable end-of-life criteria. These details help determine whether the reported results are comparable and relevant to your application.