{"id":10506,"date":"2026-10-16T01:20:55","date_gmt":"2026-10-16T01:20:55","guid":{"rendered":"https:\/\/www.herewinpower.com\/blog\/battery-voltage-vs-capacity-normal-ocv-degraded-capacity\/"},"modified":"2026-10-16T01:20:55","modified_gmt":"2026-10-16T01:20:55","slug":"battery-voltage-vs-capacity-normal-ocv-degraded-capacity","status":"publish","type":"post","link":"https:\/\/www.herewinpower.com\/it\/blog\/battery-voltage-vs-capacity-normal-ocv-degraded-capacity\/","title":{"rendered":"Why Battery Voltage Can Look Normal Even When Capacity Has Degraded"},"content":{"rendered":"
\"A<\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

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\u2019s condition and cannot establish how much charge or energy the pack can still deliver.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

OCV, SOC and SOH: What Each Measurement Tells You<\/h2>\n\n\n\n

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.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Open-circuit voltage (OCV)<\/h3>\n\n\n\n

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\u2013SOC relationship is available for that cell chemistry and condition. The OCV\u2013SOC 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<\/a>.<\/p>\n\n\n\n

The relationship is not uniform along the curve, either. A 2026 open-access study found that a weak local gradient of the OCV\u2013SOC 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\u2013SOC 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.<\/p>\n\n\n\n

State of charge (SOC)<\/h3>\n\n\n\n

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.<\/p>\n\n\n\n

State of health (SOH)<\/h3>\n\n\n\n

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 \u00d7 100%<\/strong>, 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.<\/p>\n\n\n\n