A battery can report a full charge and still deliver less runtime, more voltage sag, or less peak power than it did when new. That is why State of Health (SOH) is more useful than a simple capacity label—but also easier to misunderstand.
SOH is not one universal physical measurement. It is an estimate of how a battery’s current condition compares with a defined new or reference condition. Capacity is often the headline input, but a meaningful health assessment can also involve resistance, power capability, usable energy, efficiency, temperature behavior, and cell balance.
The short answer: SOH is a comparison, not a single sensor reading
A common capacity-based expression is:
SOH = present maximum capacity ÷ reference capacity × 100
If the reference is 100 amp-hours and testing estimates 90 amp-hours available under the specified conditions, capacity-based SOH is 90%.
That number only has meaning alongside its reference and test conditions. “Reference capacity” might mean the manufacturer’s rated capacity, a measured beginning-of-life capacity, or a value stored by a battery-management system. Temperature, charge and discharge rate, cutoff limits, rest time, and the test method can all affect the result.
SOH is therefore best understood as a model output based on observable behavior—not a quantity that a handheld meter reads directly. Battery Design’s breakdown of capacity, resistance, power, and energy SOH shows why different systems may use different health sub-metrics.
What capacity-based SOH actually tells you
Capacity-based SOH, sometimes called SOH-C, answers an energy-duration question:
How much charge can the battery hold and deliver under the defined test conditions compared with the reference battery?
It is useful for estimating runtime and identifying gradual capacity fade. If a battery’s usable capacity has fallen, a load that once ran for ten hours will not run for ten hours under the same conditions.
But capacity is not the same as instantaneous performance. A battery may retain much of its former amp-hour capacity while its internal resistance has increased. It can still store a substantial amount of energy, yet show a larger voltage drop when a motor, inverter, winch, or other high-current load starts. It may reach a low-voltage limit sooner even though a slow capacity test looks acceptable.
Capacity-based SOH also does not automatically describe the energy available to your equipment. Watt-hours depend on voltage throughout the discharge as well as amp-hours, and the usable window may be narrowed by temperature limits, protection settings, or the application’s minimum voltage.
The dimensions that capacity leaves out
Internal resistance and voltage sag
Internal resistance describes opposition to current inside the cell or pack. As resistance rises, the terminal voltage drops more under load and some input energy is dissipated as heat. The result can be weaker acceleration, earlier low-voltage shutdown, reduced charging acceptance, or less useful output during a short high-current event.
A resistance-oriented indicator compares present resistance with the new-cell reference. It is not interchangeable with capacity SOH: one describes how much charge is available; the other helps describe how readily the battery can deliver current.
Power capability
Power capability is the battery’s ability to accept or deliver a specified amount of power while staying within voltage, current, temperature, and safety limits. A battery can have adequate energy for a long trip or overnight load but inadequate power for a brief peak demand.
This is why some battery systems track a state-of-power estimate separately from capacity SOH. The practical question is not only “How long will it run?” but also “Can it meet the load when it needs to?” Battery Design notes that increasing resistance reduces voltage under a given current and can reduce charge and discharge power capability.
Usable energy and efficiency
Stored charge is measured in amp-hours; energy is measured in watt-hours. They are related, but they are not identical. A battery’s voltage changes during operation, and conversion losses occur in the battery, cables, and power electronics.
A health assessment focused on energy asks how many watt-hours can be delivered within the application’s permitted operating window. Efficiency asks how much energy is lost between charging input and useful output. These can matter more than nominal capacity in stationary storage, where round-trip energy and predictable backup duration are central decisions.
Temperature behavior and thermal margin
Temperature is not usually a single “health percentage,” but it strongly affects what the battery can safely do at a given moment. Cold and heat can change available capacity, resistance, charging acceptance, and protection limits. A battery that performs well in a moderate-temperature test may have less available power in a cold installation.
Repeated operation at stressful temperatures can also contribute to aging. For interpretation, separate two questions: is today’s limitation caused by the current temperature, or does it remain after the battery returns to a suitable test condition?
Cell imbalance and the weakest-cell effect
A pack is made from cells that do not age in exactly the same way. One weaker or more imbalanced cell can reach a voltage limit before the rest of the pack, restricting the pack’s usable capacity or triggering protection.
A pack-level SOH percentage can hide that distribution. When the system provides cell-level voltage, temperature, or diagnostic information, it can reveal whether the problem is broad, gradual degradation or one cell group limiting the pack.
SOH is not the same as State of Charge
State of Charge (SOC) is the estimated amount of charge remaining now. SOH describes how the battery compares with its reference condition over its life.
A battery can show 100% SOC because it has reached its present full-charge point while having only 90% of its original capacity. In that case, “full” means full for the battery as it exists today—not full as it was when new. Conversely, a high-SOH battery at 20% SOC may have plenty of health but little charge available at that moment.
For a useful runtime estimate, both matter: SOC tells you how much of today’s available capacity remains, while SOH helps define what “100%” currently represents. A practical explanation of the SOC/SOH distinction also emphasizes that SOH is calculated from measurable characteristics rather than directly read by a meter.
Why two SOH percentages may disagree
Different numbers do not necessarily mean one instrument is broken. Ask four questions before comparing them:
- What is the metric? Is it capacity, energy, resistance, power capability, or a composite estimate?
- What is the reference? Is the comparison against the rated specification, a commissioning test, or a learned value?
- What were the conditions? Note temperature, current, voltage limits, SOC range, and whether the battery rested before testing.
- What is the scope? Is the percentage for one cell, a module, the whole pack, or the usable system after protection limits?
A dashboard may present one simplified SOH value even though its BMS uses several models internally. The label alone does not tell you which dimension dominates the estimate.
How to interpret SOH for a real application
Start with the failure you are trying to explain:
- Runtime is shorter: prioritize a controlled capacity or usable-energy comparison.
- Voltage drops during peaks: investigate resistance, current, cable connections, and power limits—not capacity alone.
- The pack shuts down early: check cell spread, temperature, voltage limits, and protection history.
- Charging takes longer or is restricted: check temperature, charging power limits, resistance, and the charger’s behavior.
- A percentage changed suddenly: check whether the BMS recalibrated its estimate, the operating conditions changed, or a cell-level issue appeared.
Then compare like with like. Use the same equipment, limits, temperature range, and load profile when trending a battery over time. A single SOH value is less informative than a trend tied to the symptom and the operating conditions.
For safety-critical or high-value systems, rely on the battery manufacturer’s specified diagnostic method or a qualified service procedure. Do not treat a phone app percentage or open-circuit voltage as a complete health certificate.
What SOH cannot tell you by itself
SOH alone cannot promise a particular runtime, starting performance, remaining service life, or safety status. It does not identify every fault, and it does not establish a universal replacement point. End-of-life is an application decision: a battery may be unsuitable for a high-power load while still useful for a lower-power task, or it may require removal because of a specific fault even when a capacity estimate looks favorable.
The most useful reading is a matched set of evidence: the health metric’s definition, current SOC, temperature, load and charging behavior, protection events, and—when available—cell-level data.
Bottom line
State of Health measures more than how many amp-hours a battery retains, but it does not automatically measure everything at once. Capacity-based SOH describes storage potential; resistance and power capability describe delivery; usable energy, efficiency, temperature response, and cell balance explain how that potential behaves in a real system.
When you see an SOH percentage, first ask what it is comparing, under which conditions, and which practical question it is intended to answer. That turns a vague “battery health” number into a useful diagnostic clue.
