What Is EV Battery SOH? How to Check Battery Health and Understand Degradation

An electric vehicle can be fully charged and still have lost some of its original battery capacity. It may continue to drive normally for years, or it may develop performance issues that a simple charge percentage cannot reveal. That's where State of Health (SOH) comes in. If you own an EV, are planning to buy a used electric car, or simply want to understand how your battery is ageing, SOH is one of the most useful battery measurements to know. But there is a catch: a battery-health percentage doesn't always tell the whole story. Let's understand what SOH means, how it is calculated, and what you should actually look for when checking an EV battery.

1. What is SOH in an EV battery?

SOH stands for State of Health. It describes a battery's present condition relative to a defined reference condition, usually when the battery was new. Most commonly, SOH refers to how much charge-storage capacity the battery has retained as it ages. For example, imagine a battery that originally had a measured capacity of 100 ampere-hours (Ah). If its capacity under comparable test conditions falls to 90 Ah, its capacity-based SOH would be 90%. That means the battery has lost approximately 10% of its original measured charge capacity. It doesn't mean that 10% of its battery cells have failed, or that the vehicle is unsafe to drive. Some battery ageing is expected during normal ownership. One important detail: Not every manufacturer calculates or reports SOH the same way. While capacity-based SOH is common, battery health may also be evaluated using energy retention, internal resistance or other performance indicators. This is why comparing SOH percentages between two different EV models isn't always straightforward.

2. SOH vs SOC: What's the difference?

SOH and SOC might sound similar, but they answer completely different questions. State of Charge (SOC): How much charge is currently available relative to the battery's present full-charge capacity? State of Health (SOH): How much of the battery's original capacity or another defined health characteristic remains? Think of your EV battery as a water tank. SOC tells you how full the tank is. Capacity-based SOH tells you whether the tank can still hold as much water as it did when it was new. Imagine two identical electric cars:

Both vehicles indicate a full charge, but Car B has a lower measured capacity. If all other conditions are comparable, Car B may have less energy available for driving. The key takeaway: Charging a battery to 100% SOC does not restore it to 100% SOH.

3. How is EV battery SOH calculated?

One commonly used approach compares the battery's present maximum charge capacity with its reference capacity. Capacity-based SOH (%) = (Current measured capacity ÷ Reference capacity) × 100 Consider a simple example: A battery originally measured 120 Ah. After several years of use, a controlled assessment measures its current capacity at 108 Ah under comparable conditions. Its capacity-based SOH would be: (108 ÷ 120) × 100 = 90% In other words, it has retained approximately 90% of its reference charge capacity. But how does your EV know its SOH? Most electric cars have a Battery Management System, commonly called the BMS. The BMS monitors battery-related information such as voltage, current and temperature. Depending on its design, it uses these measurements along with stored data and estimation algorithms to assess battery condition. Some systems learn from charging and discharging behaviour. Others rely on mathematical battery models or additional diagnostic information. SOH is therefore often an estimated value, rather than a measurement taken through a complete capacity test each time you start the car. That distinction matters. A BMS-reported SOH can be useful, but its reliability depends on the quality of the underlying measurements, the estimation method and the conditions under which the battery has been operating.

4. What does 90% SOH actually mean?

Let's say your electric car reports 90% SOH. Should you be concerned? Not necessarily. If the reading represents capacity-based SOH, it generally suggests that the battery has retained around 90% of its defined reference charge capacity. However, the percentage alone doesn't tell you whether the battery is functioning properly. Consider these illustrative examples:

These aren't universal health grades or automatic pass/fail thresholds. A battery reporting 90% SOH might still perform reliably for daily commuting. Another battery with a similar reported SOH could have charging issues, weak cell groups or active fault codes requiring attention. Battery replacement decisions should not be made using SOH alone. Some EV manufacturers use specific capacity-retention thresholds in their battery warranty terms. Those thresholds vary by manufacturer, model, market and warranty conditions. The better question isn't simply whether your SOH is 90% or 95%. It's whether your battery still delivers the energy, power and reliability you need.

5. What causes EV battery SOH to decrease?

EV batteries don't suddenly become old after a particular mileage. They age gradually through chemical and physical changes inside their cells. Two important forms of ageing are calendar ageing and cycle ageing. Calendar ageing: Even when you're not driving An EV battery can age while the vehicle is parked. Temperature, time and the charge level at which the battery is stored can all influence this process. For example, repeatedly leaving a battery at very high SOC in a hot environment may accelerate certain ageing mechanisms, depending on the battery chemistry and thermal management. Cycle ageing: Changes caused by usage Every charge and discharge cycle contributes to the battery's operating history. The rate of degradation depends on factors such as battery chemistry, depth of discharge, operating temperature and charging or discharging currents. This does not mean charging frequently is automatically harmful. Charging behaviour must be considered in context. For example, routine shallow charging may place different demands on a battery than repeated deep discharge cycles. Does fast charging reduce SOH? High charging currents can increase battery stress under certain conditions, particularly when the battery is hot, very cold or near charging limits. But it would be misleading to claim that every DC fast-charging session causes a fixed percentage of degradation. Modern EVs manage charging power based on factors such as temperature, SOC and battery protection limits. The effect of fast charging depends on the vehicle and its battery management strategy. Does mileage determine battery health? Mileage matters because it can reflect battery usage, but it doesn't tell the whole story. Two electric cars with the same odometer reading may have different battery conditions. Their operating temperatures, charging histories, battery chemistries and usage patterns may be different. That's one reason why battery health should be assessed independently rather than estimated from age or mileage alone.

6. How can you check SOH in an electric car?

This is where things become practical. Unlike SOC, which is usually visible on the dashboard, SOH may not be directly displayed in your vehicle. There are three main ways to investigate it. Method 1: Manufacturer diagnostic tools An authorised service centre may be able to retrieve battery-health information using manufacturer-specific diagnostic equipment. Depending on the vehicle, this may include BMS-reported SOH, battery status information and diagnostic trouble codes. Ask what the reading represents and whether it is an estimate or derived from a specific test. Method 2: Compatible OBD diagnostic tools Some vehicles expose battery-related information through their diagnostic interface. A compatible tool may retrieve SOH, SOC, cell-group voltages, temperatures and other parameters supported by the vehicle. However, not every OBD scanner can read EV battery SOH. Battery parameters often use manufacturer-specific protocols and identifiers. Access may also depend on diagnostic security permissions. A scanner's ability to connect to the car doesn't guarantee that it can accurately read or interpret SOH. Method 3: Controlled battery capacity testing A capacity test measures how much charge a battery can deliver over a defined operating window under specified conditions. This can provide more direct evidence of remaining capacity than a standalone BMS estimate. However, proper testing requires suitable equipment and controlled procedures. A short drive or a single charging session generally cannot establish the battery's full usable capacity with high confidence. An independent diagnostic assessment can combine accessible battery data with other observations, while clearly stating the limits of what was actually tested.

7. Can EV battery SOH readings be inaccurate?

Yes. SOH is often estimated rather than directly measured, and estimates carry uncertainty. Imagine your EV reports 94% SOH today and 95% after a period of different charging and driving behaviour. Does that mean the battery has physically recovered 1% of its original capacity? Not necessarily. The BMS may have updated its estimate as it gathered additional information. Possible reasons for a changing SOH reading include: Recalibration of the battery estimation algorithm. Additional charging or discharging data becoming available. Changes in temperature or measurement conditions. Differences in how the diagnostic tool interprets the value. Software or battery management updates. This is why repeated battery-health measurements should ideally use comparable conditions and consistent diagnostic methods. A small change in reported SOH doesn't automatically prove a corresponding physical change in battery capacity.

8. Can two EVs have the same SOH but different battery conditions?

Yes — and this is one of the most important things to understand about EV battery diagnostics. Imagine two electric cars, both reporting 93% SOH. Vehicle A: 93% SOH Its diagnostic assessment shows relatively consistent cell-group voltages under the observed conditions, no relevant active battery faults and no unexpected power restrictions during the assessment. Vehicle B: 93% SOH Its diagnostic assessment shows unusually large cell-group voltage differences under comparable conditions, relevant battery fault history and unexpected power restrictions. The reported SOH is the same. But the diagnostic findings suggest that the vehicles may need very different levels of attention. Why? Because SOH doesn't necessarily describe every important characteristic of battery performance. A battery pack contains many cells or cell groups working together. Differences between those groups may affect how the entire pack performs. An ageing or abnormal group can sometimes restrict usable energy or power even when the battery's overall SOH estimate remains relatively high. This is also why cell-voltage differences need careful interpretation. A voltage difference observed during heavy acceleration may not mean the same thing as a similar difference measured after the battery has rested. SOC, load, temperature and battery design all influence the result. Same SOH doesn't always mean same battery condition.

9. Is SOH enough to determine EV battery health?

No. It is an important measurement, but it should be considered alongside other relevant data.

For instance, a battery may retain much of its original capacity but show elevated resistance that limits power delivery. Another battery may have reduced capacity but otherwise operate consistently within its intended limits. Neither condition can be fully understood through a single SOH percentage. A meaningful diagnostic assessment considers the available measurements, their operating context and their limitations.

10. When should you check your EV battery health?

You don't need to wait for an error message to learn about your battery's condition. A battery-health assessment may be particularly useful: Before buying a used electric car Battery condition is an important part of assessing a used EV. A diagnostic report can provide additional information beyond the vehicle's age, odometer reading and dashboard range estimate. When your driving range changes unexpectedly A noticeable range reduction doesn't automatically mean battery degradation. Driving conditions, temperature, tyre pressure and auxiliary energy consumption may also explain the change. Battery diagnostics can help investigate whether capacity loss or another issue may be contributing. When you experience unexpected SOC changes Unusual changes in reported charge level, especially when accompanied by warnings or power restrictions, deserve further investigation. When charging behaviour changes Repeated charging interruptions, unexplained charging limitations or relevant fault warnings may indicate a need for professional diagnosis. When you want to understand battery ageing over time Comparable assessments can help establish trends instead of relying on a single reading. Safety note: If your EV displays a serious high-voltage warning, experiences repeated unexpected shutdowns or exhibits other potentially hazardous symptoms, follow the manufacturer's safety guidance and seek qualified assistance. Do not attempt to open or test high-voltage components yourself.

11. What should a good EV battery health report tell you?

If you're paying for a battery-health assessment, don't judge the report solely by how many numbers it contains. Ask whether it explains what those numbers actually mean. A useful report should identify the diagnostic information available for your vehicle, highlight relevant abnormalities and distinguish confirmed findings from issues that require further testing. It should also explain the test conditions and limitations. For example, a BMS-reported SOH reading is not the same as a physically measured capacity result. A momentary voltage difference is not automatically evidence of a defective cell. And the absence of a fault code doesn't guarantee that every battery component is healthy. Understanding these distinctions makes the report more useful when deciding whether to buy, maintain or further inspect an electric vehicle.

Read more: How to Check EV Battery Health: What SOH Can Tell You?