How to Check EV State of Health: What SOH Means and How It's Actually Measured

Your EV's State of Health is the single number that determines what the car is worth, whether it will meet your range needs, and whether it is heading toward a warranty claim. Yet most owners have never seen a reliable figure for it, only a range estimate that shifts week to week. This guide explains what State of Health actually is, the four ways to check it, and why the method you choose changes how much you can trust the answer.

What State of Health actually means

State of Health, usually written SOH, is your battery's current usable capacity expressed as a percentage of its capacity when new. A battery at 100 per cent SOH holds exactly what it did the day it left the factory. A battery at 90 per cent holds ninety per cent of that, which shows up in the real world as roughly ten per cent less range.

Degradation is normal and expected. Every lithium battery loses capacity gradually as it ages and cycles, and a healthy EV battery loses that capacity slowly. The point of checking SOH is not to find a perfect number, it is to know where your specific battery sits on that curve, because that position determines the car's value, its remaining useful life, and its warranty status.

SOH is different from state of charge, which people often confuse it with. State of charge is how full the battery is right now, like a fuel gauge, and it changes constantly as you drive and charge. State of Health is how large the tank is in the first place, and it changes only slowly over years. A battery can be at 100 per cent state of charge and 82 per cent State of Health at the same time: completely full, but of a tank that has shrunk.

What causes State of Health to decline

Four factors drive battery degradation, and understanding them helps you interpret any SOH figure you get.

Time alone degrades a battery even if you never drive it, through a slow chemical process called calendar ageing. Charge cycles add wear, so higher-mileage cars generally show more degradation. Heat accelerates everything, which is why EVs in consistently hot climates tend to degrade slightly faster than the same model in temperate conditions. And charging habits matter at the margins: regular DC fast charging and routinely charging to 100 per cent and running to near zero both add stress compared to gentler daily charging in the middle of the range.

Under normal conditions, fleet data from Geotab puts average EV degradation at roughly 1.8 to 2.3 per cent per year. Battery chemistry changes this: LFP batteries, used in BYD's Blade Battery and some Tesla Standard Range variants, typically degrade more slowly, around 1 to 1.5 per cent per year, than the NMC chemistry used in most longer-range EVs.

Method 1: The dashboard estimate

Every EV shows battery information on its dashboard, usually as an estimated range at the current charge level. Charge to a known percentage and compare the range shown against the car's rated range when new.

This is the easiest check and the least precise. The range figure is the car's own estimate, influenced by recent driving style, temperature, and the algorithm's assumptions, and it can swing noticeably without the battery physically changing. It gives you a rough directional sense, nothing more. Treat a dashboard range reading as a hint that something might be worth investigating, not as an answer.

Method 2: The manufacturer app and monitoring tools

Manufacturer apps expose more than the dashboard on some brands, and third-party monitoring apps like Tessie for Tesla go further, tracking SOH trends over time and displaying the battery management system's own health estimate.

These are genuinely useful for watching your battery over the months and years you own the car. The limitation is what they are reading: the BMS estimate. The battery management system continuously estimates SOH for the purpose of operating the car safely, and that estimate is reasonable but not exact. It drifts, it recalibrates after certain charge cycles, and it can move by several percentage points without any physical change to the battery. For tracking a trend on your own car, it is fine. As a definitive figure, it is soft.

Method 3: OBD2 diagnostic tools

An OBD2 adaptor plugged into the diagnostic port, paired with an app like Car Scanner or Leaf Spy Pro, reads SOH data directly from the battery management system. This is more direct than the dashboard, gives an actual percentage, and costs little beyond a cheap adaptor.

The catch is twofold. Compatibility varies significantly by make and model, so a tool that works on one car may return nothing on another. And you are still reading the BMS self-assessment, just accessing it more directly, so it carries the same drift and recalibration caveats as Method 2. For a technically confident owner willing to source the right adaptor, it is a solid step up from the dashboard, but the number it produces is still the car grading its own exam.

Method 4: An independent API-measured audit

The methods above all read the car's own self-reported estimate. An independent audit measures the battery instead, by observing what it actually does during real charging.

Here is the difference in plain terms. Instead of asking the battery management system what it thinks the SOH is, an API-based audit watches a real charging session through the manufacturer's official data connection, measures how much energy actually went into the battery, and measures how far the state of charge moved as a result. Those two figures, corrected for the energy every charge loses to heat and electronics, reveal the battery's true current capacity independently of what the BMS reports.

EV TrustSeal works this way. You connect your car's manufacturer account online, free, in about 2 minutes through the Smartcar API, then drive and charge normally. The audit measures across your ordinary charging sessions and produces a State of Health figure accurate to plus or minus 2 to 4 per cent, with that accuracy range stated openly on the result. Most results are ready within hours, 48 at the outside, and the full process is walked through in the EV battery audit from home guide.

The reason this method matters beyond accuracy: it is the only one that produces a result someone else can trust. The dashboard, the app, and the OBD reading all live on your side of the transaction. An independent measured audit produces a figure that a buyer, an insurer, or a warranty assessor can rely on precisely because you did not generate it yourself, a distinction explored in full in why the buyer doesn't care what your dashboard says.

What the number actually means

Once you have an SOH figure, here is how to read it against the grade bands used on TrustSeal certificates.

95 per cent and above is EXCELLENT: minimal degradation, performing close to new. 85 to 94 per cent is STRONG: normal, healthy wear for most vehicles a few years old. 75 to 84 per cent is MODERATE: noticeable degradation that affects range and resale value, worth factoring into decisions. 60 to 74 per cent is DECLINING: significant degradation, and warranty position is worth checking. Below 60 per cent is REPLACE territory: the battery is near the end of its useful life.

Read the number against the car's age, mileage, and chemistry, not in isolation. A three-year-old NMC vehicle at 93 per cent is performing normally. A seven-year-old LFP vehicle at 90 per cent is doing well for its age. The same 90 per cent means different things on different cars, which is why context matters as much as the figure.

FAQ

How often should I check my EV's State of Health?

For general peace of mind, once a year is plenty, since degradation is slow. Check it specifically before selling, before a warranty period ends, or if you notice range dropping faster than expected.

Is the SOH on my dashboard accurate?

It is a reasonable estimate for operating the car, but it drifts and recalibrates and should not be treated as a precise figure. For anything that matters financially, a measured independent reading is worth the difference.

Can checking my State of Health damage the battery?

No. Every method here is read-only, including the API-based audit, which uses the manufacturer's own read-only data connection and cannot change any vehicle setting or affect the battery in any way.

What is a good State of Health for a used EV?

For a car a few years old, anything in the STRONG band of 85 to 94 per cent is healthy and normal. Below 80 per cent on a relatively young car warrants a closer look at charging history and warranty position.

Why do different tools give me slightly different numbers?

Because most of them read the same BMS estimate at different moments, and that estimate drifts. An independent measured audit reduces this by observing actual charging behaviour rather than reading the car's self-assessment, and states its accuracy range so you know the margin.

Know the number, then know what to do with it

Checking your EV's State of Health is easy at the rough level and worth doing properly when it matters. The dashboard tells you roughly. The BMS tools tell you more precisely but still through the car's own eyes. An independent audit measures what the battery actually does and produces a figure you, and anyone else, can rely on.

If you want a measured, independently verifiable State of Health reading for your EV, getting a TrustSeal audit is the right move. Connect your car free at evtrustseal.com.