Battery Health: What the Percentage Actually Means

Battery health is a measure of how much usable capacity your EV's battery still has, compared with when it was new. It is shown as a percentage called State of Health, or SoH. A car showing 92% SoH still has 92% of its original range available. The other 8% is gone for good, worn away by ordinary charging and driving, not any fault. Most EVs lose roughly 2 to 3% of their capacity a year. A healthy five-year-old battery in the high 80s or low 90s is completely normal. It is not a warning sign.

Why you are checking this number

The dashboard range estimate moves around constantly. It drops in the cold. It jumps after a fast charge. It swings with how you have been driving for the last hour. None of that is battery health. That is why so many owners end up searching for a battery health check. The number that actually matters sits a layer deeper than the one the car shows you by default.

The worry usually comes from one of two places. Either your range feels worse than it used to, and you want to know if that is normal ageing or a fault. Or you are buying a used EV, and the seller's word is the only evidence you have. Both are solved the same way: find the actual SoH figure, not the range estimate. It is one of the topics covered across our wider EV ownership and maintenance guides. Neither situation needs guesswork once you know where to look.

Why batteries lose capacity

The mechanism

Every charge and discharge cycle causes tiny, permanent changes inside a lithium-ion cell. This is normal chemical ageing. It is the same process that makes a phone battery hold less charge after two years. It cannot be stopped, only slowed. What is different about EV batteries, compared with phones, is scale and management. A large pack with active thermal control ages far more gently than a small, unmanaged one. That is a big part of why EV batteries typically outlast the rest of the car. Most drivers change their car long before its battery becomes the limiting factor.

What speeds it up

Geotab analysed over 22,700 electric vehicles across 21 models. It found an average annual degradation rate of 2.3%. That average hides a large spread, though. Frequent DC rapid charging above 100 kW roughly doubled the rate. Owners who mostly used slower charging saw about 1.5% a year. Those who leaned heavily on rapid chargers saw 3.0% a year. Heat made a measurable difference too. Cars used where temperatures topped 25°C on more than a third of days degraded around 0.4 percentage points faster each year. Heavy daily use added roughly another 0.8 points a year, compared with light use.

How you charge changes the number

Charging habits matter as much as any single factor. The RAC's advice for UK drivers is simple. Keep day-to-day charging between roughly 20% and 80%. Save 100% charges for long trips only. Charge overnight, when it is cooler, rather than in the day's heat. Do not leave a car sitting for weeks at a full charge. Closer to half charge is kinder, if it will not be driven for a while.

What is normal for your car's age and mileage

These brackets come from real fleet and ownership data, not a single guess. Recurrent's tracking of Tesla Model 3s shows around 95% retained at three years, and 93% at five, with a median of 87.4% among cars past 200,000 miles. The RAC cites an eight-year-old Nissan Leaf at average UK mileage typically showing around 20% loss, putting it near 80%. Geotab's wider dataset puts the average car at roughly 81.6% after eight years, at its 2.3%-a-year rate. A reading noticeably below its bracket is worth investigating. One comfortably inside it is not. Treat the table as a sense check, not a strict pass-or-fail line, since every car and climate is a little different.

What battery health means for what you paid

Most manufacturer warranties, including Tesla's, guarantee a minimum of 70% capacity for eight years. Fall below that floor while still inside the warranty term, and a claim usually restores the pack for free. Above the floor but still degraded, battery health does not affect a claim. It simply affects your usable range day to day, and eventually, what a buyer is willing to pay for the car.

That second effect is becoming easier to prove. A documented SoH reading is increasingly treated the way a service history is treated on a petrol car. It is evidence a buyer can check, rather than a claim they have to trust. Keep your own record of readings over time, even informally, well before you plan to sell. A single screenshot from today is worth far more at resale time than a vague memory of "it seemed fine".

When a health check is not what you need

A slow, steady decline across months or years is ordinary ageing. Monitoring it is all you need to do. A sudden drop is a different problem. Several percentage points in days or weeks, especially alongside a warning light, is not normal wear. It needs a dealer or battery management system diagnostic, rather than repeated home checks, because it usually points to a fault. Checking the number weekly will not change the outcome; checking it after any unusual symptom will.

It is also worth being clear about laptops and phones. Their battery percentage is a related idea, not the same measurement. Those devices use much simpler battery management than an EV. A laptop-style reading does not transfer across. Use your car's own tool, or an independent EV specialist, instead. Comparing the two directly is a common mistake, and it leads owners to worry about numbers that mean something different in each context.

What to do next

Find your car's actual State of Health reading this week, even if nothing feels wrong. That gives you a baseline to compare future readings against. If the figure sits outside the bracket for your car's age in the table above, get an independent opinion before assuming the worst.

Two related habits matter just as much. Learn how to look after an EV battery day to day. And remember that winter cold measurably reduces range on every EV, regardless of battery health, so a worse number in January is not automatically bad news. It is also worth reading why EV tyres wear faster than a petrol car's, since it is the other consumable that quietly costs owners money.

Sources

• Charged EVs — New Geotab Data Shows How Frequent DC Fast Charging Affects EV Battery Health
• RAC Drive — How Long Do Electric Car Batteries Last?
• Recurrent — Tesla Model 3 Battery Degradation
• Recurrent — Checking Tesla Battery Health: Measure, Monitor, and Protect