The most expensive part of a used electric car is the one you cannot see
Mileage tells you almost nothing here. The number that matters is one the seller has probably never looked up.
On a petrol car, the odometer is a rough proxy for everything you cannot see. On an electric car it is not, because the part that carries most of the value does not wear out by distance. A battery ages by the calendar, by heat, by how full it has been kept and by how much energy has passed through it, and two identical cars with identical mileage can sit ten percent apart on capacity because one lived in an underground car park in Ghent and the other spent four summers parked at 100 percent on a driveway in Andalusia. That gap is not a detail. On a five-year-old family EV it is the difference between a car that still does its old commute and one that needs a charging stop it never used to need, and it is worth more than a full service history. The number that describes it is called state of health, it is a percentage, and there are four ways to get it out of the car before you hand over any money.
What state of health actually measures
State of health is the usable capacity the pack holds today, expressed as a percentage of the capacity it held when it was new. A car sold with a 58 kWh usable battery that now measures 52 kWh is at roughly 90 percent, and its range at any given speed and temperature has fallen by about the same proportion. What it does not measure is power, and the two are separate: a pack can hold 90 percent of its energy and still deliver full acceleration, or hold 95 percent and have lost a chunk of its peak charging speed because a few cells have drifted out of line with the rest. Degradation also does not run in a straight line. Most electric cars lose a visible slice in the first year or two as the pack settles, then flatten out, which is why a jump from 100 to 96 percent in eighteen months alarms owners far more than it should while a slow drift from 92 to 88 over the following four years goes unnoticed. Large fleet studies now put average loss at under two percent a year across modern EVs, so the arithmetic to carry into a viewing is simple: expect roughly ten percent gone by year five and something in the high eighties by year eight. A car meaningfully below that line has a story, and you want to hear it.
The four ways to find the number, from free to definitive
The first is the car itself. A growing number of models will simply tell you, in a service or battery menu buried a few taps deep, and on cars type approved under the newest European rules the manufacturer has to make that figure available to the driver rather than only to a dealer. Find out before the viewing whether the model you are looking at has such a menu, because knowing which three taps to make while the seller watches is worth more than any amount of confident chat. The second is an OBD reader. A dongle costs less than a tank of fuel, plugs into the port under the dashboard and, paired with an app that has a profile for the make, reports capacity, cell voltages and often the total energy the pack has ever delivered. It is the single best forty euros a used EV buyer can spend, and it works on almost everything with a socket. The third is a manufacturer battery certificate, which several brands will issue through a dealer, sometimes free with a service. It carries weight in a negotiation precisely because it comes from the people who would have to honour the warranty. The fourth is an independent test from a specialist such as AVILOO in Austria or Volytica in Germany, which either reads the pack quickly through the OBD port or, in its thorough form, records a controlled drive from full down to nearly empty and issues a certificate. That is the definitive answer, it costs real money, and on a car priced in the tens of thousands it is cheap insurance.
Why two tools can give two different answers on the same car
Buyers regularly find one app saying 94 percent and another saying 89, and conclude that one of them is broken. Usually neither is. Every pack is sold with a buffer, a slice of capacity the software keeps in reserve at the top and bottom to protect the cells, so a car advertised with 77 kWh may hold 82 kWh gross. Some tools report against the gross figure and some against the usable one, and the same battery will score differently depending on which yardstick the app picked. The battery management system is also making an estimate rather than a measurement, and that estimate drifts until the car sees a full charge or a deep discharge to recalibrate. A car that has spent six months shuttling between 40 and 60 percent can read optimistically, which is why a reading taken straight after a charge to 100 percent is the one to trust. Cars with lithium iron phosphate chemistry are the awkward case, because their voltage barely changes across most of the range and the software has less to work with, which is exactly why their makers ask owners to charge to 100 percent every week or two. The rule that keeps you out of trouble is to compare like with like: one tool, the same state of charge, and if you are choosing between two cars, the same test on both.
What ages a pack, and why the high-mileage car is often the healthy one
Heat and time at a high state of charge are the two big villains, and neither leaves a mark on the odometer. A car kept plugged in at 100 percent in a hot climate ages faster standing still than a motorway car doing 40,000 km a year at moderate charge levels, which is the opposite of the instinct every used car buyer has trained for two decades. Rapid charging matters less than its reputation suggests on any car with proper liquid cooling, and the fleet data backs that up, but a car whose whole life was DC charging because its owner had no home socket has still had a harder time than one that trickled overnight. Ask where it was charged rather than how often it was charged fast. Chemistry plays in too: lithium iron phosphate packs shrug off being kept full and generally age gently, while the nickel-based chemistries in most longer-range cars prefer to live between 20 and 80 percent. And ask about the software, because a manufacturer that has quietly revised its charging strategy or recalled modules will have changed what the pack does, and the way a car preconditions its battery before a fast charge often changes with it.
The checks that matter more than the last two percent
Once you are within a few points of what is normal for the age, stop haggling over the decimal and look at three other things. Cell balance is the first, and any decent OBD app will show it: a healthy pack has its cells within a few tens of millivolts of each other, and a wide spread points at a module going bad, which is a far more expensive problem than gentle capacity loss. Charging behaviour is the second. Take the car to a fast charger during the test drive, arrive with the battery below 30 percent and watch the peak power it accepts. A car that should pull 100 kW and settles at 50, on a warm day, with no queue and a charger you have seen working, is telling you something the capacity figure did not. The third is recall and repair history. Several manufacturers have replaced modules or capped charging in software while they investigated a fault, and you want to know whether the car in front of you had the work done, whether the pack is the original one and whether any cap was ever lifted. A pack replaced under warranty is good news, not bad, as long as you have the paperwork proving it and the date it happened, since the clock on the replacement usually runs from the original registration rather than from the repair.
The paperwork: warranty, battery leases and what Europe now demands
Almost every electric car sold in Europe carries a separate battery warranty, typically eight years or 160,000 km, whichever arrives first, and it transfers to you as the second owner. Read what it actually promises, because it does not cover ordinary degradation. It covers falling below a stated floor, and that floor is usually 70 percent, which means a pack sagging to 74 percent is disappointing but entirely uncovered. Note the date it expires and treat any car within a year of that date as a car whose remaining protection is worth little. Then check whether the battery is even included in the sale. A generation of Renault and Nissan models sold across France, Belgium, the Netherlands and Germany came with the battery on a monthly rental, which cuts the purchase price and adds a bill that follows you for as long as you own the car, and the contract has to be formally transferred rather than simply inherited. It is not automatically a bad deal, since the rental normally came with a guarantee to replace the pack if it dropped below about 75 percent, but you need to know which kind of car you are buying before you compare its price with anything else. The rules are moving in the buyer's favour, too: European type approval requirements arriving with the next emissions standard oblige new cars to hold a minimum share of their original capacity at five and at eight years, and to report their own state of health, which over the next decade will turn today's detective work into a figure printed on the screen.
What to actually do
Half an hour of preparation and a cheap dongle turn the biggest unknown in a used electric car into a number you can negotiate with. Do it in this order, and do the physical checks on the same walk round, including how old the tyres actually are, since electric cars are harder on them than most owners expect.
Ask the seller to have the car charged to 100 percent for the viewing. It gives the battery management system a fresh calibration and it makes every reading you take afterwards worth trusting.
Look up, before you go, whether that model shows state of health in its own menus, and buy an OBD dongle and the app that supports the make. Budget about forty euros for both.
Compare the reading against the age rather than the mileage. Under two percent a year is normal, so roughly 90 percent at five years and the high eighties at eight.
Check the spread between the highest and lowest cell voltage in the app. Tens of millivolts is healthy, a wide gap is a module problem and a reason to walk.
Take the test drive to a fast charger with the battery under 30 percent and watch the peak power. A car well short of what its model should accept is hiding something the percentage did not show.
Find the battery warranty expiry date and the capacity floor it protects, usually 70 percent, and confirm it transfers to you.
Establish whether the battery is owned or rented, and if it is rented, get the transfer of the rental contract agreed in writing before you pay.
On anything expensive, pay for an independent battery certificate. It costs a fraction of one percent of the car and it is the only reading a seller cannot argue with.
Frequently asked questions
What state of health should I expect for the age of the car?
Modern electric cars average a little under two percent capacity loss a year, and the curve is front loaded, so a car that dropped three or four percent in its first eighteen months and then barely moved is behaving exactly as designed. As a working expectation, look for the low to mid nineties at three years, around 90 percent at five and the high eighties at eight, with cars from hot climates and cars that lived at a fast charger sitting at the lower end. Very early electric cars with air cooled packs are the exception and age considerably faster, which is why a decade-old EV has to be judged on its measured capacity alone rather than on any rule of thumb. What matters more than the absolute number is whether it fits the car's history. A three-year-old car at 88 percent is not a disaster in itself, but it is a car you want an explanation for before you agree the price.
Does frequent fast charging ruin a used battery?
Far less than most buyers assume, provided the car has a liquid cooled pack, which almost everything sold in the last several years does. Large sets of real-world data show only a modest difference between cars that charged mainly on DC and cars that charged mainly at home, because the thermal management is doing exactly the job it was designed for. The real damage comes from heat with nowhere to go: repeated fast charges on a hot day in a car with a passive pack, or a car left sitting at 100 percent in the sun afterwards. So the question to ask a seller is not how many rapid charges the car has had, but whether it spent its life parked full and hot. If you want to understand what actually governs charging speed on a healthy car, the charger itself is rarely the deciding factor.
Can I just look at the range shown on the dashboard at 100 percent?
No, and it is the most common mistake at a viewing. The predicted range on almost every electric car is calculated from recent consumption, so a car that has spent a fortnight on short cold trips will show a pessimistic figure and one that has been pottering around town in mild weather will flatter itself, on the same battery, on the same day. Winter alone can move that number by a quarter without any capacity having been lost at all. Some cars offer a fixed, rated range display that ignores recent driving, and that one is a usable rough proxy, but only against the same model's original rated figure. If all you have is the guessed range, treat it as a conversation starter and then go and read the actual capacity, because the difference between a car at 95 percent and a car at 85 percent is thousands of euros and the dashboard will not reliably tell you which one you are standing next to.
Is the battery warranty really transferred to the second owner?
In Europe, yes, as standard. Battery warranties run with the vehicle rather than with the person, they are typically eight years or 160,000 km whichever comes first, and they survive a private sale. Two conditions are worth verifying rather than assuming. The first is the servicing requirement: many manufacturers make the warranty conditional on the car having had its scheduled maintenance at an approved workshop, so a gap in the service book can cost you the cover. The second is the threshold. The warranty replaces or repairs a pack that falls below a stated percentage of its original capacity, commonly 70 percent, and gradual loss above that line is considered normal wear and is not a claim. Ask for the warranty booklet, find the paragraph, and check the expiry date against the date of first registration rather than the date the current owner bought the car.
The car I am looking at has a rented battery. Should I avoid it?
Not automatically, but price it properly. A rented battery lowers the asking price and replaces it with a monthly payment that continues for as long as you keep the car, often tiered by the annual mileage you commit to, so the honest comparison is the purchase price plus the rental over the years you plan to own it against the price of an equivalent car with the battery included. The upside is genuine: these contracts normally guarantee to repair or replace the pack if it drops below roughly 75 percent of capacity, which removes precisely the risk this whole article is about. The catch is administrative. The rental agreement is with the finance arm, not the seller, and it has to be formally transferred into your name, which means the sale is not complete until that transfer is approved. Never hand over money on a promise that the paperwork will follow, and if the seller cannot produce the contract, treat the car the same way you would treat one with missing registration documents.
