Eighty Per Cent of What? Why a Used EV's Battery Screenshot Won't Survive a British Winter

There is a certain kind of used EV advert doing the rounds. A phone screenshot, a big green number — 87%, 82%, occasionally a triumphant 91% — and a caption along the lines of "battery health confirmed". It is designed to end the conversation. In November, with the windscreen iced up and the heater pulling hard, it tells you almost nothing.

The 80% figure is a warranty floor, not a certificate

Most manufacturers treat 80% state of health as the line where a battery might qualify for a warranty replacement. That makes it a pass mark, not a gold star. A car sitting at 81% is, by the letter of the warranty, fine. It is also a car that has lost nearly a fifth of its original usable capacity before you've driven it home.

Worse, the number itself is an estimate. Different cars calculate state of health in different ways. Some report usable capacity, some report total capacity, and some just show the last full-charge estimate from the BMS. A slow balance charge can nudge the figure up. A cold pack can drag it down. A screenshot taken in July is not a report, and an app that reads the car's own guess is not an inspection.

Winter is the honest test

Cold weather reduces usable range. How much depends on the car, the heater, the tyres and your right foot. A car that managed 200 miles in June might do 140 in January with the climate set to 21°C and the lights on. That is normal. It is not a fault. But it does mean an 80% battery in winter feels a lot smaller than an 80% battery in summer.

Look for a heat pump if you do a lot of motorway miles in the cold — it can cut the penalty, but it won't remove it. Check whether the car has preconditioning and whether it works on your home charger. Heated seats use less energy than a full cabin heater, which sounds trivial until you're trying to eke out the last 20 miles on the A1.

Start the test drive cold

Ask the seller not to pre-warm the car. Turn it on from cold, note the range estimate, reset the trip, and drive a route you know — a bit of town, a bit of dual carriageway, a few miles at 60. Put the heater on. Watch the consumption figure, not the guess-o-meter.

If the car has a winter range display or an energy monitor, use it. If it doesn't, calculate: miles driven divided by percentage used, then multiply by the usable capacity. It won't be exact, but it will be more honest than a screenshot. If you can, do a rapid charge mid-route and watch the charging curve. A car that tapers hard or throws a thermal warning is telling you something.

Charge limits and rapid-charge habits

Many EVs let you set a daily charge limit — usually 80% or 90%. A car that has lived on that setting is generally kinder to its pack than one that was topped to 100% every night and left there. You can't always see the history, so ask. A private seller with a home charger and a routine is a better bet than an ex-lease car that did 40,000 miles on motorway rapids.

Fast charging isn't the villain it's made out to be, but repeated high-rate charging on a car without good thermal management does add stress. If the car has a charging history in the app or the infotainment, look at it. If it doesn't, ask what the previous owner did. Check the maximum AC rate too — 7kW versus 11kW matters if you're paying by the hour at a public post.

A proper health report needs cell data

An OBD dongle and a free app can tell you a lot. They can also tell you very little. A proper EV inspection should include state of health, cell balance, module voltages and any stored BMS codes. A weak module can drag down the whole pack and trigger a limp mode or a reduced-range warning — often in cold weather, when the pack is already under pressure.

Pay for a specialist inspection or a manufacturer dealer health check. Ask for the battery's manufacturing date and, if available, the number of DC charge cycles. Get the results in writing. If the seller won't allow it, that's your answer.

Read the warranty small print

Most used-car warranties exclude gradual battery degradation. The manufacturer's battery warranty — often eight years or 100,000 miles — may still apply, but it usually only covers a drop below a threshold, often 70%, and it comes with conditions: servicing on time, sometimes an annual health check, sometimes a mileage cap.

If the car is six or seven years old, that safety net is nearly gone. Check whether the warranty transfers to you, and what it actually says about capacity loss. "Battery covered" in an advert is not the same as "battery replaced if it drops below 70%". Some warranties need to be registered in your name within days of purchase — do it.

The boring checks still count

EVs are heavy, so tyres wear faster and unevenly. Check the tread and the shoulders, and make sure it's not sitting on a set of cheap ditchfinders — an EV on bad rubber is a handful in the wet. Look at the brake discs, because regenerative braking means they can rust through lack of use. Check the 12V battery, because a flat auxiliary battery will strand an EV just as effectively as an empty traction pack. Check the charging port for wear, and make sure both cables are present. Check the service history for software updates and recalls.

The November advantage

Buying a used EV in winter is not a disadvantage. It's the most honest test you'll get. A car that behaves in a cold snap will behave in July. Take the screenshot as a starting point, insist on a proper report, and do the cold drive. If the numbers hold up when the heater is on, you've found a good one.

At BTC Motors, we'd rather see a cold-start range figure than a screenshot any day.