Forecourt Detective: How to Judge a Used EV's Battery in Five Minutes Flat

Used electric cars in 2026 are no longer a niche curiosity. They are on ordinary forecourts, in ordinary adverts, with ordinary finance quotes attached. That is good news for buyers, but it puts a new burden on them: the battery is the single most expensive part of the car, and it is also the part most likely to be misunderstood. A shiny paint job tells you nothing about how the previous owner treated the cells under the floor.

The good news is that you do not need a degree in electrochemistry to spot trouble. You need five minutes, a little scepticism and a phone. This is the forecourt routine we would use before signing anything.

The battery is the car

On a petrol car, a tired engine is annoying. On an EV, a tired battery is the difference between a usable car and an expensive ornament. State of health, or SoH, is the headline number: it compares the battery's current usable capacity with its original capacity. A car at 90% SoH has lost roughly a tenth of its capacity, though the range loss you actually feel can be worse in winter, at motorway speeds, or with the heater on full blast.

Do not treat SoH as a simple linear countdown. Degradation curves are usually steeper early in life and then flatten out. A car that has dropped from 100% to 93% in three years is not necessarily heading for 50% in another three. But a car with no report, a suspiciously high reading, or a dealer who cannot explain where the number came from deserves extra scrutiny.

The five-minute forecourt routine

Minute 1: paperwork and VIN

Ask for the VIN and check it matches the advert, the service book and any battery report. Confirm the battery warranty is transferable and still in date. A typical UK EV battery warranty runs for eight years or 100,000 miles to around 70% capacity, but the devil is in the terms. Some warranties cover capacity loss, some only cover outright failure. Some require main-dealer servicing. Some exclude gradual degradation altogether.

Minute 2: the state-of-health report

Ask the dealer to show a state-of-health report from the car's own app or a diagnostic tool. A screenshot is better than nothing; a live screen in the car is better. If the dealer cannot produce one, ask why. A private seller may not have the kit, but a retail forecourt should. Cross-check the reported range with the dashboard and with the car's official figures, remembering that official range numbers are optimistic.

Minute 3: range, temperature and tyres

The range guess-o-meter is not a promise. Compare it with what you know about the model, the weather and the tyres. A car showing a suspiciously low range on a cold day may simply be cold. One showing a suspiciously high range may have been reset or driven very gently on a warm day. Check the tyres too: low pressures and cheap rubber can knock range and mask battery condition.

Minute 4: charge port and rapid charging

Look inside the charge port for scorching, bent pins, dirt or moisture. Ask for a rapid-charge demonstration if the dealer has a charger. Check that the car accepts both AC and DC charging. If it refuses DC, walk away or budget for a repair. On older EVs with passive battery cooling, ask about rapid-charge history. Lots of DC charging on a hot day is harder on the battery than slow overnight charging.

Minute 5: underbody and cooling

Look under the car for kerbed battery cases, loose panels or signs of coolant leaks. On cars with active battery thermal management, check the coolant level and service history. A battery that has been cooked by a failed cooling system can look fine on a short test drive and cause trouble later. If the car has a towbar, ask what it has been towing; heavy loads are not automatically a problem, but they are part of the history.

Warranty small print that matters

A battery warranty is only useful if it survives the handover. Check whether it is transferable to you, the second owner. Check the capacity threshold: 70% is common, but some brands use 75% or 66%. Check whether the warranty covers gradual degradation or only sudden failure. Check whether diagnostics are free or chargeable. And check whether the car must be serviced at a main dealer to keep the cover alive. If you are buying on PCP or hire purchase, remember that a degraded battery can affect the balloon payment and future trade-in value.

Red flags worth walking away from

No SoH report at all. A report that cannot be tied to the VIN. A dealer who will not let you plug in a scan tool or see the car's own battery screen. A battery case with impact damage. A car that has clearly been used as a taxi or private hire vehicle with very high mileage and no battery documentation. A salvage or insurance write-off history that has not been properly repaired. And any story about a battery replacement that comes without paperwork.

The money conversation

Use the battery as a negotiation tool, not a scare tactic. A car with 88% SoH and a full report is a known quantity. A car with no report is a gamble. Replacement batteries can cost thousands, so even a small discount for missing documentation is reasonable. If the dealer will not move on price, ask for an extended warranty or a fresh health check as part of the deal.

The bottom line

Five minutes on the forecourt will not replace a proper inspection, but it will filter out the obvious lemons. Ask for the data, cross-check it, and be prepared to walk. In the used EV market, the battery is the car. Treat it that way and you will save yourself a very expensive headache.