With a combustion car you check the engine. With an EV, 80% of the value and all of the risk sit in one place: the battery. Everything else - bodywork, suspension, interior - is judged the same way as on any used car. But if you miss the state of the cells, you can buy a car that in two years has half its advertised range, or one facing a pack replacement that costs half the value of the vehicle. After looking at dozens of EVs I’ve learned that “looks nice and drives fine” means nothing here. One number matters: SOH.

SOH - the only number that really counts

SOH (State of Health) is the percentage health of the battery relative to factory condition. A new car is at 100%. Over years and miles the capacity drops - that’s physics, not a fault. SOH of 90% means the pack holds 90% of its original energy, so real-world range is roughly 10% lower too.

This is the value you need to know before you pay. The seller will show you a nice range estimate on the dash, but that figure is based on recent trips - easy to “inflate” with gentle driving before your visit. SOH can’t be faked that easily.

How to read SOH in practice

There are several routes, easiest first:

Diagnostic app + OBD2 adapter. A cheap Bluetooth adapter plus an app like Car Scanner, LeafSpy (Nissan Leaf) or a brand-specific app reads SOH straight from the battery controller. This is the most honest source.
The car’s service menu. Some Teslas and other brands show battery condition in hidden menus or after a dealer service test.
Capacity test at the brand dealer. Paid, but it gives you a document with a concrete result. Worth it on a pricier car.
Real range test. Charge to 100%, check the displayed range and compare it with the factory figure for that year and capacity.

If the seller won’t let you connect OBD or drive to a dealer for a test - that’s already your answer.

How much degradation is normal

Use sensible benchmarks, not marketing promises:

First 1-2 years / up to ~20k miles: a drop of up to ~5%. Some of this happens quickly at the start, then slows down.
After 5 years / ~60k miles: an SOH of 88-93% is a good result for a car with active battery cooling.
100-125k miles: 80-88% can be normal on a well-treated pack.

The psychological and warranty threshold is usually 70% - below that, manufacturers consider the battery worn out. Treat a car with SOH 75% at 75k miles as a warning, not a bargain.

Charging history: DC vs AC

Two identical cars can have different SOH purely because of how they were charged. Frequent DC fast charging (50-150+ kW stations) heats the cells and accelerates degradation. A car charged mostly from a home AC wall box, and only rarely “fast”, ages more slowly.

Ask directly: was the car charged mainly at home, or did it live on fast chargers along the motorway? Fleet and long-distance cars often mean a life on DC. Some apps even show the number of cycles and the share of DC charges.

Real range vs advertised - especially in winter

The brochure figure (WLTP/EPA) is a lab number. In the real world, subtract your own. In winter, range can drop by 20-35% - the battery dislikes cold, and cabin heating draws power straight from the pack (unless the car has a heat pump - a big plus). A car claiming 250 miles will realistically do 160-175 miles on a January motorway.

If you’re viewing the car in summer, deliberately assume the winter scenario and check whether that real range is even enough for you. This isn’t a fault of the specific car - it’s the nature of an EV, but it’s better to know before, not after.

Battery warranty - check what’s left

This is your safety net. Most makers cover the pack for 8 years or 100,000 miles (whichever comes first), usually guaranteeing capacity won’t fall below 70%. It’s a separate, longer warranty than the rest of the car.

Check:

The first registration date - the 8 years count from there, not from your purchase.
The mileage against the limit.
Whether the warranty is transferable to the next owner (usually yes, but confirm it).
Whether services were done per the brand’s requirements, so the warranty isn’t voided.

A car with 2 years of battery warranty left is a very different risk from one with 6 years remaining.

Battery replacement cost - why it’s brutal

If the pack fails out of warranty, the bill can be brutal: on many models a battery replacement equals 30-50% of the used car’s value, sometimes more on older examples. It’s the one repair that can wipe out the economics of the whole purchase.

That’s why SOH and the remaining warranty aren’t trivia - they’re the foundation of your valuation. A cheap EV with a dead battery and no warranty isn’t a bargain - it’s a potential anchor around your neck.

Software, connectors and provenance

Software updates. Battery management, range and features depend on the software. Check that the car is up to date and that the brand still supports that model year.
Charging connector condition. Inspect the port (Type 2 and CCS): burnt, discoloured or worn contacts are a sign of charging problems or overheating.
Ex-taxi and ex-fleet cars. High mileage in a short time, heavy DC use, charging to 100% daily - that’s a recipe for faster degradation. Low mileage isn’t always better, but a “taxi/fleet” history is a reason to be especially thorough about SOH.

Specific models - what to watch

Nissan Leaf (especially older ones): no active battery cooling. A passively cooled pack degrades faster, particularly in a warm climate and with frequent DC. Check SOH here without exception (LeafSpy).
Tesla Model 3 / Y / S: good thermal management, but check the DC (Supercharger) history and SOH via the service menu or a readout.
VW ID.3 / ID.4: pay attention to the software version - early examples had plenty of fixes delivered via updates.
Kia e-Niro / Hyundai Kona EV: generally hold the pack well, but the same rules apply - read the SOH and check the warranty.

Red flags

The seller won’t let you connect OBD or drive to a dealer for a battery test.
Real range at 100% deviates sharply from the factory figure for the year.
Burnt or discoloured contacts in the charging port.
A history of heavy DC, taxi or fleet use, with the mileage concealed.
A battery just out of or beyond warranty sold as “like new” with no SOH document.
The car won’t charge to 100% or shows high-voltage system errors.
Frequent module replacements in the service history - a sign the pack has caused trouble.

How to line it up before you buy

Before you get in the car, combine two things: the vehicle history (origin, mileage, damage, warranty status) and a hard measurement on site (SOH, connectors, range test). With MotoPPI you get the vehicle history and a ready inspection checklist tailored to an EV - with items on the battery, connectors and warranty - so you don’t forget any of them at the lot.

An EV can be a superb, cheap-to-run car - as long as the battery is healthy and the warranty is real. Don’t buy the brochure range. Buy the SOH you saw with your own eyes.