sohpass

Battery SoH: what it is and how to find it out

Three letters that decide the price of a used electric car — and that almost nobody explains to you before selling you a measurement.

What the figure means

State of health, or SoH, compares the capacity the battery can still store with what it stored when new. At 85%, it returns about 85% of the original range. That is all, and it is already a lot: on a used car, this percentage carries more weight than the mileage.

It should not be confused with state of charge, or SoC, which is how full the tank is right now. Yet SoC is what most dashboards display, and this confusion leads people to buy cars blind.

What battery degradation looks like

A rapid loss in the first year, then a plateau. That plateau explains why an eight-year-old car often holds up far better than its age suggests — and why the warranty threshold is crossed much later than people imagine.

Expected battery state of health by ageThe range starts at 98 to 100% when new and falls to 66 to 79% at twelve years. The line at 70% marks the usual threshold of manufacturer warranties. The values are written out in full below the chart.100%90%80%70%60%new3 years6 years9 years12 years70%WarrantyExpected range
NMC battery at the average French annual mileage of 9,321 miles. The axis starts at 60%: below that, a battery is a case for replacement, not assessment. LFP chemistry holds up a few points better, an LMO pack markedly worse.
The same values written out
  • New98 to 100%0 mi
  • 2 years91 to 97%18,641 mi
  • 4 years86 to 94%37,282 mi
  • 6 years81 to 90%55,923 mi
  • 8 years76 to 86%74,565 mi
  • 10 years71 to 83%93,206 mi
  • 12 years66 to 79%111,847 mi

The thresholds that matter

  • 70% — usually the level below which the manufacturer’s warranty kicks in. Above it, falling capacity counts as normal wear, and nobody will pay in your place.
  • A little over 2% a year — the average rate of degradation observed in the fleets studied. An eight-year-old car at around 80% is therefore behaving normally.

The three ways to measure it

They differ in cost and in the weight they carry as evidence.

  1. The on-board reading. A few models display an indication of capacity, such as the Nissan Leaf with its twelve-bar gauge. Some manufacturers issue a certificate from their app. Free and instant, but often rough.
  2. Reading the battery management unit. A diagnostic tool plugged into the vehicle’s port reads the capacity the unit estimates. Quick and inexpensive, but the value depends on how well the unit is calibrated.
  3. The charge and discharge test. The most reliable, and the longest: the battery is drained and then recharged under supervision, which gives the capacity it actually delivers. This is what specialist testing services sell, for anything from a few tens to just over a hundred euros in France.

Why we give a range and not a figure

Without a measurement on the vehicle, nobody knows the SoH of a specific car. Two examples of the same year, at the same mileage, are commonly ten points apart depending on climate, use and charging habits. A website that tells you "87%" without having touched the car is making up the figure you were going to negotiate on.

So we calculate the range in which a comparable battery usually sits, based on age, mileage and chemistry. We issue no certificate and measure no battery.

Estimate the state of health of a specific car

Frequently asked questions

What does SoH mean for a battery?

SoH stands for "state of health". It is the ratio between the capacity the battery can still store and the capacity it stored when new, expressed as a percentage. A battery at 85% returns 85% of its original energy, so roughly 85% of the range quoted at the time.

At what SoH should you start to worry?

There is no universal threshold. Two benchmarks matter: manufacturer warranties usually kick in below 70% capacity, and a battery loses on average a little over 2% a year. An eight-year-old car at around 80% is therefore normal, not alarming.

Is SoH shown on the dashboard?

Rarely, and that is the main trap. Most cars only show the state of charge, which is something else entirely. A few models are the exception, such as the Nissan Leaf with its twelve-bar gauge. Some manufacturers issue a certificate through their app or at the dealer. For the rest, you need a diagnostic tool or a testing service.

Does a drop in range prove the battery is worn?

No. The range displayed depends on the cold, driving style, tyres and the on-board computer’s calculation, which only adjusts slowly. On LFP batteries in particular, the estimate drifts as long as the car has not recently been charged to 100%. Only a capacity measurement can be relied on.

Can a battery’s SoH be brought back up?

Not the actual capacity: chemical degradation is irreversible. However, a poorly calibrated battery management unit can underestimate capacity, and a recalibration or a manufacturer update then corrects the value read. This happened on a large scale on some 30 kWh Nissan Leafs.