What Is State of Health (SoH)?

State of Health (SoH) is a measure of how much of your EV battery's original capacity remains after months and years of use, expressed as a percentage. A brand-new battery starts at 100% SoH. As it ages through charge cycles, temperature exposure, and time, that figure gradually falls. SoH directly affects how far you can travel on a full charge, so tracking it gives you an honest picture of your battery's condition. Most manufacturers provide SoH data through their app or a dealer diagnostic tool, and some vehicles display it directly on the dashboard or charge point screen.

How is state of health measured?

SoH is calculated by the battery management system (BMS) by comparing the battery's current maximum capacity against its original rated capacity. The BMS tracks every charge and discharge cycle, monitors cell voltages and temperatures, and uses this data to estimate how much the pack has degraded. Because this is a modelled estimate, different diagnostic tools may return slightly different SoH figures for the same battery. An independent EV health check service can provide additional clarity if you want a second opinion.

What causes battery degradation?

Degradation happens through a combination of chemical processes that occur naturally over time and use. The main contributors are:

• Charge cycles: Each full cycle causes minor wear. High volumes of fast charging add heat, accelerating the process.
• High states of charge: Storing the battery at or near 100% for extended periods stresses the cells more than keeping it at moderate levels.
• Deep discharges: Regularly running the battery very low puts additional strain on the chemistry.
• Temperature extremes: Sustained heat and repeated cold exposure both reduce capacity over time.

Does SoH affect the warranty?

Yes. Most EV manufacturers offer a separate battery warranty covering the high-voltage pack to a minimum SoH threshold over a set number of years or miles, whichever comes first. If your battery falls below that threshold within the warranty period, the manufacturer is typically obliged to repair or replace it. Contact your dealer if you notice a sudden or unexpected drop in range, as this may indicate a warranty-eligible fault rather than normal degradation.

How can I check my battery's state of health?

There are several ways to check SoH depending on your vehicle:

• Manufacturer app: Many EVs report SoH or a capacity figure directly in the official app.
• OBD diagnostic tool: Third-party OBD adapters combined with specialist apps can read raw BMS data for many models.
• Dealer diagnostic scan: Any authorised dealer can run a full battery diagnostic using the manufacturer's software.
• Independent EV health check: Specialist EV garages and some breakdown providers offer dedicated battery health assessments.

Can I slow battery degradation?

You cannot stop degradation entirely, but you can meaningfully slow it. Keep daily charging in the 20-80% range, avoid prolonged exposure to extreme heat or cold when parked, minimise frequent rapid charging when slower overnight charging is available, and avoid leaving the battery at very high or very low charge levels for extended periods. Explore our EV guides for more practical battery care advice.

Frequently asked questions

What is a normal state of health for a used EV?

A used EV with SoH in the high 80s to mid-90s after several years of use is broadly considered healthy. Significant degradation below 80% within the first few years of ownership is unusual and may fall within warranty coverage. Always check battery health before purchasing a used EV.

Is SoH the same as SoC?

No. State of Charge (SoC) is how much energy is in the battery right now, as a percentage of the current maximum. State of Health (SoH) describes what that current maximum is compared to the original rated capacity. SoC changes every time you drive or charge; SoH changes slowly over months and years.

Can a degraded battery be repaired or replaced?

If degradation falls within the warranty threshold, the manufacturer will repair or replace the battery. Outside of warranty, specialist garages can sometimes replace individual faulty modules rather than the entire pack, reducing cost. Whole-pack replacement is also available but is typically a significant expense that varies by vehicle make and model.

Does fast charging damage state of health?

Occasional rapid charging has a limited impact on modern batteries, which are designed to handle it. Frequent reliance on ultra-rapid charging over many years adds more heat and stress than slower home charging, which can modestly accelerate degradation. For day-to-day use, a home charger is gentler on the battery long term.

How does SoH affect the resale value of an EV?

SoH is increasingly important in the used EV market. Buyers and dealers use it to assess how much usable range remains. A documented high SoH reading supports a stronger asking price, while a low or unknown SoH reduces buyer confidence. Some remarketing platforms now require a battery health certificate as part of a used EV sale.

At what SoH should I replace the battery?

There is no fixed rule. Many owners find a battery in the 70s percentage still usable for everyday driving, particularly if regular journeys are well within the reduced range. If you are regularly struggling to complete necessary journeys, a battery assessment is the sensible next step.

Keeping an eye on your battery's state of health protects the long-term value and usefulness of your EV. If you are buying used or want reassurance about your current car, a battery health check is always time well spent. Find the right Type 2 charging cable for your car, or browse the full EV guides library for more ownership advice.