Battery Degradation vs. Battery Failure

Every high-voltage battery in an electric vehicle or hybrid loses some capacity over time. That gradual decline is very different from a battery that actually fails. Knowing the difference helps you judge whether your car has a problem and whether a warranty is likely to help.

What battery degradation means

Degradation is the slow, expected loss of a battery's ability to hold charge. In an EV, you notice it as somewhat less range on a full charge than when the car was new. In a hybrid, it may show up as the engine running more often or slightly lower fuel economy.

Degradation is a normal result of chemistry. It is influenced by factors such as:

• Age, since batteries lose capacity over time even with light use
• Charge cycles, the number of times the battery is charged and discharged
• Heat, especially long exposure to high temperatures
• Charging habits, such as frequently charging to 100 percent or relying heavily on DC fast charging
• Sitting at extreme states of charge, either very full or very empty, for long periods

Degradation is typically gradual. The car still works normally, just with less capacity.

What battery failure looks like

A failure is a problem that stops the battery from working properly, not just a gradual decline. Signs can include:

• A sudden, large drop in range
• Warning lights or messages about the high-voltage system
• The car refusing to charge, or charging very slowly without explanation
• Reduced power or a limp mode
• The car not starting or not going into ready mode
• In hybrids, the engine running constantly and the hybrid system shutting down

Failures can come from a faulty cell or module, a cooling system problem, a damaged connection, or a fault in the battery management system. Sometimes the battery itself is fine and a related component, such as a sensor, contactor or onboard charger, is the culprit.

How to tell which one you have

If you notice reduced range, start with a few checks:

1. Consider the season. Cold weather temporarily reduces range in EVs. That is not degradation.
2. Look at your driving. Highway speeds, heavy climate control use and cargo all reduce range.
3. Compare over time. Degradation is slow. A sudden change points to a problem.
4. Check for warnings. Any high-voltage system message deserves a professional look.
5. Ask for a battery health report. Many dealers and some specialty shops can measure state of health.

How factory warranties treat each one

Factory warranties on high-voltage batteries generally cover defects and failures. Some also cover excessive capacity loss below a stated threshold during the warranty period, but that is not universal, and the threshold varies. Read your warranty booklet carefully for the exact wording.

Battery coverage often lasts longer than the basic warranty. For example, Tesla covers the battery and drive unit for 8 years, with a mileage limit that depends on the model. Polestar covers the battery for 8 years or 100,000 miles, and Lucid covers the battery and drive units for 8 years or 100,000 miles. Terms can vary by model year, and hybrid battery coverage varies by brand and sometimes by state.

Normal degradation that stays above any stated threshold is generally not considered a defect.

How service contracts treat batteries

Vehicle service contracts vary widely here. Some cover high-voltage battery failure under hybrid and EV components, and some exclude the battery or limit coverage while the factory battery warranty is active. Very few cover gradual capacity loss.

When reading a contract, look for:

• Whether the high-voltage battery is covered at all
• Whether related parts, such as the onboard charger, inverter and battery cooling system, are covered
• Any exclusions for capacity loss or "normal deterioration"
• Any charging equipment exclusions

Habits that slow degradation

• Follow your automaker's charging recommendations, including daily charge limits.
• Avoid leaving the car at a very high or very low charge for long periods.
• Park in shade or a garage in hot weather when possible.
• Use DC fast charging when you need it, not out of habit, if your manual suggests limiting it.
• Keep software up to date, since updates sometimes improve battery management.

How Warranty Wizards can help

We help EV and hybrid owners compare service contracts from third-party providers and explain how each one handles battery and electric drive components. Learn more about auto warranty options or request a free quote at /get-a-quote/.