State of Health

What state of health (SoH) measures, how cycling and calendar aging cause degradation, and how SoH is calculated.

Batteries don’t last forever. Over time, they gradually lose capacity, efficiency, and power output—a process known as battery degradation or aging. This decline affects everything from electric vehicle range to smartphone battery life. It’s even the primary reason why satellites go out of service!

State of Health (SoH) quantifies how much a battery has degraded compared to its original condition.

What Causes Battery Degradation?

Battery degradation occurs due to a combination of physical and chemical changes within the cell. Contributing factors include:

• Repeated charge and discharge cycles
• Extreme operating temperatures
• Passage of time (even when unused)

Types of Aging

Type	Cause
Cycling aging	Degradation caused by battery operation—charging and discharging over time
Calendar aging	Degradation due to the passage of time, even if the battery sits unused

Key degradation mechanisms include the growth of the solid electrolyte interphase (SEI), lithium plating, and loss of active material. These are covered in detail in the degradation overview.

Defining State of Health (SoH)

State of Health (SoH) is a metric that quantifies how much a battery has degraded compared to its original state. It is typically expressed as a percentage:

• 100%: Brand-new battery
• Lower values indicate reduced capacity or performance

Methods for Measuring SoH

Capacity-Based SoH

The most common definition measures how much of the original capacity remains.

$\text{SoH} = \frac{Q_{\text{current}}}{Q_{\text{original}}} \times 100\%$

For example, if a battery was originally rated for 100 Ah and now delivers only 80 Ah, its SoH is 80%.

This is measured under specific discharge conditions, and different conditions may yield different SoH measurements.

Resistance-Based SoH

Since internal resistance increases with aging, some methods estimate SoH by tracking the rise in resistance over time. Higher resistance leads to:

• Greater energy losses
• Reduced power output

Typically, resistance-based SoH is used in conjunction with capacity-based SoH to provide a more complete picture of battery degradation, never estimating SoH from resistance alone.

Model-Based SoH Estimation

Advanced battery management systems (BMS) use a combination of:

• Real-time data
• Historical usage patterns
• Electrochemical models

to estimate SoH dynamically.

Why SoH Matters

Monitoring SoH is crucial for ensuring reliable battery operation:

Application	Impact of Low SoH
Consumer electronics	Shorter battery life
Electric vehicles	Reduced driving range and charging speed
Grid storage	Lower energy availability and system efficiency
Temperature control	Degraded cells generate more heat, risking imbalance or thermal runaway

Knowing when a battery has reached an unacceptable SoH threshold helps determine when it should be:

• Replaced
• Repurposed (e.g., for second-life applications)
• Recycled

Understanding Degradation Mechanisms

Understanding battery degradation is key to extending battery lifespan, improving reliability, and designing better energy storage systems. The degradation overview takes a closer look at the mechanisms behind battery aging.