What is State of Health (SOH)?
State of Health (SOH) is a critical figure of merit used to describe the condition of a battery or a battery cell compared to its ideal conditions. Unlike State of Charge (SOC), which tells you how much energy is currently in the tank, SOH tells you how much the tank has shrunk over time due to aging and usage.
An SOH of 100% indicates that the battery meets its original factory specifications. As batteries undergo charge and discharge cycles, internal resistance increases and the ability to hold a charge diminishes, causing the SOH percentage to drop.
How to Calculate State of Health
The most common way to calculate SOH is by comparing the current full-charge capacity to the original rated capacity provided by the manufacturer. The formula is simple:
SOH (%) = (Current Capacity / Rated Capacity) × 100
For example, if you have an electric vehicle battery originally rated for 75 kWh, but after three years it can only hold a maximum of 60 kWh, your SOH would be 80%.
Why Tracking SOH is Important
Understanding the SOH of your battery systems is essential for several reasons:
- Predictive Maintenance: Knowing when a battery is reaching its end of life (usually around 70-80% SOH for EVs) helps you plan for replacements before a failure occurs.
- Resale Value: In the used electric vehicle market, SOH is one of the most significant factors in determining the vehicle's value.
- Safety: Batteries with very low SOH may exhibit volatile behavior, increased heat generation, or swelling.
- Efficiency: Systems with poor health require more frequent charging and can lead to increased energy costs over time.
Frequently Asked Questions
What is a "Good" SOH?
Generally, 80% to 100% is considered good to excellent. When a battery drops below 70%, it is often considered at the "end of life" for its primary application (like driving) but may still be useful for secondary storage.
Can I improve my battery SOH?
Unfortunately, SOH degradation is a chemical process that cannot be reversed. However, you can slow down the decline by avoiding extreme temperatures, limiting fast charging, and keeping the state of charge between 20% and 80%.
How often should I check SOH?
For heavy-use systems like solar storage or EVs, checking every 6 to 12 months is recommended to track the degradation curve.