Battery State of Health (SOH) Calculator
Estimate a battery's state of health by blending capacity fade and internal resistance drift into a single composite percentage.
Educational engineering estimate. Validate with laboratory-grade cycling equipment before making warranty or safety decisions.
Examples
- Rated 100 Ah, measured 88 Ah, 4.0 mΩ rated, 5.0 mΩ measured ⇒ 84.00% SoH
- Rated 60 Ah and measured 57 Ah with resistance fields blank ⇒ 97.50% SoH
FAQ
What does the percentage represent?
The output is an approximate state of health, combining capacity retention and resistance drift relative to the battery's original specification.
Can I ignore resistance measurements?
Yes. Leaving the resistance fields blank assumes no change in internal resistance and weights the result toward capacity retention only.
How should I handle multi-cell packs?
Use the pack's rated capacity and an averaged measured capacity. For resistance, enter equivalent pack resistance or leave blank if unavailable.
Does the calculator cap impossible results?
It prevents negative percentages but otherwise reports the computed value so you can investigate readings above 100%.
Additional Information
- Capacity ratio uses measured ampere-hours at the reference discharge rate divided by the nameplate capacity.
- Resistance factor defaults to 1.00 when resistance inputs are blank, so the result reflects capacity alone.
- Composite score averages capacity retention and the inverse of resistance growth before converting to percent.