What is battery state of health?

Battery state of health describes its condition relative to a new reference, often expressed through retained capacity. A complete assessment includes more than capacity alone [S1]. In an assumed example, 80 kilowatt-hours remaining from an original 100 means 80% capacity retention. Agree the reference and test conditions before using a dashboard percentage for warranty decisions. Checked October 2026.

Last checked 19 October 2026

What does battery state of health mean for your site?

Battery state of health (SoH) describes its condition relative to a new reference, commonly using remaining capacity. A fuller assessment also examines resistance, charge acceptance and discharge capability [S1]. For your site, the capacity measure helps check whether storage still meets the original operating brief.

Keep it separate from state of charge, which describes the present charge level. SAM models the available capacity shrinking through calendar and cycle ageing [S3]. Filling an aged battery does not restore its original storage capacity.

How can you interpret a retained-capacity percentage?

For an illustration, assume the commissioning test recorded 100 kilowatt-hours (kWh), a measure of energy. A later test under equivalent conditions records 80 kWh. Capacity retention = 80 ÷ 100 × 100 = 80%.

Those figures are assumptions, not an expected degradation rate or warranty threshold. Ask which starting capacity the reported percentage uses and whether both tests measure the same operating window. Compare the result with your battery sizing brief.

How should health be measured and used in a warranty?

Laboratory guidance says repeatable capacity testing needs specified temperature, charge and discharge rates and operating limits [S1]. Health can also be estimated during operation using algorithms and recorded electrical responses [S2]. Request the measurement method and supporting records, rather than treating every software estimate as a contractual capacity test.

Ask for the warranty’s retained-capacity definition, permitted use, throughput limit, exclusions and claim test. Keep the commissioning baseline and operating logs. The maintenance report warns that operating outside specified conditions can affect warranty claims [S1]. Check disputed contract wording with your solicitor and use the warranty and degradation guide for the procurement checklist.

What should you do next?

Bring the test report to the site check and get matched with an installer that covers your area.

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