TECHNICAL FUNDAMENTALS
SoCSoHEstimationPlanning
State of Charge vs State of Health: Reading Battery Telemetry
Two numbers describe a battery at any moment — how full it is, and how young it still is. Confusing them leads to bad planning.
01Introduction
What this article covers
Every storage conversation eventually comes down to two metrics. State of Charge (SoC) is the energy available right now; State of Health (SoH) is how much of the pack's original capacity remains. Both are estimates, and both deserve attention.
02Technical
State of Charge (SoC)
SoC is the usable energy in the pack expressed as a percentage of rated capacity. It is inferred from coulomb counting (tracking current in and out) plus voltage and temperature correction. Accuracy matters because the EMS uses SoC to decide when to stop discharging or start charging.
03Technical
State of Health (SoH)
SoH compares the pack's current maximum capacity to its rated capacity when new. As cells age, available capacity and internal resistance drift, so SoH falls. It is the number that tells you how much life the battery has left.
04Technical
How they are estimated
Neither is measured directly; both are modelled. Coulomb counting, open-circuit-voltage lookup, and impedance tracking feed an estimator. Good estimators fuse several methods and are corrected over time by the BMS as it observes real cycles.
05Technical
Why SoH matters for planning
A system sized for 100% of rated capacity on day one will deliver less years later. Planning for a modest SoH margin — and specifying depth-of-discharge limits that respect aging — keeps performance acceptable across the project life.
06Related Products
Products referenced
Referenced in this article
Battery Storage Components
Modules and BMS where SoC/SoH estimation is specified.
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Commercial & Industrial ESS
C&I systems reporting SoH for lifecycle planning.
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10FAQ
Frequently Asked Questions
What is the difference between SoC and SoH?
SoC is available energy now; SoH is remaining capacity versus new. One is about the moment, the other about the lifetime.
Can I read SoH directly?
No — it is estimated from capacity, resistance, and cycle history. A good BMS refines the estimate as it observes real use.
Why does SoH drop over time?
Cycling, temperature, and depth of discharge gradually reduce capacity and raise resistance. The rate depends on how the pack is managed.
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