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Lithium Knowledge · Systems · Last reviewed 2026-09-14

SOC vs SOH: The Two Numbers a Battery Management System Reports

A battery management system reports two numbers that sound similar but answer different questions: how full the battery is, and how healthy it is. Confusing them is one of the most common mistakes in reading a battery's condition.

Direct answer: SOC (state of charge) is the fuel gauge — the percentage of usable capacity available right now. SOH (state of health) is the aging report — how much of the battery's original capacity it still holds. They are independent: a battery can read 100% SOC while its SOH has fallen to 80%, because it still charges "full" — but that full is a smaller tank than it was when new.

What SOC measures

State of charge is the momentary energy level: 100% means fully charged, 0% means the usable capacity is exhausted. It is what a phone or EV dashboard shows as the battery bar. The BMS estimates it from cell voltage, current integration (coulomb counting) and temperature, and it resets to 0–100% on every cycle.

What SOH measures

State of health is the battery's remaining capacity relative to when it was new. A battery with 80% SOH has lost 20% of its original capacity to irreversible aging — mostly electrode degradation and electrolyte breakdown. SOH only moves in one direction (down) and does not reset.

Why the two decouple

The decoupling is the key insight. A degraded battery still charges to its own (now smaller) 100%, so SOC reads full while SOH has fallen. That is why "100% charged" says nothing about whether the battery is healthy — a 100% SOC / 60% SOH pack is full, but only holds a little over half its original energy. The usable energy is the product: SOC × SOH × original capacity.

Why this matters in practice

  • EV range — a car at 100% SOC with 80% SOH has lost ~20% of its original range, even though the gauge says full.
  • Second-life / resale — SOH is the number that determines a used battery's value, not SOC.
  • Warranty — many battery warranties are written against SOH (e.g. "70% SOH after 8 years"), not SOC.

Common misconceptions

  • "100% charged means the battery is healthy." No — it means full. Health is SOH, a separate number.
  • "SOC and SOH are the same thing." They answer different questions: how full vs how degraded.
  • "SOH is measured directly." It is estimated — from capacity tests, internal resistance and voltage behaviour, which is why different tools can report slightly different SOH.

Key takeaways

  • SOC = how full (fuel gauge); SOH = how degraded (aging report).
  • A battery can be 100% SOC but 80% SOH — full, but a smaller tank.
  • Usable energy = SOC × SOH × original capacity.
  • SOH is the number that matters for range loss, resale value and warranty.

The Author's Take

Position: The SOC-vs-SOH distinction is the single most useful concept for judging a used battery — because "100% charged" is a statement about the last hour, while SOH is a statement about the battery's whole life.

Reasoning: Sellers quote SOC because it is flattering; buyers should ask for SOH because it is the truth. A used EV advertised as "fully charged" tells you almost nothing about what you are buying — the SOH tells you how much of the original battery you are actually getting. Learn to ask for SOH, and the ambiguity disappears.

This is the author's editorial view, not a purchasing guarantee.

Sources

Cross-verified from battery engineering references and BMS documentation. SOC/SOH definitions are standard battery-management terminology.

  • IEC 62660 / ISO 12405 — lithium cell and pack test standards (reference).
  • BMS and EV-manufacturer documentation — SOC and SOH estimation.
  • Battery engineering references — state-of-charge and state-of-health definitions.

Last reviewed: 2026-09-14