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.
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.
What do people get wrong about SOC and SOH?
- "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 two numbers side by side
| SOC | SOH | |
|---|---|---|
| Question | How full is it now? | How much of its original self remains? |
| Scale | 0–100% of current capacity | 0–100% of original capacity |
| Estimated by | Voltage, coulomb counting | Capacity test, resistance trend |
| Moves | Every charge and discharge | Slowly, downward, with age |
What This Means
Where I stand: 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.
Why: 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 my analysis, not a verified fact or purchasing guarantee.
Frequently asked questions
What is the difference between SOC and SOH?
SOC (state of charge) is how full the battery is right now — a fuel gauge. SOH (state of health) is how much of its original capacity it still holds — an aging report. A battery can be 100% SOC but only 80% SOH, meaning it charges full but that full is a smaller tank.
Why does a battery show 100% SOC but 80% SOH?
Because the two measure different things. SOH reflects irreversible capacity loss from aging; SOC reflects the current charge relative to that reduced capacity. A degraded battery still charges to its (now smaller) full, so SOC reads 100% while SOH has fallen.
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