State of charge (SoC)
The remaining charge in a battery expressed as a percentage of its current full capacity.
State of charge is the battery equivalent of a fuel gauge: 100% means fully charged, 0% fully discharged within the manufacturer's voltage limits. It is the complement of depth of discharge.
SoC = 1 - DoD
SoC cannot be measured directly, so a battery management system (BMS) estimates it by combining coulomb counting (integrating current over time, which drifts) with open-circuit-voltage lookup tables and model-based filters such as extended Kalman filters. Estimation is harder for LFP, whose voltage barely changes between about 10% and 90% SoC; NMC has a steeper, more informative curve.
SoC management matters for longevity: lithium-ion cells age faster when held at high SoC and high temperature, so EV makers recommend daily charging to 80–90% for NMC, while LFP packs are often charged to 100% regularly to recalibrate. Grid batteries typically operate within an SoC window (for example 5–95%) and reserve headroom for frequency response. A related metric, state of health (SoH), is the current maximum capacity relative to the original rating.
Related terms
All terms →- Depth of discharge (DoD)
The fraction of a battery's capacity that has been discharged, or the maximum fraction a manufacturer allows to be used per cycle.
- Lithium-ion battery
A rechargeable battery in which lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode; the dominant chemistry for EVs and grid storage.
- LFP (Lithium Iron Phosphate)
A lithium-ion cathode chemistry (LiFePO4) prized for safety, long cycle life and low cost, at the expense of lower energy density than nickel-based cells.
- Cycle life
The number of full charge-discharge cycles a battery can deliver before its capacity falls to a defined end-of-life threshold, usually 70–80% of rated capacity.
- BESS (Battery Energy Storage System)
A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.