What Is State of Charge (SoC)?

State of charge (SoC) is a battery's remaining usable energy, expressed as a percentage of its full capacity. It's the single number that drives most charging, dispatch, and scheduling decisions for both EVs and stationary battery storage.

How SoC Is Tracked

Most battery management systems estimate SoC through coulomb counting, tracking the current flowing in and out of the cell over time, cross-checked periodically against voltage readings. Voltage alone isn't reliable across a lithium-ion battery's full range, since voltage stays relatively flat through much of the middle SoC band, which is why counting current flow is the primary method.

Why SoC Drives So Many Decisions

• Charging speed: SoC is the main input to the charging curve, determining how fast a battery can safely accept power right now.
• Battery health: consistently sitting at very high or very low SoC accelerates long-term degradation more than cycling through a moderate range.
• Dispatch decisions: for a BESS, current SoC determines whether it can discharge for peak shaving or needs to charge first.

SoC vs. Battery Capacity

SoC is a percentage of whatever capacity the battery currently has, not a fixed absolute number. A battery that has degraded to 90% of its original capacity still reports 100% SoC when full, it's just 100% of a smaller number than when new. This distinction matters when comparing battery health across a fleet or storage asset over years of use.

Neutron's Systems

Our energy platform tracks SoC continuously across connected EV chargers and battery storage assets, using it to schedule charging, dispatch storage for grid services, and protect long-term battery health.

How is state of charge measured?

Most systems use coulomb counting, tracking current flowing in and out of the battery over time, cross-checked periodically against voltage readings, since voltage alone is not a reliable indicator of SoC across a lithium-ion battery's full range.

Why do fleet operators care about SoC beyond just "how full is the battery"?

SoC drives charging speed (via the charging curve), influences long-term battery health (extreme high or low SoC accelerates degradation), and feeds directly into dispatch decisions for depot scheduling and BESS-based services like peak shaving or arbitrage.