The State of Charge (SoC) describes the current charge level of a battery as a percentage of available nominal capacity. An SoC of 100% means fully charged, 0% means fully discharged.
The BMS determines SoC through several methods: Coulomb counting (current measurement over time), voltage-based estimation (OCV curve), Kalman filter algorithms, and impedance-based methods. The accuracy of SoC determination is critical for safe and efficient operation.
In damage cases and performance evaluations, the historical SoC profile is a key piece of evidence. Continuous operation at extreme SoC levels (>90% or <10%) significantly accelerates degradation. PV-BESS-Assessor analyses SoC logs as part of capacity and lifespan assessments.
For maximum lifespan, the SoC should be kept between 20% and 80%. Continuous operation at SoC >90% or <10% accelerates calendar ageing due to increased electrode stress. With LFP cells, the window is somewhat more tolerant than with NMC.
SOC Miscalibration: 340 Residential Storage Units with Deep-Discharge Damage
Real anonymised investigation case with measurement data, timeline, and economic evaluation.
Last updated: 2026-06-16