Solar Lattice

Concept

SoC estimation methods

ID
concept:soc-estimation
IRI
https://solarlattice.org/kb/concept/soc-estimation
Status
active
Also known as
coulomb counting, ampere-hour counting, open-circuit voltage method, Voltage recalibration, OCV
Category
Storage
Concept kind
technology

SoC estimation methods are the algorithms a battery inverter or BMS uses to work out how full a battery is, because state of charge cannot be measured directly. Ampere-hour counting, also called Coulomb counting, integrates the battery current over time. On its own it drifts, because the starting SoC is usually unknown and the DC current measurement is not accurate enough. To limit this drift, SMA's version also estimates the battery's internal leakage current and uses full-charge detection, which looks at temperature, voltage, charging current and time and lets the algorithm learn and adjust its parameters. The Sunny Island reports its estimated SoC error as BatSocErr. The error is lowest right after a full or equalization charge and grows again over the following cycles. Small SoC jumps during recalibration are normal. Large or fast jumps suggest wrong settings, early battery aging or a battery error, and the first step SMA recommends is an equalization charge. Wrong installation or parameter settings make SoC and state of health estimates less accurate. Only a capacity test gives the true values, and most field sites cannot run one.

<!-- review notes

Claims

explains

c1 drafted · lead State of charge

SoC cannot be measured directly; it is estimated from measurable values, algorithms and parameter settings.

related_to

c2 drafted · lead SOC error estimate monitoring proposed

SMA reports the estimated SoC error as parameter BatSocErr; it is lowest after a full or equalization charge.

c3 drafted · lead Battery SOC recalibration

Voltage recalibration corrects the counted SoC using battery voltage; an expert-mode option recalibrates on open-circuit voltage only.

c4 drafted · lead Sudden SOC jump proposed

Small jumps come from normal recalibration; large or fast jumps point to wrong settings, early battery aging or battery errors.

c5 drafted · lead Battery equalization charge proposed

Recommended first action when SoC deviations occur; also resets the estimated SoC error to its minimum.

c6 drafted · lead Battery capacity test proposed

Only capacity tests give actual SoC and SoH, but most field applications cannot perform them.

c7 verified · fact State of health

verified by: review-workflowverified on: 2026-10-24

c8 drafted · lead Lead-acid battery

Full-charge detection relies on the typical charging characteristic of lead-acid batteries.

c9 drafted · lead Lithium iron phosphate

A LiFePO4 tracker controller battery uses OCV plus Coulomb counting for SoC.

c10 drafted · lead Sunny Island 4.4M / 6.0H / 8.0H

Sunny Island acts as BMS for VRLA batteries and combines ampere-hour counting with voltage recalibration.

c11 drafted · lead Solar FlexRack TDP / BalanceTrac single-axis tracker proposed

Referenced by

Data