In the financial modeling of BESS assets, state of health (SoH) is the most critical variable. It dictates your remaining asset value and your ability to meet grid discharge contracts. However, there is a silent decay occurring in almost every battery rack: SoH Drift.

When your battery management system (BMS) reports a health percentage based on software models rather than physical reality, you are operating on "Ghost Capacity”. If the software says 92% but the chemistry only supports 86%, you are one peak-event away from a contractual default. Managing a BESS means moving past the digital estimate and verifying the physical energy throughput.

What is State of Health (SoH) drift in BESS?

The Direct Answer

State of Health (SoH) drift is the growing discrepancy between the battery health reported by the BMS and the actual physical capacity of the cells. This occurs because BMS algorithms often rely on "look-up tables" and voltage models that fail to account for non-linear degradation factors like micro-cycling, ambient thermal stress, and electrolyte loss. Without regular physical calibration, the BMS "drifts" away from reality.

The Financial Risk of "Model Lag"

Relying on drifted data isn't just a technical oversight; it’s a financial liability.

• Contractual Penalties: Most grid services – such as frequency regulation or peak shaving – require a guaranteed energy delivery. If you bid 10MWh into a market based on an inflated SoH, but your "drifted" battery can only deliver 9MWh, the resulting non-performance fines can wipe out months of revenue.
• Warranty Documentation: Battery OEMs provide capacity guarantees, but the burden of proof lies with the operator. Without a verified, timestamped history of physical performance tests, proving a premature failure for a warranty claim becomes a legal uphill battle.
• Asset Valuation: For infrastructure funds, an inflated SoH creates a "False Positive" on the balance sheet. When the drift is eventually caught, it leads to massive, unbudgeted write-downs that can shake investor confidence.

How do you recalibrate BESS State of Health data?

The Direct Answer

Recalibrating SoH requires a Reference Performance Test (RPT). During an RPT, the BESS is taken through a controlled, full discharge from 100% to 0% State of Charge (SoC) at a constant C-rate, followed by a full recharge. By measuring the actual energy throughput in kilowatt-hours (kWh) via high-precision metering and comparing it to the original nameplate capacity, the BMS "Actual Capacity" value is reset to match the physical chemistry.

The Role of Physical Execution in Data Integrity

Software cannot "self-heal" SoH drift. Recalibration requires a physical "Deep Cycle" protocol that must be executed by qualified field teams.

• The Limitation of SCADA: Remote monitoring is excellent for day-to-day oversight, but it cannot simulate the stress of a full RPT. Only a physical intervention can force the cells to reveal their true remaining capacity.
• As-Found vs As-Left Documentation: To build a defensible audit trail, technicians must log the "As-Found" capacity (what the BMS thought) and the "As-Left" capacity (the physical reality). This delta is the only way to track the true Degradation Slope of your asset over its decade-long lifecycle.

Looking to bring more precision to your BESS maintenance? Or need a reliable system of record for your performance data? Book a free FieldEx demo today, or simply get in touch. We’re here to help.