Maximize Your Battery Power: The Secret to Accurate SOC for Grid-Scale Energy Storage
For grid-scale Battery Energy Storage Systems (BESS), accurate site capacity information is critical. It enables the system operator to utilize the asset to its fullest potential and maximize return revenue. Specifically, it is typical that a given plant will be required to report an available energy (in MWh) and/or State of Charge (SOC, in %) metrics at regular intervals.
SOC metrics can be inherently inaccurate with traditional SOC calculation methods typically resulting in +/-15% error. This means that either:
- SOC metric is underreported: the system operator believes the site is fully discharged (due to a 0% SOC reported) when there is still 15% of the site capacity available for discharge. This leaves the BESS underutilized, with a clear negative financial impact, especially when considering periods of high value of dispatched electricity.
- SOC metric is overreported: The system operator believes that there is still 15% of the site capacity available (15% SOC is reported) when the batteries are depleted. This can lead to costly penalties for the site for failing to meet committed discharge energy.
Similar scenarios can also occur when the BESS is charging: failure to have an accurate SOC measurement could leave the BESS less than 100% full, hindering the amount of energy that can be discharged when called upon. The project can also face imbalance changes if the operator sees the system reading at 85% SOC but it’s already at 100% charge and no more energy can be added.
In addition to real-time reporting for state of charge, day-of and day-ahead energy forecasting is becoming more critical in many markets. In these cases, BESS assets are required to advertise the amount of charge energy and discharge energy the asset will be able to provide in the future – either later in the day or the following day. An erroneous SOC calculation can skew the forecasts either leading to underutilization of the asset or inability to fulfill commitments.
Challenges with SOC calculations
SOC is not a directly measurable quantity, and must be calculated using the available voltage, current, and temperature measurements of the battery.
The most common BESS battery chemistry, Lithium Iron Phosphate (LiFePO4) or LFP for short, poses a unique set of challenges in calculating SOC as compared to other Li-Ion battery chemistries. The voltage that can be measured on the cell – the Open Circuit Voltage (OCV), while it can be the best proxy available by which an SOC can be inferred in the absence of any other data, is not a deterministic indicator of SOC because the curve of Voltage vs SOC has a large flat region between 5% and 95% SOC (as seen in Figure 1). This means that the voltage reading at 40% SOC will be nearly identical to the voltage reading at 65% SOC.
A typical Voltage vs SOC curve is shown Figure 1 (Voltage being Single Cell Open Circuit Voltage).
Another approach for SOC calculations for LFP batteries is the Coulomb Counting method, which uses the measurement of the battery current as a proxy for the net change in SOC. However, not all the current that is measured as entering the battery will be used to change the state of charge of the battery – some will be lost due to inefficiency. The system cannot measure the amount of lost current between the entrance to the battery and the final charge that settles into the cells. Therefore, the errors that are endemic to the current measurement will accrue over time as the battery is operated within the flat portion of the curve shown in Figure 1.
Recalibration of the SOC can be done but requires that a BESS sit idle for a period (typically at least 30 minutes), such that the Open Circuit Voltage (OCV) can be measured. In addition, due to the non-deterministic middle range between 5% and 95% SOC on LFP batteries, the BESS must be either discharged to below 5% or charged above 95% to recalibrate to initiate this recalibration.
A more accurate SOC calculation for BESS
Dynamic recalibration, which can be used to recalibrate the SOC calculation in real time, is proving to be the most accurate and efficient way to perform the calculation because it does not require the system to be at rest or at a specific state of charge.
To dynamically recalculate SOC, the system keeps a constantly updated mathematical model of each battery string and what its measurable parameters should be, which can then be compared with available measurable parameters from the battery to more accurately understand the true SOC of the battery.
The more accurate the mathematical system models, the more accurate predictive values that are generated, which in turn makes a more accurate SOC reading. IHI Terrasun’s calculation can leverage site-wide historical data to dynamically update the parameters that go into the mathematical model, thereby increasing the mathematical model’s accuracy and the SOC accuracy.
The diagram of this process is shown in Figure 2.
The accuracy of SOC calculations is paramount for the successful operation of grid-scale BESS. IHI Terrasun's SOC calculation method stands out as an essential feature for our customers, as it can significantly enhance the precision of SOC readings. This increased accuracy not only optimizes the utilization of the BESS but also increases the opportunity for generating additional revenue from an asset. Accurate SOC calculations ensure that the system operates efficiently, meets energy commitments, and avoids costly penalties, ultimately leading to better project outcomes.
