This chapter addresses the second of the four ESS axes — BMS and PCS. The BATTERY_MANAGEMENT_SYSTEM (BMS) is the brain that monitors voltage, current, and temperature at the cell, module, and rack levels in real time, estimating SOC, SOH, and SOP, and issuing shutdown commands the moment abnormal operation is detected. The POWER_CONVERSION_SYSTEM (PCS) is the heart that bidirectionally converts battery direct current (DC) into grid alternating current (AC). This chapter harmonizes BMS estimation algorithms and PCS inverter requirements with IEEE 1547, UL 1741, and IEC 61727.
Even safe cells (Chapter 2) are useless without a precise BMS, and a precise BMS threatens the grid if the PCS is inadequate. This chapter defines the cooperation interface between the brain and the heart, ensuring that the two middle axes operate without disconnection.
The BMS performs five core functions. (1) Measurement — real-time measurement of voltage, current, and temperature for each cell. (2) Estimation — derivation of SOC, SOH, and SOP from the measurements. (3) Protection — issuing pre-alarms before the cell exits its safe operating area (SOA), and immediate shutdown commands when it does. (4) Cell balancing (CELL_BALANCING) — active or passive correction of voltage imbalance among cells. (5) Thermal management (THERMAL_MANAGEMENT) — cooperation with cooling and heating subsystems to keep cell temperatures within normal range.
The BMS architecture is typically three-tier. The lowest tier, the cell-monitoring IC (BMIC), directly measures 4 to 16 cells. The middle tier, the module BMS (MBMS), aggregates several BMICs into module-level information. The highest tier, the system BMS (SBMS), integrates all modules and communicates with the PCS and the operating system. Inter-tier communication uses CAN, SPI, UART, or Ethernet. For reliability and security, this standard recommends a combination of CAN 2.0B at 125 kbps and 100BASE-TX Ethernet.
SOC (State-of-Charge) is the current charge level expressed as a percentage of rated capacity. Three estimation methods are recognized: (1) open-circuit voltage (OCV) method, which uses the relationship between resting-state voltage and SOC — accurate but requires a 30-minute or longer rest period; (2) Coulomb counting, which integrates current to compute net charge or discharge — excellent real-time behavior but susceptible to drift; and (3) the Kalman filter, which combines OCV and Coulomb counting to minimize error. This standard recommends the Extended Kalman Filter (EKF) or the Unscented Kalman Filter (UKF) as default algorithms.
SOH (State-of-Health) is the residual-life indicator; the BMS must deploy at least one of the three models from §2.9 (empirical, electrochemical, machine learning). SOP (State-of-Power) is the maximum charge or discharge power the cell can immediately deliver, communicated by the BMS to the PCS in real time and used to determine PCS operating limits.
In Korea, KAIST and KETI jointly developed the K-BatteryNet LSTM-based SOH prediction model, and KIER is researching SOC estimation algorithms for flow battery and sodium-ion chemistries. The SOC, SOH, and SOP algorithm requirements in this standard directly reflect these Korean research outputs.
When many cells are connected in series and parallel within a module or rack, voltage imbalance arises naturally. Accumulated imbalance causes some cells to leave the SOA prematurely, reducing the usable capacity and life of the entire system. Cell balancing corrects this imbalance.
Two balancing methods are recognized. Passive balancing dissipates the energy of higher-voltage cells through resistors — simple but inefficient. Active balancing transfers energy from higher-voltage cells to lower-voltage cells — efficient but with more complex circuitry. This standard recommends active balancing for GRID_SCALE and UTILITY applications and passive balancing for RESIDENTIAL and UPS applications.
All three major Korean manufacturers possess production-grade active balancing circuitry. LG Energy Solution's RESU FLEX adopts a hybrid scheme combining module-level active balancing with cell-level passive balancing, achieving an effective balance between efficiency and cost.
Thermal management maintains cell temperature within the SOA. Normal operating temperature varies by chemistry (see §2.12); operation outside this range increases life degradation and thermal-runaway risk. Four thermal-management methods are recognized: air cooling, liquid cooling, phase-change materials (PCM), and heat pipes.
Air cooling is simple but offers low cooling efficiency, and is mainly used in small RESIDENTIAL and UPS applications. Liquid cooling provides high efficiency and precise control, and is standard for GRID_SCALE and UTILITY applications. PCM is recommended as a passive supplement to active cooling. Heat pipes are used in aerospace applications and are rare in stationary ESS.
For liquid cooling, this standard recommends coolant inlet-to-outlet temperature differential of 5℃ or less, inter-cell temperature variance of 3℃ or less, and maximum operating temperature of 45℃ or less. Hanwha Solutions and Hyosung Heavy Industries are major global suppliers of liquid-cooled ESS, and KETI has developed thermal-behavior simulation models for liquid cooling systems.
The PCS is a BIDIRECTIONAL_CONVERTER device that converts battery DC into grid AC and vice versa. Its core components are: (1) the input-stage DC_DC_CONVERTER, which boosts battery voltage to the PCS internal DC-link voltage (typically 750–1500 V); (2) the intermediate DC link, where smoothing capacitors stabilize the DC voltage; (3) the output-stage INVERTER, which converts the DC link to 60 Hz or 50 Hz three-phase AC; and (4) filters and a transformer, which remove output harmonics and match the grid voltage.
Key performance metrics for the PCS are: conversion efficiency at or above 96% (GRID_SCALE) or 94% (RESIDENTIAL); response time under 100 ms (frequency regulation); output harmonic distortion (THD) at or below 3%; power factor at or above 0.95; and insulation resistance at or above 1 MΩ. This standard integrates UL 1741, IEEE 1547, IEC 61727, and IEC 62103.
The PCS is not merely a power converter; it operates as part of the grid. Accordingly, it must meet strict requirements to avoid degrading grid stability:
In Korea, KEPCO's Distributed Energy Resource Distribution-Grid Interconnection Standard (2024 revision) specifies these requirements for the Korean grid environment. This standard cross-walks KEPCO's criteria with IEEE 1547 and UL 1741 so that Korean operators can satisfy both international and domestic standards simultaneously.
PCS inverters fall into three topology classes. Two-level topology is simple but produces high output harmonics, typically used in small RESIDENTIAL systems. Three-level Neutral-Point-Clamped (NPC) topology offers superior output quality and is standard for GRID_SCALE and UTILITY. Modular multilevel converter (MMC) topology is suited to high-capacity and high-voltage applications.
Hyosung Heavy Industries, LS Electric, and Hanwha Solutions are major Korean global PCS suppliers. Hyosung mass-produces 4 MW NPC-topology PCS units. LS Electric offers modular designs covering a wide output range from 100 kW to 10 MW. Hanwha Solutions focuses on residential and small-commercial applications in the RESIDENTIAL and COMMERCIAL_INDUSTRIAL segments.
KEPCO's "I-SMART" integrated operations platform monitors and controls distributed resources (solar, wind, ESS) in real time. The BMS and PCS data interfaces defined in this standard are fully compatible with I-SMART's communication protocol (IEC 61850-7-420). The Korea Electrotechnology Research Institute (KERI) operates LVRT and FRT test facilities for PCS, and this standard adopts KERI's test procedures as recommended practice.
KEMRI evaluates PCS R&D programs; the PCS performance requirements in this standard align with the KEMRI evaluation matrix. KETI provides the K-BMS Reference Design as a publicly available BMS design guide, which this standard adopts as the starting point for recommended BMS architectures.
Cooperation between BMS and PCS depends absolutely on the reliability of the communication interface. This standard specifies: (1) communication period of 100 ms or less; (2) communication outage detection within 200 ms; (3) PCS entry into safe mode (zero output) within 500 ms of communication loss; and (4) communication-data integrity protection via CRC-32 or HMAC-SHA-256.
Recognized protocols are CAN 2.0B, Modbus TCP, and IEC 61850-7-420. IEC 61850-7-420 is standard for GRID_SCALE and UTILITY applications; CAN or Modbus TCP is used for RESIDENTIAL and COMMERCIAL_INDUSTRIAL applications. KEPCO's I-SMART adopts IEC 61850-7-420, and this standard recommends it as the default protocol for GRID_SCALE applications.
The BMS is not merely a measurement-and-control device but a component of an industrial control system (ICS), and is therefore exposed to cyber threats. An attacker accessing the BMS could falsify SOC estimates or disable protection alarms, deliberately inducing overcharge, overdischarge, or thermal runaway.
This standard defines BMS cyber-security requirements as follows: (1) integrity and confidentiality of communication data via HMAC-SHA-256 and TLS 1.3; (2) digital signature verification of BMS firmware (Secure Boot); (3) multi-factor authentication (MFA) and least-privilege for remote access; (4) tamper-evident (WORM) preservation of incident logs; and (5) periodic vulnerability assessment and patch management.
In Korea, the Korea Internet & Security Agency (KISA) and the KEPCO Cyber Security Center have issued guidelines for ESS and distributed-resource cyber security. This standard is aligned with these guidelines and meets the cyber-security obligations of Electric Utility Act §32-2. The Anchored tier additionally recommends Common Criteria (CC) EAL 4+ certification.
PCS conversion efficiency directly affects ESS economics. This standard defines three efficiency tiers: Standard (94% or above at rated output), High (95.5% or above), and Ultra (97% or above). The latest models from Hyosung and LS Electric meet the Ultra tier, and the Korean government's "ESS R&D Support Program" weights its evaluation according to the efficiency tier.
Conversion efficiency is evaluated not only at rated output but also at partial load. This standard recommends recording efficiency at 25%, 50%, 75%, and 100% load in the manifest. Particularly for GRID_SCALE applications, which operate at an average load factor near 50%, partial-load efficiency is decisive for overall economics.
The BMS issues graded alarms as measurements approach configured thresholds. This standard defines five alarm tiers: (1) Info — near the SOA boundary, awareness only; (2) Warning — approaching the SOA limit, active monitoring; (3) Alarm — temporary SOA exit, automatic curtailment; (4) Trip — clear SOA exit, immediate PCS shutdown command; (5) Emergency Shutdown — thermal-runaway indicators detected, all power circuits cut and fire-suppression activated.
Tier 1 typically corresponds to events such as SOC reaching 95% or temperature reaching 40℃, while tier 5 corresponds to imminent thermal runaway (cell temperature exceeding 80℃, abrupt voltage drop, gas-sensor response). Tier thresholds are set per chemistry and application. Simulator Panel 2 automatically computes recommended thresholds for each chemistry-application combination.
Korean KESCO pre-use inspection requires that tier-5 emergency shutdown respond within 500 ms. The communication interface requirements in this standard (100 ms communication period, 500 ms shutdown response) are designed to satisfy this. KETI has integrated 100 test vectors for tier-5 alarms into simulator Panel 2.
The PCS is a bidirectional converter capable of four-quadrant operation. Quadrant 1 (P+, Q+) delivers active power and supplies reactive power; quadrant 2 (P-, Q+) absorbs active and supplies reactive; quadrant 3 (P-, Q-) absorbs both; quadrant 4 (P+, Q-) delivers active and absorbs reactive. Stationary ESS can leverage all four quadrants to enhance grid stability.
Reactive-power compensation in particular greatly elevates the value proposition of an ESS. KEPCO launched a 2024 pilot in which ESS units near wind and solar farms operate as reactive-power compensators (substituting for STATCOMs). This standard defines four-quadrant capability as a recommended PCS feature.
BMS and PCS communicate with a variety of external systems: operational systems (SCADA, EMS); grid operators (Korea Power Exchange, KEPCO I-SMART); fire-detection and suppression systems; remote-monitoring access by insurers and auditors; and cloud analytics services for predictive maintenance and AI optimization.
Each interface uses standard protocols such as IEC 61850, OPC UA, MQTT, and OpenADR 2.0b. This standard recommends IEC 61850 and OPC UA for GRID_SCALE applications, and MQTT and OpenADR for RESIDENTIAL applications. KEPCO's I-SMART adopts IEC 61850-7-420, and GRID_SCALE PCS conforming to this standard can communicate directly with I-SMART.
External communications are a prime target for cyber attack, so this standard mandates TLS 1.3 or IPSec on every external interface, aligned with KISA's ICS Security Guidelines and Electric Utility Act §32-2 cyber-security obligations.
BMS and PCS performance, safety, and grid-interconnection conformance are evaluated at dedicated test facilities. In Korea, three institutes operate ESS-related test infrastructure: KERI, KTC, and KTR. KERI's Distributed Resource Test Center in Changwon performs PCS LVRT, FRT, and anti-islanding tests on units up to 5 MW rated output; not only Korean but also Japanese and Southeast Asian operators use KERI facilities, and the results feed IEEE 1547 and UL 1741 certification. KTC performs BMS communication-reliability, cyber-security, and electromagnetic compatibility (EMC) testing. KTR performs cell testing and PCS insulation testing, having standardized a 24-hour insulation-resistance monitoring procedure that underpins this standard's 1 MΩ requirement.
BMS and PCS technology will evolve along three axes: AI-driven predictive operation, wide-bandgap (WBG) semiconductor adoption, and digital-twin integration. AI-driven operation deploys LSTM or Transformer networks within the BMS to deliver more accurate SOC, SOH, and SOP estimation. K-BatteryNet by KAIST and KETI exemplifies this; this standard defines AI model validation procedures (MAE bounds and retraining cycles) to ensure "verifiable AI" operation. WBG semiconductors (SiC and GaN) raise PCS conversion efficiency above 97% while reducing PCS size and weight by roughly 30%. Hyosung and LS Electric plan SiC-based PCS production starting in 2025. This standard's Ultra-tier efficiency (97% or above) presumes WBG adoption. Digital twins mirror an ESS's live state into a virtual model for incident simulation, predictive maintenance, and optimization. KETI and KIER are developing ESS digital-twin platforms, and this standard recommends integrating the requisite data interfaces into the manifest.
BMS and PCS reliability is maintained through routine inspection. This standard recommends daily checks (alarm history, communication continuity, mode confirmation), weekly checks (consistency of SOC, SOH, and SOP estimates; cell-voltage spread; cooling-system operation), monthly checks (insulation resistance, ground resistance, conversion efficiency, cyber-security log review), and annual checks (BMS firmware update, PCS protection-setting review, KESCO pre-use inspection renewal). Inspection results accumulate in the manifest, with Anchored-tier requiring Sigstore signatures over inspection records. Korean operators may submit monthly inspection reports to KESCO's KESIS, with this standard's manifest format aligned to the KESIS template.
This chapter has defined the second of the four ESS axes — BMS and PCS. Four core messages: (1) the BMS is the brain that guards cell safety, and the precision of SOC, SOH, and SOP estimation determines overall ESS safety and economics; (2) the PCS is the heart that links the battery to the grid, and must strictly satisfy anti-islanding, LVRT, FRT, and related grid-interconnection requirements; (3) the BMS-PCS communication interface prevents incidents via 100 ms communication and integrity protection; and (4) Korea's KEPCO I-SMART, KERI test facilities, KETI K-BMS, and KEMRI evaluation matrix are directly aligned with this standard, enabling Korean operators to achieve multi-jurisdiction certification through a single procedure.
Chapter 4 addresses ESS safety and fire prevention. The BMS alarms and PCS shutdown commands defined here are the first line of defense determining life and property losses in a fire. In the spirit of "Hongik Ingan," this standard guarantees ESS safety through the precise cooperation of brain (BMS) and heart (PCS).
弘益人間 (Hongik Ingan) — Benefit All Humanity