Open Rack V3 48V BBU (Battery Backup Unit Module) Specification, Rev 1.4

Overview

The document defines a single battery module that slots into the Open Rack V3 BBU shelf. Six BBUs sit in the shelf with redundancy, and the system works on a narrow-range 48 V architecture with 47.5-48 V output. Each module contains a battery pack with BMS, a bidirectional charger and discharger, and auxiliary blocks. The specification was contributed by Meta and published under OWFa 1.0; several values are still marked TBD in the text.

The electrical core is the set of operating modes: sleep (storage and transport), standby (most of the module's life), discharge, charge, state-of-health (SOH) test, periodic charge, fault and system-control modes. Standby consumption, fan included, must stay below 15 W. If bus voltage falls below 48.5 V for 2 ms the BBU takes over within 2 ms, and the bus may never drop under 46 V during the transition. The discharger delivers up to 3000 W and 63.2 A, with peaks up to 150% of rated power.

The reference cell is an 18650 Li-ion (at least 1.5 Ah, 4.2 V maximum) and the recommended pack is 11S6P. After at least 4 years at 35 C the pack must still supply 3 kW for at least 4 minutes at the PCM threshold (for example 3.9 V per cell). Charge current is chosen as 1 A or 2 A from the previous discharge energy (200 kJ threshold), and the rack monitor can override it between 0 and 5 A. Maximum recharge time from 0% to 100% is 6 hours, and charging is delayed 1 minute after a discharge event.

Monitoring and maintenance logic is detailed. Every 90 days the BBU runs an SOH test by forced discharge; only one module per shelf tests at a time, and the others join the discharge so shelf backup is preserved. Target SOH accuracy is +/-3% of full charge capacity. A forced-discharge mode supports peak-power shaving and a forced-detached mode allows remote rack power cycling. Communication runs over Modbus and CAN, and firmware updates must take under 2 minutes per BBU.

Environmental, reliability and safety sections define acceptance criteria: 0-40 C operation, 15-35 C long-term ambient, altitude up to 3000 m and an 85 dBA noise limit. The fan needs an 8-year (70,000 h) L10 life at 40 C, all electrolytic capacitors must exceed 8 years, and the BBU excluding battery needs at least 1,000,000 h MTBF at 45 C and full load. Safety requires UL/IEC/EN 62368-1, IEC 62619, UL 9540 and UL 9540A at cell, module and unit level; a module must neither explode nor spread fire during cell thermal runaway.

Key points

1. The battery pack must deliver 3 kW for at least 4 minutes at the PCM threshold (e.g. 3.9 V/cell) after at least 4 years at 35 C (Section 4.2.1).
2. Reference cell is 18650 Li-ion and the recommended pack is 11S6P; discharger input 28.6-44.0 V, input current up to 110 A.
3. Output is 48.0 V at 0% load and 47.5 V at 100% (0.5 V droop); output power 3000 W / 63.2 A; discharger efficiency at least 97%, charger at least 95%.
4. Discharge starts when bus voltage stays below 48.5 V for 2 ms; startup time is at most 2 ms and the bus must never fall below 46 V.
5. Peak load may reach 150% of rated power; in overload the droop extends linearly (47.25 V at 150% load).
6. Maximum recharge time is 6 hours (0-100%, 2 A per pack); charge power is 130 W at 2 A and 265 W at 5 A; charging is delayed 1 minute after a discharge.
7. The SOH test runs every 90 days, one module per shelf at a time, with depth of discharge limited so 90 s of 3 kW backup remains.
8. Standby power is below 15 W including the fan; fan L10 life is 8 years (70,000 h) at 40 C; BBU life is at least 8 years.
9. Safety: UL/IEC/EN 62368-1, IEC 62619, UL 9540 and UL 9540A at cell, module and unit level; no explosion or fire spread on thermal runaway.
10. Forced discharge mode raises BBU output by 3 V for peak shaving; forced detached mode defaults to a 30-minute timeout for remote rack reboot.

Using it in design

• Use 3 kW, 4 minutes and 6 hours recharge as default parameters for ORV3 BBU modules in the rack-power calculator.
• Model the period after an outage during which the rack runs without full backup until recharge completes, especially for generator re-transfer and repeated-outage scenarios.
• Include the 8-year BBU replacement cycle in lifecycle cost and embodied-carbon estimates.
• When distributed lithium BBUs are used, make UL 9540A evidence (cell, module, unit) mandatory in the fire-strategy checklist.
• Where many racks request charge at once, stagger recharge load using the variable charge current (1 A / 2 A) and the rack monitor's 0-5 A override.
• Keep module ambient inside the 15-35 C long-term range; the battery aging assumptions rest on it.

Who should read it

Power-system engineers evaluating rack-level battery backup of the Open Rack V3 type, rack and BBU suppliers, and fire-safety owners.

Document contents

1. License
2. Compliance with OCP Tenets
3. Scope
4. Overview
5. Acronyms
6. Electrical Requirements
7. BBU Operation Modes
8. Battery Pack
9. Discharger
10. Charger
11. Transition between Power Shelf and BBU Shelf
12. SOH test
13. Current Sharing and Redundancy
14. Redundancy and Hot Swap
15. Fuse and Protections
16. Signal Pin Descriptions
17. Communication and Firmware
18. Environment
19. Thermal
20. Mechanical
21. Reliability and Quality
22. Compliance requirements

Related standards

• OCP Open Rack V3 BBU Shelf Specification
• IEC 62619
• UL 9540
• UL 9540A
• IEC 62368-1
• IPC-9592B

Read the document

Publication details

Authors

Sun, D.; Shapiro, D.; Kim, B.; Athavale, J.; Mercado, R. (Meta)

Publisher

Open Compute Project Foundation

Year

2023

Document number

Rev 1.4

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

OWFa 1.0 — Open Compute open specification licence; redistribution permitted

Licence evidence

www.opencompute.org

Citation