Cell platform

21700 LFP: the cell under the backup.

Every VoltBridge BBU battery backup unit draws from one cell platform: cylindrical 21700 LFP cells at 3.2V nominal. This page explains why the cylindrical format is returning to the data hall — and what the platform guarantees at cell level.

3.2V nominal per cell

≥6,000 cycle platform rating

-20~60°C discharge window

Trays of 21700 LFP cylindrical cells lined up before grading

FIG.05 — 21700 LFP cell trays before grading · 1536x1024

Industry context

Why cylindrical is coming back to the data hall

Prismatic formats owned the last decade of large batteries. AI backup is pulling cylindrical back into the building — the reasons are rate capability and manufacturing maturity.

Samsung SDI is reported to supply LFP cells for AI data-center backup (industry reports)
CBAK has previewed high-rate LFP cylindrical cells aimed at BBU and UPS duty (industry reports)
Trade press including TheElec has flagged the 3.2V LFP voltage trade-off in server backup — managed at string level
21700 carries the high-rate manufacturing toolchain that consumer cells matured, redirected at stationary backup

Macro shot of a single 21700 LFP cell with visible casing markings

FIG.06 — 21700 LFP cell macro · 1024x1536

Cell quality

Graded, matched, and documented

Cells for one shelf come from one manufacturing lot, are graded for capacity and internal resistance, and are matched into 16S strings so the pack ages evenly. Every cell is UN 38.3 tested — the transport baseline — and the Test Summary travels with the order on request.

Single-lot sourcing per production run
Capacity and internal-resistance grading before stringing
Cells are UN 38.3 tested; Test Summary on request
In-house pre-compliance bench before pack assembly

Cell datasheet

21700 LFP platform, cell-level view

Parameter	21700 LFP platform
Format	Cylindrical 21700
Chemistry	LiFePO4 (LFP)
Nominal voltage	3.2V per cell
String mapping	16S = 51.2V nominal shelf bus
Discharge capability	High-rate continuous, pulse-capable
Cycle life	≥6,000 (platform rating, 25°C / 80% DOD basis)
Discharge temperature	-20~60°C
Charge temperature	0~55°C
Transport status	Cells are UN 38.3 tested; Test Summary on request
Supervision	BMS above the cells — CAN / RS485 / SNMP at rack level, PMBus in-node

Cell-level figures; shelf-level ratings per product datasheet (VB-5125 / VB-E3 / VB-H400)

From cell to shelf

What the platform buys you at pack level

Voltage

The 51.2V standard

Sixteen 3.2V cells in series land exactly on the 51.2V rack battery backup standard — compatible with -48V DC plant windows per ETSI EN 300 132-2.

Rate

1C continuous, 2C pulse

At shelf level the platform delivers 100A continuous and 200A for 10 seconds on the VB-5125 — cell headroom is why the pack stays cool doing it.

Longevity

6,000-cycle service life

≥6,000 cycles at 25°C and 80% DOD — cycled once a day, the rating comfortably outlasts the 5-year warranty.

Ask for the cell file with your sample.

Every VB-5125 and VB-E3 sample ships with lot documentation, and the UN 38.3 Test Summary is available on request — ask for it with your RFQ.

See the shelf it powers