Server Batteries & BBWC — RAID Controller Backup

Battery backup units and flash-backed capacitors for RAID controllers

About Server Batteries & BBWC

A RAID controller battery backup unit (BBU) protects cached write data during a power loss. When the battery is healthy, the controller operates in write-back mode — writes are acknowledged as soon as they hit the cache, delivering 2-5x faster write performance. When the battery is dead or missing, the controller falls back to write-through mode, where every write must be committed to disk before acknowledgment, causing severe performance degradation.

Modern controllers use two backup technologies: traditional lithium-ion batteries (BBU/BBWC) that recharge and deplete over 12-24 months, and flash-backed write cache (FBWC) with supercapacitors that use a capacitor to power the NAND flash long enough to dump cache to non-volatile storage. FBWC/supercap modules last 3-5 years and are the preferred solution in current-generation servers.

Popular RAID Battery Replacements

  • Dell PERC H730/H740 Battery (0T40JJ) — standard BBU for PowerEdge 14th gen
  • Dell PERC H330/H730 Cable Battery (0668J7) — 13th gen RAID battery with cable
  • HP Smart Array P440ar FBWC (727258-B21) — 1 GB flash-backed capacitor module
  • HP Smart Array P420i Battery (660093-001) — BBWC for Gen8 servers
  • LSI/Broadcom CacheVault CVPM05 (05-50039-00) — supercapacitor for MegaRAID 9400/9500
  • LSI/Broadcom BBU09 (LSI00279) — battery for older MegaRAID 9271/9361 controllers

Replacing a RAID battery is a hot-swap operation on most controllers — no downtime required. Contact us with your RAID controller model for the correct battery part number.

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Frequently Asked Questions

How do I know if my RAID battery needs replacing?

The RAID controller logs a warning when the battery is degraded or failed. In Dell OpenManage: "Battery capacity below threshold." In HP iLO: "Cache module status: degraded." In the RAID BIOS/CLI: the battery status shows "Failed" or "Replacement Required." You will also notice significantly slower write performance because the controller has switched from write-back to write-through mode.

What is the difference between BBU and FBWC?

A BBU (Battery Backup Unit) is a rechargeable lithium-ion battery that keeps the volatile cache RAM powered during a power outage (24-72 hours). An FBWC (Flash-Backed Write Cache) uses a supercapacitor that provides just enough power (30-60 seconds) to flush the cache contents to onboard NAND flash memory, where data persists indefinitely without power. FBWC is more reliable, lasts longer (3-5 years vs 1-2 years for BBU), and does not require periodic learn cycles that temporarily disable write-back cache.

Can I run a RAID controller without a battery?

Yes, but write performance will be significantly reduced. Without a battery, the controller operates in write-through mode — every write is committed directly to disk before being acknowledged. This is 2-5x slower than write-back mode for random write workloads (databases, VMs, email servers). Read performance is unaffected. For read-heavy workloads, the impact may be minimal, but for any write-intensive application, replacing the battery is critical.

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