IBM BladeCenter S RAID Battery 4.8V 3500mAh 17P8979

IBM BladeCenter S RAID Controller — 4.8V Ni-MH Replacement Battery (17P8979)

This 4.8V, 3500mAh Ni-MH battery backs up the RAID controller cache in IBM BladeCenter S blade server chassis. It holds cached write data in memory during unexpected power loss, giving the controller enough power to flush pending writes safely to disk. It replaces OEM part numbers 17P8979, 45W5002, 45W4439, 00Y3447, and several others used across BladeCenter S configurations.

• BladeCenter S RAID controller fit: The BladeCenter S SAS RAID controller modules — including 43W3584, 43W3630, and 22R6649 — all share this same 4.8V cell pack and connector footprint. The controller's BMS reads cell voltage and internal resistance at startup; a matched Ni-MH cell pack at the correct voltage prevents a failed handshake that would lock the controller in write-through mode.
• Bench tested on actual hardware: We ran the cell pack through charge, discharge, and BMS negotiation cycles. The BMS accepted the pack without fault codes, and the controller transitioned from write-through to write-back after the learn cycle completed — confirming the cell voltage range and internal resistance sat within the controller's acceptance window.
• Post-installation learn cycle on BladeCenter S: After fitting this battery, initiate a learn cycle from the controller management interface. Use StorCLI /cx bbu show to confirm learn cycle status. The controller stays in write-through mode — and reports the backup window as unknown — until the learn cycle finishes recalibrating against the new cell's actual capacity.

RAID controller staying in write-through mode after battery swap

Write-through mode is not a fault — it is the controller's default safe state when the backup window is unverified. After a battery swap, the controller does not automatically trust the new cell's stated capacity. It runs a learn cycle that discharges and recharges the pack under monitored conditions to calculate a real backup window. Until that cycle completes, the controller holds in write-through and write performance drops noticeably. Trigger the learn cycle manually via StorCLI: /cx bbu startlearning — the controller moves to write-back once it confirms the pack can sustain the cache voltage above 4.2V through the full discharge curve.

Management console showing battery error days after battery installation

This catches many people off guard — the battery appears fine at boot but a fault appears two or three days later. The BladeCenter S controller runs its battery self-assessment on a timed interval, not at every boot. That scheduled test is what surfaces the error, not the installation itself. If the learn cycle was not completed after the swap, the controller lacks a valid backup window estimate and flags the battery as unverified. Run /cx bbu show to check learn cycle state; if it shows "Requested" or "Unknown," start the cycle and allow 24–72 hours for it to complete under normal server load.