IBM 571F RAID Controller Replacement Battery 3.7V 3600mAh

IBM 571F / 572F RAID Controller — 3.7V Li-Polymer Replacement Battery (42R3965)

This 3.7V, 3600mAh lithium-polymer cell replaces the backup cache battery in IBM RAID controllers including the 571F, 572F, 5739, and 5778 series. It powers the controller's write-back cache during a power loss event, holding cached data in memory until it can be flushed to disk. Without a functional battery, the controller falls back to write-through mode, cutting cache performance significantly.

• 571F / 572F / 5739 / 5778 cache module fit: These controllers share the same backup battery rail at 3.7V and use the same physical connector and BMS handshake tied to OEM part 42R3965. Cross-references 74Y5665 and 42R3969 map to the same cell specification and are interchangeable across this controller family.
• Bench tested on actual hardware: We cycled this cell through charge, load, and BMS communication checks. The protection circuit responded correctly to undervoltage cutoff and communicated state-of-charge data back to the controller interface without error flags.
• Post-swap learn cycle required: After installation, trigger a battery learn cycle from the controller management interface — use system controller battery show in ONTAP or StorCLI /cx bbu show for Broadcom/LSI. The controller will not recalibrate its backup window estimate until the learn cycle completes, which keeps it in write-through mode until that process finishes.

RAID controller staying in write-through mode after battery swap

Swapping the battery does not automatically return the controller to write-back mode. The controller holds write-through until it completes a full learn cycle and recalculates the backup window against the new cell's actual capacity. This process runs under normal load and takes 24–72 hours depending on the controller model and charge state. Until the cycle completes, the controller treats the backup window as unknown and refuses to enable write-back cache.

Management console reporting a battery fault days after installation

IBM RAID controllers run their battery health assessment on a timed interval, not at boot. If the console surfaces a battery error two or three days post-swap, the assessment has likely run against a cell still mid-learn-cycle. The BMS reports a degraded backup window because recalibration has not finished. Let the learn cycle complete fully, then check status again with system controller battery show — the error clears once the controller has a validated backup window reading.