IBM System Storage DS3400 Replacement Battery 2.5V 6600mAh

IBM System Storage DS3400 / DS3500 Series — 2.5V Li-ion Replacement Battery (39R6519)

This is a 2.5V 6600mAh Li-ion battery for the RAID controller cache backup circuit in IBM System Storage DS3400, DS3500, DS3512, DS3524, and compatible units. It keeps write-cache data safe during a power loss by holding charge long enough for the controller to flush cache to disk. Capacity figures are taken from product data, not estimated from web sources.

• DS3400 / DS3500 controller compatibility: These models share the same cache backup battery slot, connector pinout, and BMS handshake protocol. Swapping across this family works because the controller firmware reads the same battery registers regardless of enclosure size.
• Bench tested on actual hardware: We ran the battery through a full charge cycle and monitored BMS communication with the controller. The backup window estimate stayed suppressed — as expected — until the learn cycle recalibrated the new cell's capacity.
• Post-installation learn cycle requirement: After fitting this battery, initiate the learn cycle from the controller management interface. On StorCLI-managed systems, run StorCLI /cx bbu show to confirm cycle status. The controller holds in write-through mode until the learn cycle completes and a valid backup window is established — this is normal and takes up to 72 hours under typical load.

RAID controller staying in write-through mode after battery swap

Write-through mode after a battery replacement is not a fault — it is the controller protecting cache data while it has no confirmed backup window. The controller will not switch back to write-back until it has completed a full learn cycle against the new cell. On LSI/Broadcom-based controllers, StorCLI reports the learn cycle status under the BBU section. Until the cycle finishes, write performance will be lower than normal, but no data is at risk.

Management console showing a battery error days after installation

IBM DS-series controllers run their battery health assessment on a timed schedule, not at boot. If you swap the battery and the console flags an error two or three days later, the controller has just reached its next scheduled assessment window and found the learn cycle incomplete. This is not a faulty cell. Let the learn cycle run to completion — the error clears automatically once the controller logs a valid backup window, typically after the battery reaches full charge and the discharge test confirms capacity above the controller's minimum threshold.