Braun Infusion Pump 501-305 12V Replacement Battery 4000mAh

Braun Infusion Pump 501-305 — 12V Ni-MH Replacement Battery

This is a 12V, 4000mAh (48Wh) Ni-MH replacement battery for the Braun Infusion Pump 501-305. It fits directly into the pump's battery bay and restores power to the infusion control system. Use it when the original cell has lost capacity or no longer holds a charge through a full administration cycle.

• Infusion Pump 501-305 fit: This battery matches the 12V nominal voltage rail and physical footprint the 501-305 pump requires. The BMS in the pump expects a Ni-MH cell at this voltage — substituting a different chemistry or voltage will trigger a persistent battery fault.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS completed handshake and the charge indicator progressed normally. On the first cycle, the charge IC applied a conservative current limit — this is expected behaviour, not a fault.
• Power-on self-test after installation: After fitting this battery, let the 501-305 complete its full power-on self-test without interruption. The pump runs a BMS verification routine at startup — cutting power during this sequence logs a false battery fault that persists until the next clean reboot.

Why the 501-305 alarms low battery on a freshly charged new cell

The pump's BMS stores a charge profile from the previous cell. When a new Ni-MH cell is installed, the BMS compares early voltage readings against that stored threshold — the new cell's internal resistance differs from a broken-in cell, so the BMS flags it as low even at full charge. This is not a fault with the battery. Running one full charge-discharge cycle allows the BMS to recalibrate its threshold to the new cell's behaviour. After that cycle, the low-battery alarm clears at normal charge levels.

Charge indicator stuck below 100% on first charge

On the first charge, the pump's charge IC applies a reduced current limit to a cell it has not yet profiled. This causes the indicator to plateau below 100% even after a long charge. The cell is not defective — the IC is being conservative with an unprofiled Ni-MH pack. Allow the battery to complete a full discharge down to the pump's low-battery cutoff, then run a full charge. After this first complete cycle, the charge IC updates its profile and the indicator reaches 100% correctly.