McGaw Horizon 501-305 Infusion Pump Replacement Battery 12V 4000mAh

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

This is a 12V, 4000mAh (48Wh) Ni-MH replacement battery for the McGaw Horizon Infusion Pump 501-305. It restores portable operation when the original cell has degraded to the point where the pump can no longer sustain delivery away from mains power. Dimensions are 127.90 x 52.00 x 51.15 mm — confirm clearance before installation.

• Infusion Pump 501-305 fitment: The 501-305 draws a consistent low-current load profile during infusion cycles, with brief spikes at motor actuation. Ni-MH chemistry handles that duty cycle without the voltage sag that older Ni-Cd cells showed at end-of-charge.
• Bench tested on actual hardware: We cycled this cell through charge and load on a pump-simulated bench profile. The BMS accepted charge, completed cutoff at full capacity, and held voltage within the 501-305 operating window across the full discharge arc.
• Post-installation self-test protocol: After swapping this battery in, let the 501-305 complete its full power-on self-test without interruption. The pump's BMS runs a verification sequence at startup — cutting power mid-cycle flags a false battery fault that persists until the next clean reboot.

Why the 501-305 alarms low battery immediately after a confirmed full charge

The pump's charge IC compares cell voltage against thresholds calibrated for a well-cycled OEM cell. A new Ni-MH cell has a slightly different voltage profile in the first few cycles — the resting voltage reads low relative to those thresholds even when capacity is full. The BMS interprets this as insufficient charge and raises the alarm. Running one complete charge-discharge cycle normalises the cell's voltage signature and clears the false alarm on subsequent boots.

501-305 will not power on after the replacement battery sat in storage

Ni-MH cells self-discharge during storage at a faster rate than lithium chemistries. If the cell voltage drops below the pump's BMS recovery threshold — typically around 10.0–10.5V on a 12V Ni-MH pack — the device will not initialise. Place the battery on a compatible external charger first and bring it to at least 11.5V before inserting it into the pump. Once the pump sees sufficient voltage at startup, normal charging and boot sequence will proceed.