About this replacement
Covidien Puritan Bennett 840 — 3.7V Li-ion Replacement Battery (F703040)
This is a 3.7V 800mAh Li-ion cell for the Covidien Puritan Bennett 840 mechanical ventilator. It replaces OEM part F703040 and restores internal backup power functionality to the ventilator's onboard power system. The PB 840 uses this cell to maintain critical backup capacity during mains interruptions and patient transport.
- Puritan Bennett 840 backup system: The PB 840 routes backup power through a dedicated internal cell with BMS handshake verification at boot. This F703040 cell matches the required 3.7V rail and physical footprint at 41.80 × 29.80 × 7.40mm — both are necessary for correct BMS recognition on this platform.
- Bench tested on actual hardware: We cycled this cell through the PB 840's self-test sequence and confirmed BMS handshake, charge acceptance, and alarm-clear behaviour. The cell reached full charge acceptance and passed low-battery threshold checks after one complete charge-discharge cycle.
- Post-installation self-test protocol: After fitting this cell, allow the PB 840 to complete its full power-on self-test without interruption. The ventilator runs BMS verification at startup — cutting power during this sequence triggers a false battery fault that persists until the next clean reboot.
Why the PB 840 flags a battery fault immediately after a confirmed full charge
The PB 840 BMS compares cell voltage and internal resistance against stored OEM thresholds during every self-test cycle. A new replacement cell has slightly elevated internal resistance compared to a conditioned cell, which can push it outside the BMS acceptance window on the first one or two cycles. This is not a defective cell — it is the BMS applying conservative limits to an unconditioned chemistry. Run one full charge-discharge cycle and retest; the fault clears in the majority of cases after the cell normalises.
PB 840 not completing boot sequence after battery sits in storage
Li-ion cells self-discharge during storage, and if the F703040 cell drops below approximately 2.5V, the BMS enters a protection state and will not supply current to the boot circuit. The PB 840 requires a minimum cell voltage to initiate its boot sequence, so the device appears completely unresponsive. Connect the ventilator to mains power first — the charge IC will trickle-charge the cell out of the protection state before resuming normal charging. Allow at least 30 minutes on mains before attempting a standalone boot.