About this replacement
Pax S90 3G — 7.4V Li-ion Replacement Battery (S90-MW0-363-01EA)
This 7.4V 1800mAh Li-ion battery replaces the OEM pack in the Pax S90 3G portable payment terminal. It fits the S90 3G only — confirm your model label before ordering. Capacity is 1800mAh (13.32Wh), matching the original specification.
- S90 3G platform fit: The S90 3G uses a specific connector pinout and BMS handshake tied to the terminal's power management IC. This replacement uses the same contact configuration, so the terminal recognises the pack and allows normal boot without triggering a hardware fault.
- Bench tested on actual hardware: We cycled this pack through full charge and discharge sequences on the S90 3G platform. The BMS negotiated correctly with the terminal's charge IC, and no overcurrent trips occurred during the receipt printer and wireless radio draw peaks.
- First deployment tip: After fitting the battery, complete one full transaction cycle — including a receipt print — before putting the terminal into live service. The S90 3G maps battery capacity during normal operation and needs at least one complete power cycle to calibrate the charge indicator accurately.
Pax S90 3G rebooting mid-transaction after a battery swap
The S90 3G draws a sharp current spike when the receipt printer motor fires. On a new battery that hasn't completed its first full charge cycle, the BMS may read this spike as an overcurrent event and cut power momentarily. This resets the terminal mid-sale. The fix is straightforward: charge the replacement pack to 100% before the first transaction, then run one complete print cycle to let the BMS learn the load profile. After that conditioning step, mid-transaction reboots from printer spikes stop.
Terminal not completing PCI boot sequence after sitting unused
If the S90 3G has been stored without use for weeks or months, the battery may self-discharge below the terminal's minimum operating threshold — typically around 3.0V per cell. At that voltage, the PCI boot sequence stalls because the terminal won't advance past the security check on an undervoltage supply. Connect the terminal to its dock charger and leave it charging for at least 90 minutes before attempting to power on. Once the cells recover above 3.2V per cell, the boot sequence completes normally.