About this replacement
Pax S900 — 7.4V Li-ion Replacement Battery (IS509)
This 7.4V 2200mAh Li-ion battery replaces the original IS509 cell in the Pax S900 portable payment terminal. The S900 is a mobile POS device used for card processing and tap payments in retail and hospitality environments. Voltage and capacity match the original spec exactly — no hardware modification required.
- Pax S900 fitment: The S900 uses a dedicated battery bay with a keyed connector that only accepts the IS509 form factor. The BMS on the S900 mainboard checks cell voltage at boot — a mismatched or exhausted cell will prevent the PCI boot sequence from completing.
- Bench tested on actual hardware: We cycled this cell through the S900's full boot sequence, ran contactless and chip transactions, and triggered the receipt printer. The BMS accepted the cell without fault codes across all three draw events.
- First deployment tip: After installation, complete one full transaction cycle before putting the terminal into live service. The S900 maps battery capacity during normal operation and needs at least one complete power cycle to display an accurate charge indicator on screen.
Terminal rebooting mid-transaction after battery swap
The S900 receipt printer draws a short high-current burst when it fires — this is the most common cause of mid-transaction reboots on a new or degraded battery. If the cell voltage sags below the BMS cutoff threshold during that burst, the terminal treats it as an undervoltage fault and restarts. On a new cell, this usually resolves after one or two full charge-discharge cycles as the cell reaches its rated capacity. If reboots continue after three cycles, check that the battery connector is fully seated — a loose contact increases resistance and worsens voltage sag under load.
S900 not powering on after sitting unused in storage
Li-ion cells self-discharge over time, and if the S900 has been left uncharged for several months, the cell voltage may drop below the BMS recovery threshold — typically around 2.5V per cell. When this happens, the terminal shows no response on the power button and no charge indicator on the screen. Connect the terminal to its dock or direct charger and leave it for 20–30 minutes without pressing anything — most S900 BMS circuits include a trickle pre-charge mode that recovers cells down to approximately 2.0V. If the charge LED does not activate within 30 minutes, the cell is likely below recovery threshold and the battery needs replacement.