About this replacement
Pax A920 / A920C — 3.7V Li-ion Replacement Battery (IS900)
This is a 3.7V, 5250mAh Li-ion replacement battery for the Pax A920 and A920C payment terminals. It replaces the original IS900 cell when the terminal's charge cycle count has degraded capacity to the point of affecting daily transaction volumes. Fits directly into the existing battery bay with no modification.
- A920 and A920C compatibility: Both models share the same battery bay dimensions and BMS handshake protocol. The IS900 cell communicates charge state data over the same SMBus line, so the terminal's charge indicator and low-battery warnings function correctly on both variants.
- Bench tested on actual hardware: We cycled this cell through the A920's full boot sequence and ran back-to-back transaction simulations, including receipt printing. The BMS held stable under the combined wireless, display, and thermal printer current draw without triggering overcurrent protection.
- First-deployment cycle on live terminals: After installing this battery, run at least one complete power cycle before putting the terminal into live service. The A920 maps cell capacity during normal operation — the charge indicator will not read accurately until the terminal has completed a full boot-and-use sequence.
Terminal not completing PCI boot sequence after battery swap
The A920 runs a PCI-mandated self-test during boot that includes a battery health check. A fresh cell has no stored charge history, so the BMS may report an unvalidated state that causes the terminal to stall or loop at the boot screen. This is not a fault — the terminal needs one full charge-to-depletion cycle to accept the new cell's data. Charge to 100%, allow the terminal to complete a full boot, and the PCI sequence will clear on the next power-on.
Battery percentage stuck at 99% after a full charge on new install
On a new Li-ion cell, the A920's charge IC enters a top-off phase once it detects the cell is near full voltage — it trickle-charges rather than terminating immediately. The fuel gauge stays at 99% while this top-off current flows, which can last 30–60 minutes depending on ambient temperature. This is normal charge IC behaviour, not a fault with the battery or the terminal. Unplug and replug the charger once the indicator has held at 99% for over an hour — that clears the top-off state and the gauge will commit to 100%.