About this replacement
Pax A77 — 3.8V Li-Polymer Replacement Battery (YW-015)
This is a 3.8V 5100mAh Li-Polymer battery for the Pax A77 mobile payment terminal. It replaces OEM part YW-015 and fits the A77 directly. Capacity is drawn from product data at 19.38Wh — not estimated from the web.
- A77 platform fit: The Pax A77 runs a single-cell Li-Polymer architecture at 3.8V nominal. The connector pinout and BMS handshake on this replacement match the A77's charge IC, so the terminal recognises the cell and accepts a full charge cycle without triggering a fault code.
- Bench tested on actual hardware: We cycled this battery through the A77's boot sequence, a card transaction, and a receipt print. The BMS held stable through the printer motor current spike — the point where many third-party cells trip an overcurrent cutoff and force a reboot.
- First deployment tip for the A77: After installation, complete one full transaction cycle before putting the terminal into a live queue. The A77 maps battery capacity during normal operation and needs at least one complete power cycle to display the charge indicator accurately.
Pax A77 rebooting mid-transaction after a battery swap
When a new cell goes in, the A77's charge IC starts in a cautious state. The receipt printer draws a short, sharp current spike — sometimes over 1A — when it fires. If the BMS has not yet profiled the cell's internal resistance, it can read that spike as an overcurrent event and cut power. This shows as a mid-transaction reboot with no error code on screen. Running two or three complete transaction cycles lets the BMS establish a baseline and stops the false trips.
Pax A77 not powering on after sitting unused in storage
Li-Polymer cells self-discharge at roughly 2–3% per month. A terminal left in a drawer for several months can drop below the BMS recovery threshold — typically around 2.5V per cell — and the device will not respond to the power button at all. Connect the A77 to its original charger and leave it for at least 30 minutes before attempting to power on. If the charge LED does not light within five minutes of plugging in, the cell voltage is likely below 2.5V and the charge IC needs a trickle-charge wake cycle — keep it connected and check again at the 30-minute mark.