Doro Primo 366 Replacement Battery RCB01P04 3.7V 1200mAh

Doro Primo 366 — 3.7V Li-ion Replacement Battery (RCB01P04)

This is a 3.7V, 1200mAh Li-ion cell built to replace the original battery in the Doro Primo 366. The Primo 366 is a simplified mobile phone aimed at users who need clear calling and straightforward texting — nothing more. When the original cell degrades and the phone cuts out unexpectedly or refuses to hold charge, this replacement restores the device to full working condition.

• Primo 366 fitment: The Primo 366 uses a removable flat-cell format at 62.18 × 38.14 × 4.50mm. This cell matches that footprint precisely, including the connector position required for the phone's charge IC to communicate with the battery correctly.
• Bench tested on actual hardware: We ran charge cycles on this cell and confirmed the BMS accepted standard charging current without triggering a protection cutoff. Capacity held steady across multiple cycles, and the cell did not trip the low-voltage lockout under simulated call and screen load.
• Fuel gauge recalibration after install: On first use, run the phone from a full charge down to automatic shutdown — do not charge before shutdown. This gives the fuel gauge IC one complete reference cycle against the new cell's discharge curve before the OS starts reporting percentage.

Why the Primo 366 reports wrong battery percentage after a cell swap

The Primo 366 uses a fuel gauge IC that builds its charge estimate against a stored discharge curve from the previous cell. Swap in a new cell and that stored curve no longer matches — the IC will report percentages that drift from actual state of charge. This shows up most often as the phone jumping from 40% to 15% with no load change, or reporting 100% when the cell is clearly not full. One full discharge-charge cycle resets the coulomb counter and re-anchors the IC to the new cell's actual curve.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under the current draw of an active call or backlit screen — a voltage cliff the fuel gauge IC did not anticipate. The phone's protection circuit reads voltage collapse as a low-battery emergency and shuts down immediately, even though the displayed percentage appeared safe. The root cause is an uncalibrated fuel gauge, not a faulty cell. Complete one full discharge cycle to let the IC map the new cell's voltage curve, and shutdowns at false percentages will stop.