BQ Aquaris 4 B45 Replacement Battery 3.8V 1700mAh

BQ Aquaris 4 — 3.8V Li-Polymer Replacement Battery (B45)

This 3.8V, 1700mAh Li-Polymer cell is a direct swap for the original B45 battery in the BQ Aquaris 4 smartphone. It fits the Aquaris 4 only — not the Aquaris 4.5 or any other model in the range. Capacity is 6.46Wh, matching the stock specification.

• Aquaris 4 fit: The Aquaris 4 uses a specific connector pitch and cell footprint — 54.90 × 54.00 × 4.85mm — that does not transfer across BQ's other Aquaris models. The B45 part number locks this cell to that platform. Cross-fitting a neighbouring model's battery risks connector stress and incorrect BMS communication.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Aquaris 4 mainboard. The BMS accepted the new cell without fault codes, and charge current stepped down correctly at the expected voltage threshold near 4.2V.
• Fuel gauge recalibration on first use: After installing this cell, disable any fast-charge accessory and run one complete discharge-to-charge cycle at standard current. This lets the fuel gauge IC map its coulomb counter to the new cell's discharge curve before higher charge rates are applied to an uncalibrated cell.

Why the Aquaris 4 reports wrong battery percentage after a cell swap

The Aquaris 4 uses a fuel gauge IC that builds a discharge model from previous charge cycles. When the old, degraded cell is replaced, the IC still references that worn curve. The phone reads voltage correctly but maps it to the wrong state-of-charge percentage. One full discharge-charge cycle — down to automatic shutdown, then a full charge — forces the IC to relearn the new cell's voltage-to-capacity curve. After that single cycle, percentage accuracy returns to normal.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC has not yet recalibrated and the phone underestimates how close the cell is to its minimum voltage floor. Under modem or screen load, the cell voltage drops sharply — faster than the stale fuel gauge model predicts — and hits the hardware cutoff before the OS registers low battery. The fix is the same recalibration cycle: one full discharge to shutdown, then charge uninterrupted to 100%. If shutdowns continue after two full cycles, check that charge voltage is reaching 4.20V at the battery terminals.