ioneer B40 3.7V Smartphone Replacement Battery 1650mAh Li-ion

ioneer B40 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1650mAh (6.11Wh) Li-ion cell that replaces the OEM B40 battery in compatible smartphones. It slots into devices that take the B40 form factor — 51.36 x 50.89 x 5.03mm. Voltage and capacity figures come from the product data, not estimated from the original cell.

• B40 form factor compatibility: Devices that use the B40 share a common physical envelope and connector pinout. The BMS in the replacement cell communicates over those same pins — voltage regulation and charge termination work through the same handshake the phone expects.
• Bench tested on actual hardware: We ran a full discharge-charge cycle and confirmed BMS cutoff at the correct low-voltage threshold. Charge termination triggered at the right point on the CC-CV curve with no thermal excursion.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging begins pushing current into an uncalibrated state.

Why the phone reports wrong battery percentage after a B40 cell swap

The fuel gauge IC — typically a coulomb counter — builds its charge model against the original cell's discharge curve over many cycles. Swap in a new cell and that model no longer matches actual cell behaviour. The IC reads voltage and infers charge state, but without a calibrated reference curve, it guesses. One full slow discharge down to automatic shutdown, followed by a full charge with fast charging off, resets the counter and rebuilds the curve against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff, not a capacity problem. Under a high-current load — mobile data radio or screen at full brightness — the cell voltage sags sharply. The phone's protection circuit sees the rail drop below its cutoff threshold (typically around 3.0–3.2V) and shuts down, even though the reported percentage looked healthy. The fuel gauge IC hasn't yet mapped where that cliff sits on the new cell. One full calibration cycle resolves this — discharge fully until the phone shuts itself off, then charge uninterrupted to 100%.