Sagem MYC-2 Replacement Battery SA-BIRD 3.7V 850mAh

Sagem MYC-2 — 3.7V Li-ion Replacement Battery (SA-BIRD / BD27-05)

This is a 3.7V, 850mAh Li-ion replacement cell for the Sagem MYC-2 mobile phone. It fits both MYC-2 and MYC2 variants and slots directly into the original battery bay. Capacity is rated at 850mAh (3.15Wh), matching the original Sagem specification.

• MYC-2 and MYC2 compatibility: Both designations refer to the same hardware platform. The battery connector, cell dimensions, and BMS handshake are identical across both variants, so one replacement covers either label you see on the back of the phone.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a MYC-2 unit. The BMS accepted charge at the expected rate and held cutoff at the correct upper and lower voltage thresholds without tripping into protection mode prematurely.
• Fuel gauge recalibration on first install: After fitting this replacement, run the phone down to automatic shutdown once before recharging fully. The MYC-2 fuel gauge IC is calibrated to a known discharge curve — skipping this step leaves the IC working against the old cell's curve, which causes inaccurate percentage readings from day one.

Sudden shutdown at 20–30% on the Sagem MYC-2 after a cell swap

This is a voltage cliff issue, not a fault with the replacement cell. Under load — active call, screen at full brightness, or background data — the cell voltage drops faster than the fuel gauge IC anticipates. When voltage falls below roughly 3.2V under that load, the BMS triggers a hard cutoff to protect the cell, even if the reported percentage still looks healthy. The fuel gauge IC hasn't yet mapped the new cell's internal resistance profile, so its voltage predictions are off. One full discharge-charge cycle corrects the IC's model and pushes the cutoff point back to where it belongs.

Phone won't power on after the replacement cell sat in storage

Li-ion cells self-discharge during storage, and if this one sat long enough, the cell voltage may have dropped below 2.5V — the point where the BMS locks out to prevent further discharge damage. The phone won't respond to the power button because the BMS is blocking all output. Connect the phone to a charger and leave it for 15–20 minutes before attempting to power on; the charge IC needs to push enough current through to lift the cell above the BMS re-enable threshold, typically around 2.9V, before the BMS will release the lock.