KDDI IS01 Replacement Battery 3.7V 650mAh SA001UAA

KDDI IS01 / SA001 Series — 3.7V Li-ion Replacement Battery (SA001UAA)

This is a 3.7V, 650mAh Li-ion cell for the KDDI IS01, SA001, SA002, and K007 Android smartphones released in Japan. It replaces part numbers SA001UAA and KY003UAA. The cell matches the original dimensions at 53.70 × 34.80 × 3.80mm and slots into the same battery bay.

• IS01 / SA001 / SA002 / K007 platform fit: These four models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell covers all four without any adapter or modification.
• Bench tested on actual hardware: We ran a charge-discharge cycle on the IS01 and confirmed the BMS accepted the cell, reported voltage correctly, and held the charge IC in CC/CV mode through a full cycle without tripping a protection cutoff.
• Fuel gauge recalibration on first use: On the first charge after installation, disable any fast-charge mode if your charger supports it and run one complete discharge-charge cycle. This lets the fuel gauge IC map its coulomb counter to the new cell's actual discharge curve before high-current charging begins.

Sudden shutdown at 20–30% on the IS01 after a cell swap

The IS01's fuel gauge IC retains the discharge curve from the old, degraded cell. When a new cell is installed, the IC does not immediately know where the real voltage cliff sits. Under screen or modem load, voltage can drop sharply at around 3.5–3.6V — below what the IC expected — and the BMS trips an undervoltage cutoff before the displayed percentage reaches zero. One full discharge-charge cycle resets the coulomb counter and moves the reported cutoff to match the new cell's actual curve.

IS01 not powering on after the replacement battery sat in storage

Li-ion cells self-discharge in storage. If the cell dropped below approximately 2.5V per cell, the BMS enters a hard lockout to prevent damage and will not respond to a normal charger. Connect the phone to a USB charger and leave it for 15–30 minutes without pressing the power button — the charge IC will trickle current into the cell at a low rate until voltage climbs above the BMS re-enable threshold, typically around 2.9V. Once the phone shows the charging indicator, proceed with a normal full charge cycle before first use.