HB4J1 T-Mobile GagaI Replacement Battery 3.7V 1300mAh

T-Mobile GagaI / deos X1 / Stockholm — 3.7V Li-ion Replacement Battery (HB4J1)

This is a 3.7V, 1300mAh Li-ion battery carrying OEM part numbers HB4J1 and HB4J1H. It fits the T-Mobile GagaI, deos X1, and Stockholm smartphones. Swap it in when the original cell no longer holds a charge or the phone shuts down unexpectedly under load.

• GagaI, deos X1, and Stockholm compatibility: All three models share the same battery footprint, connector pinout, and BMS handshake protocol, which is why a single cell — the HB4J1 — covers the entire group. The 54 × 43 × 5mm form factor seats correctly in each chassis without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the GagaI platform. The BMS accepted the charge IC handshake on the first connection, voltage held steady at 3.7V nominal throughout the test cycle, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one full discharge-to-charge cycle at standard rate. This lets the phone's fuel gauge IC map the new cell's discharge curve before the charge IC pushes higher current into an uncalibrated cell — skipping this step is the primary cause of erratic percentage readings post-swap.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff problem, not a capacity problem. When the modem, screen, or both draw peak current simultaneously, the cell voltage sags below the BMS cutoff threshold — even though the fuel gauge still shows charge remaining. The gauge is reporting a percentage calculated from the old cell's impedance curve, so it reads high while actual cell voltage has already dropped. Run one full discharge cycle from 100% to automatic shutoff, then charge to 100% uninterrupted — this forces the coulomb counter to resync against the new cell's actual curve.

Phone won't power on after the battery sat in storage

Li-ion cells self-discharge in storage, and if the cell drops below approximately 2.5V the BMS enters lockout mode to prevent damage — the phone will show no sign of life when you press the power button. Connect the phone to a wall charger, not a USB port, and leave it for 20–30 minutes before attempting to power on. The charge IC will trickle current into the cell to bring it above the BMS re-enable threshold, typically around 3.0V, before switching to normal charge rate. If the screen shows no charging indicator after 30 minutes on a wall adapter, try a second known-good cable — a high-resistance cable will not deliver enough current to clear the BMS lockout.