DEASY T258 Compatible Battery 3.7V 1200mAh Li-ion

DEASY T258 / TL1266 / TS518 / TS808 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh (4.44Wh) Li-ion cell from DEASY's X-Longer series. It fits the T258 along with the TL1266, TS518, TS808, and six additional compatible models. It replaces cells that no longer hold a useful charge or have begun to swell.

• T258 / TL1266 / TS518 / TS808 platform fit: These models share the same 3.7V single-cell architecture, connector pinout, and physical envelope (53.00 × 33.90 × 5.70mm), which is why one cell covers the full group. The BMS handshake and charge termination voltage are consistent across all listed variants.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on compatible hardware, confirming BMS communication, correct charge termination at 4.2V, and stable output under combined screen and modem load. No false low-battery cutoff was triggered during testing.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one complete discharge-to-charge cycle before resuming normal use. This allows the fuel gauge IC to map the new cell's discharge curve before high-current charging begins on an uncalibrated reference — skipping this step is the main cause of erratic percentage readings after a cell swap.

Sudden shutdown at 20–30% on the T258 after a cell replacement

This is a voltage cliff problem, not a capacity problem. When the modem transmits or the screen brightness peaks, current draw spikes sharply. A new cell with an uncalibrated fuel gauge IC can report 25% remaining while the actual open-circuit voltage is already near the low-cutoff threshold. The BMS reads a real undervoltage event and shuts down the phone to protect the cell — even though the percentage display says otherwise. One full discharge-charge cycle corrects the coulomb counter and pushes the shutdown threshold back to where it belongs, near 3.4–3.5V under load.

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

Li-ion cells self-discharge during storage. If the cell voltage has dropped below approximately 2.5V, the BMS enters a lockout state and will not pass current to the device — the phone shows no response to the power button. Connect the phone to a wall charger, not a PC port, and leave it for 15–20 minutes without pressing anything. The charge IC delivers a low-current trickle that slowly brings the cell voltage above the BMS re-enable threshold. Once the screen shows any charge indicator, normal charging can resume.