Sony SRS-X2 SF-02 Compatible Battery 3.7V 2400mAh

Sony SRS-X2 — 3.7V Li-ion Replacement Battery (SF-02)

This is a 3.7V, 2400mAh Li-ion replacement battery for the Sony SRS-X2 portable Bluetooth speaker. It fits directly into the SRS-X2 and restores battery-powered wireless audio when the original cell has degraded. OEM part numbers SF-02 and 9-885-197-08 both reference this same cell.

• SRS-X2 fit: The SRS-X2 runs a single-cell 3.7V Li-ion pack at a low enough draw to keep the compact amplifier and Bluetooth radio running simultaneously. This cell matches the original connector pinout and voltage window the onboard BMS expects.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the SRS-X2 and confirmed the BMS accepted the pack without fault codes, balanced charge termination fired correctly at 4.2V, and the protection circuit tripped cleanly on simulated deep discharge.
• Fuel gauge reset after install: After fitting this battery, run the speaker down to the low-battery warning tone before recharging fully. This lets the SRS-X2 fuel gauge recalibrate to the new cell's actual capacity and avoids false full-charge readings from the first cycle onward.

Capacity fade from constant top-off charging on the SRS-X2

Most SRS-X2 owners charge the speaker on a desk between every use, often topping it off from 70–80% rather than letting it discharge fully. Li-ion cells cycled in this shallow band accumulate micro-stress on the cathode that accelerates capacity loss faster than deeper cycles would. The cell's state-of-charge window narrows over time, so the speaker reads full but delivers noticeably less playtime. To slow this, let the pack drop below 20% at least once a month before plugging in.

Audio distorting before the battery indicator reaches empty

When the Li-ion cell voltage sags under combined amplifier and Bluetooth radio draw, the SRS-X2 amplifier hits its minimum supply voltage before the battery LED sequence signals low charge. The result is audible clipping or distortion at mid-to-high volume while the indicator still shows charge remaining. This is voltage sag under load, not a capacity problem. A fresh cell with lower internal resistance eliminates the sag — confirm the replacement cell rests at 3.7V open circuit before installation.