About this replacement
Sony SRS-XB40 / SRS-XB41 — 7.4V Li-ion Replacement Battery (ID770)
This is a 7.4V 6800mAh Li-ion battery for the Sony SRS-XB40 and SRS-XB41 portable Bluetooth speakers. It replaces OEM part numbers ID770, JD770B, and ID659B. Swap it in when the original cell no longer holds charge or the speaker shuts down early during playback.
- SRS-XB40 and XB41 platform fit: Both models share the same battery bay dimensions, cell voltage, and BMS handshake protocol. The 67.94 × 37.58 × 37.02mm pack seats the same connector orientation, so the speaker's protection circuit reads charge state without recalibration.
- Bench tested on actual hardware: We ran this pack on an SRS-XB40 and confirmed the BMS accepted charge from both the AC adapter and USB input, balanced the dual cells correctly, and held the speaker at full output voltage under amplifier load without triggering an early low-voltage cutoff.
- Fuel gauge reset after installation: After fitting this battery, run the SRS-XB40 down to the flashing low-battery LED — do not cut it short at 30%. Then charge uninterrupted to full. Skipping this step leaves the fuel gauge out of sync with the new cell's actual capacity, causing the indicator to misread state of charge from day one.
Why the SRS-XB40 cuts out at high volume on a battery that shows half charge
At peak Extra Bass output, the amplifier draws a sharp current spike that the degraded original cell cannot sustain without voltage sagging below the BMS cutoff threshold. The protection circuit interprets that sag as a dangerously low cell voltage and shuts the speaker down — even though the fuel gauge still shows 40–50%. A fresh 6800mAh cell at 7.4V nominal has enough internal resistance headroom to absorb those spikes without tripping the cutoff. If the speaker still cuts at high volume after fitting this battery, check that the charge cycle reset described above is complete before assuming a fault.
Audio distorting before the battery indicator reaches empty
As cell voltage falls toward the lower end of its discharge curve — roughly 6.4V on a 7.4V nominal pack — the amplifier's supply rail drops with it. The XB40's amplifier stage begins clipping the audio signal once rail voltage dips below what the output stage needs to reproduce bass transients cleanly. You hear distortion or a muddy low end, but the battery LED still shows one bar. This is voltage sag under combined amplifier and radio draw, not a speaker fault. Replace the cell if the distortion onset is creeping earlier across charge cycles — that pattern confirms capacity fade, not a driver problem.