About this replacement
JBL PartyBox 300 — 7.4V Li-ion Replacement Battery (SUN-INTE-125)
This 7.4V 13400mAh (99.16Wh) Li-ion battery replaces the original pack in the JBL PartyBox 300 portable Bluetooth speaker. It fits both the PartyBox 300 and JBLPARTYBOX300CN variants. When the original cell degrades after repeated charge cycles, this pack restores full wireless operation to the speaker.
- PartyBox 300 and JBLPARTYBOX300CN fit: Both models share the same 7.4V cell configuration, connector pinout, and BMS handshake. The SUN-INTE-125 part number covers both. No adapter or modification needed.
- Bench tested on actual hardware: We ran this pack through charge and discharge cycles on the PartyBox 300. The BMS communicated correctly with the speaker's onboard controller — charge acceptance, fuel gauge reporting, and protection cutoffs all behaved as expected with the original firmware.
- Monthly deep-discharge cycle: The PartyBox 300 is often kept plugged in or topped off before it drops below 50%. Do this at least once a month: let the speaker drain below 20% before recharging fully. Constant shallow cycling causes fuel gauge drift and accelerates cell capacity fade on this pack.
Why the PartyBox 300 clips and distorts before the battery indicator reads empty
At high volume, the PartyBox 300's Class D amplifier draws heavy current in short bursts. A degraded or partially discharged pack cannot sustain the voltage under those peaks, so cell voltage sags below the amplifier's stable operating threshold. The speaker's protection circuit interprets this as acceptable — the fuel gauge still reads 20–30% — but the amp is already clipping the signal. Replacing the pack resolves this if internal resistance has risen; check resting voltage after a full charge, which should sit at or above 8.3V.
PartyBox 300 won't wake from USB-C charge after sitting unused for weeks
Li-ion cells left at low state of charge for extended periods can drop below the minimum voltage that USB-C PD chargers will accept — typically under 2.5V per cell, or around 5V at the pack level. The charger handshake fails before any current flows, so the speaker appears completely dead. Use a compatible barrel or DC input if available to force a trickle charge, or connect the original JBL charger which has a lower acceptance threshold. Once the pack climbs above 6V, standard USB-C charging resumes normally.