JBL Boombox 4 Replacement Battery 21.6V 5000mAh FG6CELL21700G

JBL Boombox 4 / PartyBox 520 / PartyBox 720 — 21.6V Li-ion Replacement Battery (FG6CELL21700G)

This is a 21.6V Li-ion replacement battery rated at 5000mAh (108Wh), using OEM part number FG6CELL21700G. It fits the JBL Boombox 4, PartyBox 520, and PartyBox 720 portable Bluetooth speakers. It replaces a degraded original pack and restores full charge capacity to the speaker.

• Boombox 4, PartyBox 520 and 720 compatibility: All three models share the same 21.6V battery rail, connector pinout, and BMS handshake protocol. The fuel gauge IC in each speaker reads the same authentication sequence from the pack, so one battery serves all three platforms without modification.
• Bench tested on actual hardware: We ran this pack through charge and discharge cycles on a PartyBox 520 unit. The BMS negotiated charge termination correctly at full voltage, and the speaker's protection circuit tripped as expected at the low-voltage cutoff threshold — no false shutdowns during playback at high output.
• Monthly deep-discharge cycle for PartyBox and Boombox users: These speakers are often left plugged in or topped off before the pack drops below 50%. Do this at least once a month: run the speaker until the battery indicator drops below 20% before reconnecting the charger. This prevents fuel gauge drift and slows capacity fade on the 21700-format Li-ion cells.

Audio distortion before the battery indicator reaches empty on the PartyBox 520 and 720

The PartyBox 520 and 720 run high-wattage Class D amplifiers that pull heavy current during bass transients. As the pack discharges toward the lower end of the voltage curve, internal cell resistance rises and voltage sag increases under those current spikes. The amplifier clips before the BMS low-voltage cutoff triggers — so the speaker distorts while the fuel gauge still shows charge remaining. This is a voltage sag issue, not a firmware or speaker fault. Replacing a degraded pack with a fresh 5000mAh cell reduces internal resistance and pushes clipping onset back toward the actual low-voltage threshold.

USB-C charging won't start on a deeply discharged Boombox 4 or PartyBox

If the speaker has been stored discharged for several weeks, individual cells can drop below the USB-C Power Delivery minimum acceptance voltage — typically around 2.5V per cell. At that level, the BMS blocks inbound current as a protection measure, and the charger sees no load, so it never negotiates a voltage contract. The fix is to apply a low-current pre-charge using the original JBL DC barrel adapter if available, which bypasses PD negotiation and trickle-charges the cells back above 3.0V per cell. Once the pack recovers to that threshold, USB-C charging resumes normally.