About this replacement
Harman/Kardon Onyx Studio 9 — 3.7V Li-ion Replacement Battery (I0709A)
This is a 3.7V, 5000mAh Li-ion cell built to fit the Harman/Kardon Onyx Studio 9 portable Bluetooth speaker. It replaces OEM part number I0709A and matches the original's physical dimensions of 73.70 x 25.70 x 23.00mm. If your Onyx Studio 9 no longer holds a charge or powers off unexpectedly, this is the direct replacement cell.
- Onyx Studio 9 fitment: The Studio 9 uses a single-cell 3.7V pack with a specific connector and BMS handshake tied to the speaker's charging circuit. This cell matches the voltage rail and physical form factor that circuit expects — swapping an incorrect cell can trigger protection lockout on the board.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Studio 9 platform, confirming the BMS accepted the cell without fault codes and that the protection circuit tripped correctly at low-voltage cutoff rather than allowing deep discharge.
- Fuel gauge calibration on the Studio 9: Once installed, let the speaker discharge to below 20% at least once a month before recharging. Constant top-off charging without a full discharge cycle causes fuel gauge drift, which makes the indicator read full while actual capacity has quietly dropped.
Why the Onyx Studio 9 audio distorts before the battery indicator reaches empty
As the cell approaches its lower voltage range, the amplifier inside the Studio 9 can no longer draw the current it needs to reproduce loud or bass-heavy audio cleanly. The battery indicator may still show one or two bars, but the cell voltage has already sagged below what the amp stage needs. The result is clipping — distortion that sounds like a crackle or a flattened bass response — even though the speaker hasn't shut off yet. If you hear this, the cell is near end-of-charge; plug in rather than pushing the volume higher.
USB-C won't wake a deeply discharged Onyx Studio 9
USB-C Power Delivery negotiation requires the device to respond to the charger's handshake — but if the cell has dropped below roughly 2.5V, the speaker's microcontroller can't power up to complete that negotiation. The charger sees no valid response and stops sending current. To recover, try a 5V 1A USB-A to USB-C cable instead of a PD charger — the lower, non-negotiated voltage can sometimes trickle enough current into the cell to bring it above the BMS acceptance threshold. Once the indicator light activates, switch back to your normal charger.