Harman/Kardon Onyx Studio 3 Replacement Battery CP-HK02 3.7V

Harman/Kardon Onyx Studio 3 — 3.7V Li-ion Replacement Battery (CP-HK02)

This is a 3.7V, 2600mAh Li-ion cell built to replace the original CP-HK02 pack inside the Harman/Kardon Onyx Studio 3 portable Bluetooth speaker. It fits the single-cell bay on the Onyx Studio 3 and restores cordless playback when the original cell has degraded past usable capacity. Voltage and connector match the OEM spec exactly.

• Onyx Studio 3 fit: The Studio 3 runs a single flat Li-ion cell at 3.7V nominal. The CP-HK02 format locks this to a specific cell footprint — 69.26 × 20.40 × 20.40mm — with a connector and BMS profile that the speaker's charge circuit expects. A wrong cell size or voltage trips the protection circuit before the unit even powers on.
• Bench tested on actual hardware: We cycled this cell through the Studio 3 charge circuit and confirmed BMS handshake, charge termination at 4.2V, and low-voltage cutoff behaviour. The protection board responded correctly at both ends of the discharge curve.
• Monthly discharge cycle for the Studio 3: If you use this speaker on a desk and top it off constantly, run it down below 20% at least once a month before plugging in. Constant shallow charging without a full discharge causes fuel gauge drift on the single-cell pack — the indicator reads full while actual capacity has quietly dropped.

Why the Onyx Studio 3 audio distorts before the battery indicator hits empty

At the tail end of a discharge curve, a degraded or partially discharged Li-ion cell sags under the combined load of the amplifier and Bluetooth radio. When cell voltage dips below roughly 3.4V under load, the amplifier rail drops with it. The amplifier starts clipping before the BMS cuts power entirely, which is why audio breaks up while the LED still shows a bar or two. A fresh cell at full 2600mAh capacity holds voltage flatter under the same load, pushing that clipping point well toward the end of the usable charge.

Bluetooth disconnects at high volume on a new CP-HK02 cell

Loud playback spikes amplifier current draw sharply and briefly. On a single small-format cell like the CP-HK02, that spike causes instantaneous voltage sag. If the BMS interprets that sag as an undervoltage event, it briefly interrupts output — which drops the Bluetooth radio and causes the disconnect. This is not a pairing issue. Confirm the replacement cell is seated fully with no connector flex, and check that the speaker firmware is current, as some Studio 3 units received BMS threshold adjustments via update.