About this replacement
JDS Labs C5 / C421 Series — 3.7V Li-ion Replacement Battery (ZH613450 1S1P)
This is a 3.7V 1300mAh (4.81Wh) Li-ion cell for the JDS Labs C5, C5D, and C421 portable headphone amplifiers. It replaces the original ZH613450 1S1P pack when the cell can no longer hold a full charge after repeated cycles. Dimensions are 51.00 × 36.00 × 6.43mm — measure your existing cell before ordering if unsure.
- C5, C5D, and C421 compatibility: All three models run the same 3.7V single-cell architecture and share the ZH613450 form factor. The BMS handshake and connector pinout are identical across the series, so one cell covers all three variants without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the C5 platform. The BMS accepted the cell cleanly, held the 3.7V nominal rail through both low-gain and high-gain output stages, and showed no false cutoffs during sustained audio load.
- First-cycle current draw: Run the amplifier at 50% volume for the first full charge cycle. Full-volume operation during the first cycle draws peak current from an uncalibrated cell and can trigger a premature BMS trip before the cell reaches its rated delivery capacity.
Amp shutting off before the battery indicator reaches empty
The C5 and C421 amplifier circuits require a minimum supply voltage to maintain stable bias in the output stage. If the cell voltage drops below roughly 3.2V under load, the amplifier cuts power before the fuel gauge or LED indicator registers empty. This is not a fault in the indicator — it is the amplifier protecting its output stage from under-voltage distortion. A worn cell with high internal resistance reaches that cutoff voltage faster under audio load than it does at rest. Replacing the cell restores the full usable voltage window from 4.2V charged down to the 3.2V cutoff.
Clipping and distortion at moderate volume on a freshly charged amp
Clipping at moderate volume on a charged battery usually points to voltage sag — not the amplifier circuit. A degraded or poorly calibrated cell cannot sustain 4.0–4.2V under the instantaneous current spikes that occur during dynamic audio transients. When the supply rail sags below the headroom the output stage needs, the amp clips the waveform before the volume control reaches its limit. Charge the new cell fully to 4.2V, run the first cycle at 50% volume, and verify the output stage has clean headroom across the full volume range before pushing to maximum gain.