About this replacement
Fishman Fender Acoustasonic / Fluence — 3.7V Li-Polymer Replacement Battery (PRO-BPK-101)
This 3.7V 1800mAh Li-Polymer battery replaces the internal cell in Fender Acoustasonic guitars and Fishman Fluence systems that run integrated electronic tuner circuits. It matches the OEM part PRO-BPK-101 and fits the 52.50 × 34.00 × 12.20mm cell bay. Use the capacity figure from the product data — 1800mAh (6.66Wh) — not third-party listings.
- Acoustasonic and Fluence platform fit: Both the Fender Acoustasonic tuner circuit and Fishman Fluence pickup system draw from the same 3.7V Li-Polymer cell format. They share voltage rail, connector pinout, and BMS handshake requirements — which is why Fishman lists one part number, PRO-BPK-101, across both platforms.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the tuner circuit. The BMS reached full charge termination cleanly, held stable voltage under signal-processing load, and triggered low-voltage cutoff at the correct threshold without false trips.
- Li-Polymer chemistry match: Do not substitute an alkaline or NiMH cell in this bay. The tuner's voltage reference circuit is calibrated for the 3.7V nominal output of a Li-Polymer cell. A different chemistry shifts the reference voltage and introduces tuning drift even when the display shows a signal lock.
Why the Acoustasonic tuner loses signal lock on a degraded cell
The tuner's signal-processing IC has a minimum operating voltage — typically around 3.0V for Li-Polymer-powered circuits. As the cell ages, internal resistance climbs. Under the brief current spike when the tuner activates, terminal voltage drops below that threshold even if the resting voltage looks acceptable. The IC resets mid-detection, showing a blank display or no needle movement. Swapping in a fresh 1800mAh cell restores the voltage headroom the IC needs to hold a lock.
Tuner display flickering before the low-battery indicator appears
Flickering digits or a briefly blanking screen usually appear before the battery icon flags low charge. This happens because the cell's voltage sags under load faster than the fuel-gauge circuit can report it — the display driver loses power for a fraction of a second on each detection cycle. It is not a display fault. If flickering starts during normal use, the cell is near end of life. Check resting voltage with a multimeter — a reading below 3.5V on this chemistry means replacement is overdue.