Callstel BFX-300 Compatible Battery 3.7V 900mAh Li-ion

Callstel BFX-300 — 3.7V Li-ion Replacement Battery (TM533443 1S1P)

This 3.7V, 900mAh Li-ion cell replaces the original battery in the Callstel BFX-300 portable car speaker. The TM533443 1S1P matches the factory cell's footprint at 46.20 × 34.14 × 5.68mm. When the BFX-300 no longer powers on or drops off early under load, this is the direct swap.

• BFX-300 fit: The BFX-300 runs a single-cell 3.7V architecture — one cell, one protection circuit. Any cell going into this speaker must match that 1S1P configuration. A mismatched cell trips the onboard BMS at startup and the speaker will not initialise.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a BFX-300 unit, monitoring BMS handshake and cutoff behaviour. The protection circuit engaged correctly at low-voltage threshold and accepted charge from the factory USB input without fault.
• First-cycle volume control: Run the BFX-300 at no more than 60% volume during the first full charge cycle. High-volume bass draws peak current before the new cell is conditioned, and the BMS can trigger early protection cutoff — leaving the speaker appearing dead before the cell is even warm.

Why the BFX-300 loses bass depth at high volume on a fresh battery

Sub-bass frequencies demand short, sharp current spikes from the cell. A 900mAh 1S1P cell has a finite discharge rate, and at high volume those spikes pull the cell voltage below the amplifier's stable operating window. The result is audible bass rolloff — not a speaker fault, but a voltage sag event. This is most noticeable on tracks with sustained low-frequency content. Keeping volume under 75% on a new cell reduces sag until the cell has completed a few full cycles.

BFX-300 speaker cuts out completely before the battery indicator shows low

The BMS protection circuit responds to instantaneous voltage drop, not average charge level. At high amplifier draw, the cell can dip below the BMS cutoff threshold — around 2.75V — even when the displayed charge reads mid-level. The speaker shuts off abruptly as protection trips. To recover, disconnect the USB charge cable, wait 10 seconds, and reconnect — this resets the BMS and allows the cell voltage to recover before restart.