Fujikura FSM-21S Compatible Battery 14.4V 3400mAh

Fujikura FSM-21S / FSM-22S Series — 14.4V Li-ion Replacement Battery (BTR-11)

This 14.4V, 3400mAh lithium-ion battery replaces the BTR-11 and LTR-21 packs used in Fujikura FSM-21S, FSM-22S, FSM-27S, and FSM-28S fusion splicers. These splicers run precise heating elements and arc motors during fiber splicing — both draws hit the battery simultaneously. At 48.96Wh, this pack meets the energy budget those dual loads require.

• FSM-21S through FSM-28S platform fit: These four splicer models share a common 14.4V battery bay, connector pinout, and BMS communication protocol. A single pack family covers the full range because Fujikura kept the power architecture consistent across that generation.
• Bench tested on actual hardware: We cycled this pack through repeated arc-and-motor sequences. The BMS handled the combined inrush current from arc initiation without tripping into protection mode, and cell voltage stayed stable across the discharge curve.
• Field deployment prep for fusion splicers: After installing this pack, run the splicer through its electrode conditioning routine before your first splice job. The splicer references battery state during that cycle, and skipping it can trigger premature low-battery warnings mid-job when cells are actually at adequate charge.

Why the FSM-21S cuts power during arc discharge

The arc electrode in the FSM-21S pulls a sharp current spike at the moment of fiber fusion — this is the highest single draw the splicer makes. If the BMS registers that spike as an over-current fault, it trips the protection circuit and shuts the pack off. Older or degraded cells can't sustain the voltage under that load, which causes the BMS to cut out even when the battery indicator shows remaining charge. A fresh pack with healthy internal resistance handles the arc current without the voltage sagging below the BMS threshold.

Splicer not recognising the new pack after the unit sat unused

If the FSM-21S shows no battery status or fails to power on after installing this pack, the BMS is likely in deep-sleep mode from prolonged storage below recovery voltage. Connect the splicer to its AC adapter first and leave it for 15–20 minutes before attempting to power on — the charger wakes the BMS protection circuit by pushing a low-current recovery charge into the cells. Once the pack responds, the splicer will display battery status normally. Cell voltage should read at or above 12.0V before the BMS fully re-initialises.