Fujikura FSM-50S Replacement Battery 13.2V 4000mAh

Fujikura FSM-50S / FSM-60S Series — 13.2V Ni-MH Replacement Battery (TYPE-39 / BTR-08)

This is a 13.2V, 4000mAh (52.8Wh) Ni-MH replacement battery for the Fujikura FSM-50S, FSM-50R, FSM-60S, and FSM-60R fusion splicers. It replaces OEM part numbers TYPE-39, TYPE-37, BTR-08, and BTR-08R. The battery slots into the splicer's battery bay and powers all core functions — arc calibration, motorised fiber alignment, and the heating oven.

• FSM-50 and FSM-60 platform compatibility: These splicer models share the same battery bay geometry, connector pinout, and BMS handshake protocol. The 13.2V rail is required — it feeds both the precision servo motors and the arc electrode system from a single pack.
• Bench tested on actual hardware: We cycled this pack through arc discharge events and motor-driven fiber alignment sequences on the FSM-60S. The BMS held the 13.2V rail steady during electrode firing and did not trip under the combined servo and heater load.
• Post-install calibration before field deployment: After fitting this pack, run the FSM's built-in arc calibration routine from the maintenance menu before heading to site. The splicer maps battery state during that sequence — skipping it causes the low-battery indicator to trigger prematurely on the first splicing session, even with a full charge.

Why the FSM-50S shuts down during arc discharge even when the battery reads full

The fusion arc draws a sharp current spike — typically lasting under a second but pulling significant instantaneous load. Ni-MH cells that have degraded internally cannot deliver that peak current without their terminal voltage collapsing below the splicer's cutoff threshold. The FSM interprets this voltage sag as a depleted pack and initiates a protective shutdown. A fresh replacement pack with healthy internal resistance handles the spike without the voltage rail dropping far enough to trigger that cutoff.

Splicer does not recognise the new pack after the unit sat unused for several months

Ni-MH packs self-discharge during storage, and if the cell voltage drops below approximately 10V total, the FSM's BMS can enter a sleep state and refuse to communicate with the charger or splicer. The splicer may power on briefly then cut out, or show no battery indication at all. Connect the pack to the splicer's OEM charger and leave it for 30–60 minutes — most BMS circuits recover once trickle current pushes the pack above the 10.8V recovery threshold. If the charger LED does not respond after that, check the charger output voltage at the connector before assuming the pack is faulty.