Sumitomo Type-25 Replacement Battery 11.1V 3400mAh BU-25

Sumitomo Type-25 / Type-25e Fusion Splicers — 11.1V Li-ion Replacement Battery (BU-25)

This is an 11.1V, 3400mAh Li-ion replacement for the Sumitomo BU-25 battery pack. It fits the Type-25, Type-25e, and related fusion splicer models used in fibre optic splicing and field cable work. Capacity figures come from the product data — 37.74Wh — not from third-party sources.

• Type-25 and Type-25e platform fit: Both models run the same 11.1V three-cell configuration and share the BU-25 form factor, connector pinout, and BMS communication protocol. A pack built for one slots into the other without modification.
• Bench tested on actual hardware: We ran this pack through the splicer's arc calibration sequence and motor alignment cycle. The BMS held stable through the combined motor and arc discharge spike, with no mid-cycle cutoff or error flags thrown on the splicer display.
• Post-storage initialisation before field use: If the pack has sat in transit or storage, run a full arc calibration cycle through the splicer menu before deploying to site. The Type-25 maps battery state during calibration — skipping it causes the low-battery warning to trigger early on the first real splicing session.

Why the Type-25 cuts out during motor alignment or arc discharge

The Type-25 pulls a brief high-current spike when the fibre alignment motors engage and again when the arc fires. If the pack's internal resistance has risen — common in aged cells — that spike drops cell voltage below the BMS cutoff threshold momentarily. The splicer reads this as a low-battery condition and shuts down to protect the splice. A fresh pack with low internal resistance handles the spike without tripping. If shutdowns occur with a new pack, check that the battery contacts on the splicer bay are clean and seated flush.

Pack will not charge after months sitting unused in the carry case

Li-ion cells that self-discharge below approximately 2.5V per cell trigger a BMS sleep state — the pack appears dead and the splicer charger shows no activity. This is a protection lockout, not cell failure. Some chargers include a recovery or boost mode that applies a low pre-charge current to bring cell voltage back above the re-initialisation threshold. If the charger does not have this mode, place the pack on charge for 15–30 minutes regardless of indicator status — enough current often bleeds through to wake the BMS. Recovery is unlikely if cells have sat below 2.0V per cell for an extended period.