Firestorm A18 18V Ni-MH Replacement Battery 3000mAh

Firestorm FS1800CS Series — 18V Ni-MH Replacement Battery (A18 / FSB18)

This is an 18V Ni-MH replacement battery rated at 3000mAh (54Wh) for Firestorm cordless power tools. It fits the FS1800CS, FS1800D, FS1800D-2, FS1800ID, and over 35 additional Firestorm models. OEM part numbers covered include A18, FSB18, FS180BX, FS18BX, and NST2118, among others.

• FS1800 platform compatibility: These models share the same 18V battery bay, terminal layout, and BMS handshake protocol. The same connector geometry and cell voltage window applies across the full FS1800 range, so one pack covers drills, fasteners, and string trimmers in the lineup.
• Bench tested on actual hardware: We cycled this pack through a full discharge and recharge sequence on an FS1800D drill. The BMS held cutoff at the correct low-voltage threshold and recovered cleanly on charge. Cell balance across the pack stayed within spec after three full cycles.
• Ni-MH break-in on first use: On first use, run the tool at half load for two cycles before maximum torque applications. This allows the BMS to profile the motor inrush current draw before setting overcurrent protection thresholds — skipping this can trigger false cutoffs under heavy drilling.

BMS cutoff on motor-start inrush surge in the FS1800 platform

When you pull the trigger on a loaded drill, the motor draws a spike of current that briefly exceeds steady-state load — sometimes by 3–5×. Ni-MH packs have a BMS overcurrent threshold tuned for the motor's expected inrush profile. If the pack is new, cold, or partially discharged, internal resistance is higher, and the voltage dip on that inrush spike looks worse than it is. The BMS reads this as an overcurrent condition and trips. Running two break-in cycles at reduced load lowers internal resistance and resets the BMS's baseline, preventing nuisance trips during normal trigger pulls.

Tool bogs and loses torque under sustained load

This is voltage sag — not a dead pack. Under sustained high-torque applications like driving large fasteners or boring through hardwood, cell voltage drops across the pack and the tool loses power before the battery is actually depleted. The most common physical cause is oxidised terminal contacts at the battery bay, which add resistance and amplify the sag. Clean the tool's battery contacts with isopropyl alcohol and a contact brush, then check that the pack snaps firmly into the bay with no play. Resting voltage after a full charge should read 21–22V on a Ni-MH 18V pack; anything below 20V at rest suggests a cell issue.