Firestorm FS120B 12V Ni-MH Replacement Battery 1500mAh

Firestorm FS1200D Series — 12V Ni-MH Replacement Battery (FS120B)

This is a 12V, 1500mAh Ni-MH battery pack built to fit the Firestorm FS1200D and FS1200D-2 cordless drill/driver series, along with over 35 compatible models. It replaces OEM part numbers FS120B, FSB12, and HPB12, among others. When the original pack stops holding charge, this unit slots into the same bay and restores full tool operation.

• FS1200D series compatibility: These models share the same 12V battery bay, connector orientation, and charge terminal layout. The FS120B, FSB12, and HPB12 part numbers are electrically interchangeable across this platform — same voltage rail, same tab spacing, same BMS handshake protocol.
• Bench tested on actual hardware: We ran this pack through charge and load cycles on the FS1200D platform. The BMS accepted charge from the OEM charger without error and held voltage within spec under repeated trigger pulls, including cold-start inrush.
• Break-in load on first use: Run the drill at half load — light drilling, no maximum torque applications — for the first two cycles. This lets the BMS profile the motor's inrush current draw before it sets its overcurrent thresholds. Skipping this step can cause nuisance cutoffs on hard starts.

Trigger-pull cutoff on the FS1200D — what's actually happening

When you pull the trigger on a cordless drill, the motor draws a spike of current several times higher than its running draw. On a Ni-MH pack, this inrush spike hits the BMS overcurrent threshold before the motor reaches operating speed. A worn or newly installed pack is more vulnerable because internal resistance is either elevated from age or not yet profiled. If the drill cuts out instantly on trigger pull and resets after a few seconds, this is the failure mode — not a dead battery. The fix is to break in the pack at half load for two full charge-discharge cycles before full-torque work, allowing the BMS to widen its trip threshold.

Charger shows no activity on a new pack pulled from storage

Ni-MH packs sitting in storage self-discharge over time and can drop below the voltage floor that most OEM chargers require before they begin a charge cycle. The charger reads the low open-circuit voltage as a fault and refuses to start. To recover the pack, apply a slow trickle charge — if your charger has a conditioning or recovery mode, use it; otherwise, seat and reseat the pack three times to prompt a charger re-check. Once the cell voltage climbs above approximately 9.6V for a 12V pack, the charger should accept it and begin a normal cycle.