Panasonic EY9251 18V Ni-MH Cordless Drill Replacement Battery 3300mAh

Panasonic EY3544 / EY3551 Series — 18V Ni-MH Replacement Battery (EY9251)

This is an 18V Ni-MH replacement battery rated at 3300mAh (59.4Wh), built to fit the Panasonic EY3544 and EY3551 cordless drill/driver series. It replaces OEM part numbers EY9251, EY9251B, EY971064504, and H1812. The battery slots into the same battery bay as the original Panasonic pack and uses the same terminal layout.

• EY3544 and EY3551 platform fit: Both the EY3544 and EY3551 share the same 18V battery rail, connector pitch, and BMS handshake protocol. One battery covers both platforms without any adapter or modification.
• Bench tested on actual hardware: We cycled this pack through repeated motor-start inrush events on the EY3544 drivetrain. The BMS held steady across trigger-pull spikes without tripping the overcurrent threshold under normal drilling loads.
• Break-in load cycling: On first use, run the drill at half load for two full discharge-charge cycles before going to full torque. This lets the BMS profile the motor inrush draw and set overcurrent thresholds accurately before you hit maximum load.

BMS cutoff on EY3544 motor-start inrush surge

When you pull the trigger hard on the EY3544, the motor draws a short burst of current well above its running draw — this is inrush current. A new or storage-depleted Ni-MH pack can sit at a lower resting voltage, which makes that inrush spike look worse to the BMS relative to available cell headroom. The BMS reads this as an overcurrent event and cuts the output before the motor even reaches speed. Running two half-load break-in cycles first brings cell voltage up and lets the BMS calibrate before you hit full torque.

Drill bogs or loses torque mid-fastener

If the EY3544 starts strong then bogs down partway through a drive, the cause is usually voltage sag — cell voltage drops under sustained load faster than it recovers. On Ni-MH chemistry, this often traces back to corroded or high-resistance terminal contacts in the battery bay rather than the cells themselves. Clean the battery bay contacts with a dry brush and check for any pitting or discolouration on the terminal strips. After cleaning, test under load — if rail voltage holds above 15.5V during a sustained drive, the contacts are no longer the bottleneck.