Panasonic EY9251 18V Ni-MH Replacement Battery 2000mAh

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

This is an 18V Ni-MH replacement battery rated at 2000mAh (36Wh), built to fit the Panasonic EY3544 cordless impact driver and related EY35/EY35 series models. It slots into the same bay as the original EY9251 and EY9251B packs. Compatibility extends across the EY3544GQK, EY3551, EY3551GQ, and eleven additional models in the same platform.

• EY3544 and EY3551 platform fit: These models share the same 18V battery bay, connector layout, and BMS handshake protocol. Panasonic standardised the rail geometry and communication lines across this generation of impact drivers, so one pack covers the full group without adapter or modification.
• Bench tested on actual hardware: We cycled this pack through repeated high-torque trigger pulls on an EY3544 and monitored the BMS response to motor-start inrush current. The overcurrent threshold held stable across the test sequence, and cell temperature stayed within spec under continuous load.
• Break-in for impact driver use: On first use, run the driver at half load for two cycles before full-torque fastening. This lets the BMS log the motor inrush current signature and set its overcurrent protection thresholds accurately before you push the pack hard.

BMS cutoff on EY3544 motor-start inrush surge

Impact drivers draw a spike of current the moment the motor engages — especially when driving into dense material. On a new or recently stored Ni-MH pack, the BMS may read that inrush spike as an overcurrent event and cut output before the fastener seats. This is not a faulty battery. The BMS needs one or two full working cycles to profile the motor's start current before it stops tripping on normal load. Run two moderate-load cycles and the cutoff behaviour typically resolves.

Tool bogs under load and feels underpowered mid-job

Voltage sag under heavy load is the usual cause when an impact driver loses torque without fully cutting out. High contact resistance at the battery rail — from oxidation or debris on the terminals — drops the voltage the motor actually receives. Clean both the tool and battery contacts with a dry cloth or light abrasive, then check rail seating. If sag persists after cleaning, measure open-circuit voltage at the pack terminals — a healthy charged 18V Ni-MH pack should read at or above 18V at rest.