About this replacement
Einhell RT-CD18i / RT-CD18I Hammer Drill — 18V Ni-MH Replacement Battery (4511894)
This is an 18V 3300mAh Ni-MH replacement battery for the Einhell RT-CD18i and RT-CD18I Hammer Drill. It replaces OEM part numbers 4511894 and 451326001004. Capacity is 59.4Wh — same voltage rail and connector as the original pack.
- RT-CD18i and RT-CD18I Hammer Drill fit: Both the standard drill/driver and the hammer drill variant share the same 18V battery bay, connector pinout, and BMS handshake protocol — one pack covers both without modification.
- Bench tested on actual hardware: We cycled this pack through full charge and load discharge on an RT-CD18i drill. The BMS held stable across repeated trigger pulls, including the inrush spike at startup, without tripping overcurrent protection.
- Break-in procedure for Ni-MH packs: Run the drill at half torque through two full charge-discharge cycles before maxing out the torque clutch. Ni-MH cells deliver more consistent capacity after the first few conditioning cycles, so skipping this leaves charge headroom on the table early on.
BMS overcurrent trip on motor-start inrush in the RT-CD18i
When you pull the trigger on an RT-CD18i, current spikes sharply before the motor reaches operating speed. This inrush can hit two to three times the running current in the first milliseconds. A new Ni-MH pack — especially from storage — has a higher internal resistance, which amplifies the voltage sag during that spike. If the BMS reads the sag as an overcurrent event, it cuts power before the motor fully starts. Running two break-in cycles at reduced load lowers internal resistance and reduces the chance of a nuisance trip on the first few uses.
Charger not recognising the pack after storage
Ni-MH packs left unused for several months can self-discharge below the voltage threshold some Einhell chargers require to begin a charge cycle. If the charger light stays off or blinks without starting, the pack voltage has likely dropped too low for the charger to accept. Place the pack on the charger and watch for 10–15 minutes — some units will retry. If it still won't accept, try a known-working Ni-MH compatible charger that supports a recovery or trickle-start mode to bring cells up past 1.0V per cell (18V total across the 15-cell pack) before switching to full charge current.