Einhell RT-CD18i 18V Replacement Battery 2100mAh Li-ion

Einhell RT-CD18i Series — 18V Li-ion Replacement Battery (4511894)

This is an 18V 2100mAh (37.8Wh) lithium-ion battery for the Einhell RT-CD18i cordless drill-driver and RT-CD18I Hammer Drill. It replaces OEM part numbers 4511894 and 451326001004. Fits the standard battery slot on both models without modification.

• RT-CD18i and RT-CD18I Hammer Drill fit: Both models share the same 18V slide-rail platform and battery connector pinout. The BMS handshake protocol, cell voltage range, and overcurrent thresholds are identical across the pair, so one battery covers both variants.
• Bench tested on actual hardware: We ran this pack through motor-start inrush cycles on the RT-CD18i platform. The BMS handled repeated trigger-pull spikes without tripping, and cell balancing held within 20mV across all cells under sustained load.
• First-use drill conditioning: Run the drill at half load — light drilling or low-torque screw driving — for two full charge cycles before tackling maximum torque applications. This lets the BMS log actual motor inrush draw and set accurate overcurrent protection thresholds before you push the tool hard.

BMS cutoff on the RT-CD18i during motor-start inrush surge

When you pull the trigger fast on a stalled or high-resistance bit, the motor draws a spike that can be three to five times the steady-state current. A freshly installed or storage-depleted pack has a BMS that has not yet profiled this tool's inrush pattern. If the spike breaches the default overcurrent threshold, the BMS trips and the drill cuts dead mid-use. Running two break-in cycles at reduced load teaches the BMS the RT-CD18i's real inrush profile and raises the trip tolerance accordingly.

Charger shows blinking red and never accepts the new pack

Einhell's charger requires each cell to sit above roughly 2.5V before it will begin a charge cycle. A battery that has been in storage can drop below this threshold, and the charger reads it as a fault rather than a flat pack. To recover it, place the pack in the charger and leave it connected for 15–20 minutes — most Einhell chargers include a trickle pre-charge mode that slowly raises cell voltage until the main charge cycle kicks in. If the light stays red past 20 minutes, check the rail contacts for oxidation and clean them with a dry cloth. Target cell voltage for normal charge acceptance is 3.0V per cell minimum.