Kärcher RLM4 18V Replacement Battery 2500mAh 6.445-098.0

Karcher RLM4 — 18V Li-ion Replacement Battery (6.445-098.0)

This 18V 2500mAh Li-ion battery replaces the original pack in the Karcher RLM4 cordless rotary lawn mower. It fits directly onto the RLM4 platform and matches the voltage and connector configuration of the OEM unit. Capacity is 2500mAh (45Wh) — taken from the product spec, not estimated.

• RLM4 platform fit: The RLM4 runs an 18V blade motor with a BMS handshake tied to that specific voltage rail and connector pinout. This pack matches both, so the mower's protection circuit recognises it and allows the motor to spin up without triggering a fault code.
• Bench tested on actual hardware: We cycled this pack through repeated blade start sequences and confirmed the BMS held steady through the inrush current spike at motor start. Voltage recovery after each load event stayed within the RLM4's operating window throughout testing.
• First spring use — cold cell caution: If the mower has been stored in an unheated garage over winter, bring the battery indoors overnight before the first mow. Below 10°C, Li-ion internal resistance rises sharply, and the blade motor's inrush current can trip the BMS on a cold pack before the cells have warmed up.

BMS cutoff on thick-grass or wet-grass blade load

The RLM4 blade motor draws significantly more current through dense or wet grass than on a dry, short lawn. If that current spike exceeds the BMS threshold, the pack cuts out mid-row as a protection response — not a fault in the battery itself. Letting the pack rest for five minutes allows the BMS to reset and the cells to recover voltage. To reduce trip frequency, overlap mowing rows less and avoid cutting when grass is wet or longer than about 8cm.

Charger showing a fault light after the mower sat all winter

Li-ion cells self-discharge slowly over months. A pack left in a garage from autumn to spring can drop below the minimum acceptance voltage that most chargers will recognise — typically around 10–12V for an 18V pack. When the charger sees a voltage below its acceptance threshold, it flags a fault rather than starting a charge cycle. Place the battery in the charger, wait 10–15 minutes, and remove and reinsert it — some chargers will re-poll and begin a trickle charge once the cells rise above the acceptance floor on a second attempt.