McCulloch ROB 500 18V 4000mAh Replacement Battery for Robotic Lawn Mower

McCulloch ROB 500 / ROB 600 / ROB 800 / ROB S400 — 18V Li-ion Replacement Battery

This 18V 4000mAh (72Wh) lithium-ion battery fits the McCulloch ROB 500, ROB 600, ROB 800, and ROB S400 robotic lawn mowers. It replaces the original pack when capacity has dropped or the mower stops holding charge between cuts. Dimensions are 138.50 x 80.20 x 37.60mm — the same form factor as the factory unit.

• ROB series compatibility: The ROB 500, 600, 800, and S400 share the same 18V battery bay, connector pinout, and BMS handshake protocol. One replacement pack covers all four models without modification.
• Bench tested on actual hardware: We cycled this pack through repeated blade-start events, including simulated heavy-load conditions. The BMS handled motor inrush current without nuisance tripping and balanced cells correctly across the charge cycle.
• Winter storage tip: Before storing the mower for the off-season, run the pack to roughly 50% charge — not flat. A fully depleted Li-ion pack left over winter can discharge below the BMS acceptance threshold, and the charger will refuse it in spring.

Charger showing fault after the mower sat in the garage all winter

Li-ion cells left in a discharged state over several months can drop below 2.5V per cell — the threshold most chargers use to confirm a safe, living pack. When cell voltage falls under that floor, the charger's protection circuit reads it as a fault and refuses to start a charge cycle. Some McCulloch chargers display a flashing red LED or no LED at all in this state. If the pack sat at low charge all winter, connect it to the charger and leave it for 30–60 minutes — some chargers apply a trickle pre-charge to recover borderline packs before switching to full charge mode.

Mower cuts out mid-row then restarts a few minutes later

This is thermal protection activating, not a faulty battery. The blade motor draws high current through dense or wet grass, which heats the cells faster than normal open-lawn mowing. Once cell temperature hits the BMS upper limit, it cuts power to protect the pack. The mower restarts when temperature drops back into the safe window. Let the mower sit for five minutes in a shaded spot, then resume — and check that the mowing zone is set to avoid patches of overgrown or waterlogged grass where load spikes are frequent.