Wolf Garten 40er Bai 12V 3000mAh Ni-MH Replacement Battery

My Wolf Garten mower starts fine then just cuts out halfway through the lawn — why does it stop before the battery is dead?

This is a thermal cutoff trigger, not a dead battery. Ni-MH packs at 12V in high-draw cutting tools heat up fast under continuous load, and the protection circuit shuts the motor down before the cells are damaged — even when charge remains. Let the pack cool for 20 minutes with it off the mower, then check resting voltage: it should sit above 13.2V on a full charge. If voltage looks healthy but the cutout keeps happening earlier each session, the cells are losing capacity and the replacement pack will solve it.

The charger light stays solid green but the mower barely moves through thick grass — did the old battery ruin something in the mower?

The mower is almost certainly fine. What you're seeing is voltage sag — the old Ni-MH pack appeared charged but could no longer hold its voltage under real cutting load, so the motor was running starved. A solid green on the charger only confirms the pack accepted a surface charge, not that the cells have usable capacity under draw. Load-test the old pack with a multimeter while the mower is running: if voltage drops below 10.5V under load, the pack is the problem, not the motor.

New replacement battery fitted but the mower is slower than it was when the original was new — is this normal for a replacement?

Ni-MH cells need several full charge and discharge cycles before they reach rated capacity — this is called conditioning and it's normal for any new pack, not a defect. We see this consistently on the bench: fresh cells often deliver noticeably less torque for the first two or three full cycles. Run the mower until the pack is fully depleted, charge it completely, and repeat that at least twice. If performance is still noticeably weak after three full cycles, check that the terminal contact points on the mower are clean and making solid contact.