My Husqvarna battery-powered garden tool keeps cutting out mid-job even though the battery shows full charge — what's wrong?

This is almost always voltage sag under load: the pack reads full at rest but drops below the tool's cutoff threshold the moment draw spikes. Li-ion cells at 18V degrade unevenly, and one weak cell dragging the group will trigger the protection circuit before the indicator catches up. Check resting voltage with a multimeter — a healthy fully charged 18V Li-ion pack should read between 20.0V and 21.6V at the terminals. If it reads below 20V straight off the charger, the pack isn't holding a true full charge and cell replacement or pack swap is the fix.

I put a brand new replacement battery in and the tool runs noticeably weaker than the original — is the battery the problem or the tool?

A new Li-ion pack sometimes ships in a partially discharged or unbalanced state, and the cells won't deliver full current until they've been cycled two or three times under real load. Run it through a full discharge and a full charge cycle twice before drawing conclusions about the pack. If the weakness persists after three cycles, measure the pack voltage under load — significant drop during operation points to a capacity mismatch or a tool motor that's already drawing more current than it should due to wear. Check the tool's air vents and blade/cutting head for debris restriction before assuming the battery is at fault.

The replacement battery gets warm really fast and the tool shuts off — is that a charger problem or a battery problem?

Rapid heat buildup in an 18V Li-ion pack during use points to high internal resistance, which is a cell condition issue — not a charger issue. The charger only contacts the pack during charge cycles; heat during tool operation is coming from inside the cells fighting to deliver current. We see this on the bench when a replacement pack has been stored discharged for an extended period before shipping, which stresses the cells before they ever see a load. Let the pack cool completely, charge it fully, and check whether the housing stays cool during the first two minutes of light use — if it's already warm by then, the pack has elevated internal resistance and the protection circuit will keep shutting the tool down.