3.6V Flymo Freestyler Compatible Battery 2400mAh

My Flymo Freestyler cuts fine for a bit then the blades just stop mid-job — battery still shows charged

This is thermal cutoff triggering under load, not a dead battery. The 3.6V Li-ion cell in the Freestyler is a small pack, and if the blades are working through dense grass or thick hedge growth, current draw spikes and the protection circuit shuts the cell down before voltage bottoms out. Let the tool sit unpowered for five minutes, then try again — if it restarts immediately, heat or overcurrent was the cause, not the battery itself. If the new battery does this from the first charge cycle, check that the blade pivot isn't stiff or gummed up with debris, because mechanical drag is the fastest way to cook a small-capacity pack like this one.

Replaced the battery in my Freestyler and now it feels weaker than the old one — blades are slower than I remember

A new Li-ion cell often needs two or three full charge-discharge cycles before it reaches rated capacity — this is normal break-in behaviour for this chemistry and is not a fault. If the blades still feel sluggish after a few cycles, the issue is more likely blade condition than the battery: dull or nicked blades create more resistance and make even a healthy pack feel underpowered. Run the tool unloaded and listen — if the blades spin freely at normal speed in the air, the battery is delivering correctly. Check blade sharpness and alignment before assuming the cell is at fault.

Charged the Freestyler overnight and it died after barely touching anything — new battery won't hold a charge

Overnight charging on a small 3.6V Li-ion pack without a termination circuit can cause mild overcharge, which degrades capacity immediately on the first cycle. Charge only until the charger light signals completion, then remove it — leaving it on past that point stresses cells at this voltage class harder than larger packs. Before writing off the battery, fully discharge it through normal use and do one complete controlled charge cycle, as a first charge that was cut short or interrupted can make capacity appear far lower than it is. If it still drops out under light load after a proper full cycle, check the contact terminals on the tool body for corrosion or poor seating.