Flymo 18V 5000mAh Cordless Trimmer Replacement Battery

My Flymo UltraTrim starts then cuts out after a few seconds of trimming thick grass — is the battery faulty?

This is thermal cutoff triggering under high load, not a dead battery. Dense or wet grass forces the motor to draw hard spikes of current, and the battery's protection circuit shuts it down before cells overheat. Let the battery cool for 10 minutes off the tool, then test it again on lighter growth — if it runs fine there, the cells are healthy and the cutoff did its job. If it trips immediately even on thin grass, put a multimeter on the terminals and check you're reading at least 17V rested; anything below 15V under light load points to a weak cell group inside.

I put the new battery on the Flymo charger and the charge light just stays green without doing anything — it's not actually charging.

A resting green light on a Flymo charger usually means the charger is detecting a pack voltage that already reads full, or the communication pin between the battery and charger isn't making contact. Reseat the battery firmly — there's a locking tab that needs to click — then check the contact pins on both the battery and charger base for dirt or corrosion and wipe them with a dry cloth. If the light still won't shift to the charging sequence, check the resting pack voltage with a multimeter; a brand-new Li-ion 18V pack should sit between 18V and 21.6V, and a reading below 16V means the pack deep-discharged in transit and needs a recovery charge on a compatible charger that supports 0V recovery mode.

My Flymo EasiCut runs slower than the old battery did even though this replacement is fully charged — the line isn't cutting cleanly.

A fully charged replacement that runs sluggish almost always means the new pack's voltage sag under load is higher than expected, which happens when a battery sits in a warehouse and the cells partially self-discharge and sulfate before first use. Run two full charge-discharge cycles — let it charge completely, use it until the low-battery indicator triggers, then charge again — this balances the cells and typically restores full working voltage under load. Also confirm the cutting line itself isn't worn down to a stub, since a short line reduces the blade speed needed for a clean cut regardless of battery condition. After cycling, if the tool still feels slow, check terminal voltage while the trimmer is spinning freely: a healthy 18V Li-ion pack should hold above 16V under that light load.