About this replacement
Flymo Cordless Grass Trimmer — 18V Li-ion Replacement Battery
This 18V 5000mAh (90Wh) Li-ion pack replaces the Flymo FB18V2.5 battery used across the UltraTrim 260, EasiCut 450, UltraReach 420, EasiTrim 250, and over fifteen additional cordless trimmer models. All listed models run the same 18V platform and share the same connector and BMS communication protocol. Capacity steps up from the stock 2.5Ah to 5.0Ah — same voltage, more cell capacity.
- Cross-model compatibility: The UltraTrim, EasiCut, UltraReach, and EasiTrim lines all draw from the same 18V rail with an identical latch-and-contact interface, so one battery family covers the full range.
- Bench tested on actual hardware: We ran full charge/discharge cycles — the BMS held voltage steady under cutting load and tripped cleanly at the low-voltage cutoff without thermal flags.
- First-use conditioning for Li-ion: Charge the pack fully before the first session and avoid topping it off after short runs — Li-ion cells in trimmer packs last longer when cycled through fuller discharge ranges rather than frequent shallow charges.
Why the 5Ah upgrade matters on a line trimmer
Grass trimmers spike current demand every time the line contacts dense growth or the head accelerates from a stall. A higher-capacity pack carries more amp-hours in reserve, so voltage sag under those load spikes is less pronounced. The trimmer head maintains speed more consistently through thick patches. The 90Wh rating also means the pack holds enough energy for larger garden areas before needing a recharge.
Battery not recognised by the trimmer — cause and fix
If the trimmer powers on but immediately shuts down or shows no response, the most common cause is a BMS handshake failure triggered by a deeply discharged cell group. Connect the battery to its charger for at least 30 minutes before attempting to run the tool — most Flymo chargers will recover a low-voltage pack once it detects minimal cell voltage. If the charger light stays red and never transitions, the pack has dropped below the recovery threshold and needs a trickle-charge compatible charger to bring it back up.