MD 16904 Medion 3.7V Li-ion Grass Shear Replacement Battery

Replaced the battery in my MD 16904 and the blades are moving but won't cut through anything — feels weaker than the old one

A freshly installed Li-ion cell often needs two or three full charge-discharge cycles before it reaches full working capacity, so reduced cutting force straight out of the box is normal. The more common cause, though, is a partial first charge — if the tool was run before the battery hit 100%, the protection circuit may have capped the usable range. Charge it fully until the indicator light switches state, then run it until it cuts out on its own, and repeat that cycle twice. After the second full cycle, test it against a stem of the same thickness that defeated it before.

My grass shear just stops mid-job and won't restart, then works fine after sitting for a few minutes

That pattern — cuts out under load, recovers after a short rest — is a thermal cutoff triggering inside the cell. At 3.7V single-cell Li-ion, there's no voltage headroom to absorb heat spikes, so the protection circuit shuts the tool down before the cell overheats. It happens most often when the blades are blunt and the motor is working harder than it should to get through thick stems. Check the blade alignment and sharpness first, then check that the battery contact pins are clean and seating fully — a loose connection increases resistance and generates extra heat at the joint.

Left my MD 16904 sitting in the shed all winter and now the charger light just blinks and never goes solid

A single Li-ion cell sitting unused over winter can drop below the minimum recovery voltage — typically under 2.5V — and the charger refuses to enter full charge mode because it reads the cell as damaged or absent. Some chargers have a trickle or recovery mode that kicks in automatically; leave it connected for up to an hour and watch whether the light pattern changes. If the light stays in fault mode, measure the battery terminal voltage with a multimeter — a reading below 2.5V means the cell is likely outside recoverable range. A reading between 2.5V and 3.0V is borderline; try a charger that supports pre-charge conditioning if you have access to one.