Gardena 8816 4.8V Hedge Trimmer Compatible Battery 3600mAh

My Gardena hedge trimmer cuts fine for a few seconds then the blades slow down and it feels like it's struggling — new battery, same problem

This is voltage sag under load, and it's common with NiMH packs that haven't been properly conditioned after a long shelf period. The cells need two or three full discharge-and-charge cycles before they deliver rated capacity and hold voltage under the blade load. Charge it fully, run it until the trimmer cuts out, then charge again — repeat that cycle twice more. After conditioning, check that the resting voltage after a full charge reads at or above 5.6V.

Charged the new Gardena battery overnight and it's still not holding a charge — dies almost immediately when I try to use it

NiMH batteries at 4.8V can arrive in a partially discharged state after storage, and some chargers won't initiate a proper charge cycle if the pack voltage has dropped too low — the charger reads it as a fault and stops early. Put the battery on the charger, wait for a full indicator, then let it rest for 30 minutes before use rather than using it straight off the charger. If the problem persists, measure the pack voltage with a multimeter immediately after the charger indicates full — it should read between 5.4V and 5.8V; anything below 5.0V means the charger didn't complete the cycle.

My Gardena 8816 trimmer worked fine last season, battery sat in the garage all winter, now the charger just clicks or flashes and won't charge it

NiMH cells that sit unused for months self-discharge deep enough that standard chargers reject them as defective — the pack voltage drops below the threshold the charger needs to start. This is called voltage depression, and the fix is a slow "recovery charge" using a charger that has a trickle or refresh mode, which feeds a small current to bring the cells back above the detection threshold. If your charger has no refresh mode, try warming the battery to room temperature first — cold cells read lower than they actually are, and a garage in winter makes this worse. Once the charger accepts the pack and completes a full cycle, run two conditioning cycles before expecting full performance.