ST6 Hedge Trimmer 7.2V 3600mAh Ni-MH Replacement Battery

My ST6 hedge trimmer blade starts moving then slows to almost nothing after a few cuts — is the battery dying already?

That's voltage sag under load, and it's the most common early sign of a Ni-MH pack that either has shallow-cycle damage from the old battery or hasn't been properly conditioned on the new one. Ni-MH cells need two or three full discharge-and-charge cycles before they deliver stable output under cutting load. Run the trimmer until it stops completely, charge to full, and repeat that cycle twice before assuming the battery is faulty. If blade speed still drops heavily after conditioning, check that the pack is reading at or above 7.2V at rest with a multimeter — anything below 6.8V at rest points to a cell that didn't recover.

I left my ST6 sitting in the shed all winter and now the new battery won't charge — charger just does nothing.

Ni-MH packs that sit discharged for months can drop below the threshold a standard charger needs to detect a valid battery, so the charger sees nothing and refuses to start. This is a deep-discharge recovery situation, not a dead battery. Some chargers have a recovery or trickle mode — check the charger indicator light behavior, as a slow or alternating blink pattern on some units signals it's attempting a recovery charge rather than a full charge. If the charger shows no activity at all, try warming the battery to room temperature first, then connect it again and wait up to 15 minutes to see if the charge light changes state.

The ST6 trimmer feels weaker cutting thick stems than my old battery did — did I get a lower quality pack?

With Ni-MH at 7.2V, perceived power difference between a new and old pack often comes down to internal resistance, not capacity. A freshly manufactured cell actually has higher internal resistance until it's been cycled, which means it delivers less peak current to the motor on tough cuts early in its life. We see this consistently on the bench — output improves measurably after the first few full cycles. Cycle the battery completely two to three times before comparing performance, and if weakness persists on thick stems, check that the blade mechanism isn't dragging due to resin buildup, which will mask what is actually a normal battery.