Stihl HSA 25 10.8V Cordless Shears Replacement Battery

My HSA 25 cuts fine for a few passes then the blade just stops mid-job — battery still shows charged

This is a thermal cutoff trigger, not a dead battery. The HSA 25 draws current in short aggressive bursts when blades contact thick stems, and the protection circuit in a degraded or undersized cell reads that spike as an overload and shuts the tool down. Let the battery sit off the tool for ten minutes and try again — if it recovers and cuts normally before shutting off again in the same pattern, the original cell is no longer holding voltage under load. Check the resting voltage with a multimeter: a healthy 10.8V Li-ion pack should read between 11.5V and 12.6V fully charged; anything below 11V at rest means the cells are not recovering properly.

Replaced the battery on my HSA 25 and now the blades feel slow and weak even on a full charge

A capacity mismatch or a cell imbalance inside the new pack will cause exactly this — the tool runs but never gets the current delivery it expects, so blade speed drops noticeably. This happens most often when a replacement pack has cells that are mismatched in internal resistance, even if the label voltage is correct. Run the shears unloaded and listen: if the blade speed sounds normal in air but bogs immediately on contact with light growth, the pack is sagging under even a small load. Fully discharge and do two complete charge cycles before concluding the pack is faulty, since new Li-ion cells sometimes need conditioning to reach rated output.

HSA 25 battery worked fine in storage all winter but now it won't take a charge at all

Li-ion cells left in a deeply discharged state over a long storage period can drop below the minimum voltage threshold that the charger's protection circuit will accept — the charger sees the pack as faulty and refuses to begin the charge cycle. This is called deep discharge lockout, and it is common in light-draw tools like shears that get stored after a season with less than a full charge remaining. Some chargers will recover a deeply discharged pack with a slow "wake-up" or trickle charge mode — check whether the charger LED is steady or flashing, as a fast blink on most Stihl chargers indicates it has detected an out-of-range pack rather than a charger fault. If the pack reads below 8V on a multimeter, cell recovery is unlikely and replacement is the correct path.