Grizzly AGS 3680-D 3.7V Replacement Battery 2900mAh

My AGS 3680-D cuts fine for a bit then the blade just stops mid-job — why does it keep dying before the battery reads empty?

This is a voltage sag issue, not a capacity problem. At 3.7V the cell has very little headroom, and as soon as internal resistance rises — from age, heat, or a partially discharged state — the motor draws more current than the pack can cleanly deliver and the protection circuit trips. Check the resting voltage of the old pack with a multimeter after a full charge: anything reading below 3.9V means the cell has degraded past the point of reliable delivery under load. Fit the new battery, run the tool, and if it cuts through the full job without stalling, internal resistance on the old cell was the culprit.

The charger finished but the grass shear attachment is noticeably weaker than the hedge trimmer blade — is something wrong with the motor or the battery?

The grass shear attachment has a smaller blade and lighter mechanical load, so if it feels weak compared to the hedge trimmer, the battery is almost certainly the cause — a degraded cell sags hardest under the brief, sharp current spikes the shear makes on each cut stroke. Both attachments run off the same 3.6V motor, so a healthy battery should power both without any difference in feel. Pull the grass shear attachment off, hold the trigger with no load, and listen — a strong motor hum with no load but weak cutting under load points directly to the pack losing voltage under draw. Replace the battery and retest both attachments back to back.

New replacement battery installed and now the charger light won't turn green — it just stays on solid red the whole time

A solid red that never transitions usually means the charger is detecting a cell voltage it doesn't recognise as within its charge window — this can happen with a new Li-ion cell that shipped in a deep-discharge state after sitting in a warehouse. Let the charger run uninterrupted for a full hour before assuming it's faulty; some 3.7V wall adapters need time to trickle the cell up to a voltage where the charge IC will hand over to normal CC/CV mode. If red persists past 90 minutes, measure the battery terminals directly — a reading below 2.5V means the cell tripped its deep-discharge lockout and may need a brief boost charge from a compatible Li-ion charger set to recovery mode before the wall adapter will accept it.