3.7V Black&Decker AS36LN Replacement Battery 2900mAh

My Black & Decker AS36LN screwdriver feels weaker than it used to — it bogs down on the same screws it handled fine before

Voltage sag under load is the most common cause on 3.6V Li-ion cells that have been through repeated shallow cycles — the battery reads "charged" but can't hold voltage once the motor draws current. Charge it fully, then check the resting voltage with a multimeter: a healthy cell sits at or above 3.6V at rest, but a degraded one will drop sharply the moment the tool is under load. If you're seeing that drop, the cell capacity has worn below the point where it can sustain torque through a full screw-driving cycle. Replace the battery and run it through two full charge/discharge cycles before judging performance.

The AS36LN charged fine for years but now it won't hold a charge at all after sitting in a drawer for a few months

Li-ion cells self-discharge slowly, and if the AS36LN sat long enough for the cell to drop below roughly 2.5V, the charger may no longer recognise it as a valid battery and will refuse to charge — this is called deep discharge lockout. Put the battery on charge and watch for any LED response within the first few minutes; no response at all usually confirms the cell has dropped below the charger's acceptance threshold. Some chargers have a recovery or "boost" mode that can wake a deeply discharged Li-ion cell — check if yours does. If the charger shows no sign of accepting the pack after 30 minutes, measure the cell voltage directly: below 2.0V, recovery is unlikely and replacement is the practical fix.

I just put a new battery in my AS36LN and the screwdriver spins but cuts out almost immediately under any load

This points to a protection circuit trip, not a bad battery — the built-in PCB cuts power when it detects an overcurrent event, which can happen if the contacts between the battery and tool are dirty or making partial contact and causing resistance spikes. Clean the battery terminals and the tool's contact points with isopropyl alcohol, then seat the battery firmly and retry. If it still cuts out, check that the replacement battery's dimensions match exactly — even a fraction of a millimetre of misalignment can cause an intermittent connection that the protection circuit reads as a fault. A fully charged replacement cell should read 4.1–4.2V before installation; if it reads lower out of the box, the cell was shipped in a depleted state and needs a full charge before the tool will run correctly.