Meister BAS 36 LI Replacement Battery 3.7V 2900mAh

My cordless screwdriver runs fine for a few screws then suddenly cuts out — is the new battery doing that?

That's thermal cutoff triggering early, and it's almost always caused by a cell that hasn't been through a full break-in cycle yet. Li-ion cells at this voltage class need two or three full charge-discharge cycles before they deliver stable current under load. Run it through a full charge, drive screws until it dies completely, then charge it fully again — repeat twice. After break-in, check whether the cutout moves later in the session or disappears entirely.

Charger shows full but the screwdriver feels weaker than it did with the original battery — what's wrong?

The replacement cell voltage and the original cell voltage don't match — the product data shows 3.7V on this replacement versus 7.4V on the original pack, which means the device is running on half the voltage it was designed for. Before installing, confirm the replacement pack is wired to match the original's series configuration inside the housing. Check the open-circuit voltage at the battery terminals with a multimeter — it should read close to 8.2V fully charged for a 7.4V nominal pack, not 4.2V.

New battery won't hold a charge — sits in the drawer for a week and it's dead when I pick it up

Li-ion cells at this capacity sitting unused will self-discharge, but losing charge in under a week points to a protection circuit that's draining the cell or a storage state issue. We see this on bench when a cell ships in a deep-discharged state and the charger handshake doesn't complete properly on the first cycle, leaving the cell partially charged even though the charger light goes green. Put it on charge for a full uninterrupted session without removing it early. If it still drops to zero within a few days of sitting idle, measure resting voltage immediately after a full charge — anything below 3.9V within 48 hours of charging indicates a faulty cell.