Dremel Lite 7760 3.7V Replacement Battery 2900mAh Li-Ion

My Dremel 7760 works fine for a few seconds then slows way down — is the new battery bad or is something else wrong?

That slow-down pattern on a rotary tool usually means the battery is hitting its thermal or current protection threshold under load — common when a fresh Li-ion cell hasn't been through a full charge cycle yet. Charge the replacement fully before first use, then let it cool completely between sessions for the first two or three runs while the cell conditions itself. If the speed drop still happens, check whether the bit is binding or the collet is overtightened, because high mechanical resistance at the shaft pulls more current than the protection circuit will allow. Run the tool unloaded at max speed — if it holds RPM there but sags under material contact, the issue is draw, not the battery.

Charged my 7760 replacement battery through USB and the light went green, but the tool won't spin at all — what's happening?

A green charge light with zero motor response usually means the cell voltage sat too low before charging started and the protection circuit is still blocking output even though the charger declared it full. Disconnect the USB cable, wait two minutes, then reconnect and let it charge again from a powered USB port — not a laptop in sleep mode, which often delivers inconsistent current to 3.7V Li-ion packs. We've seen this recovery behavior on cells that shipped partially discharged after extended storage. If the tool still won't spin after a second full charge, measure the battery output terminals with a multimeter — you're looking for at least 3.6V; anything below 3.0V means the cell didn't recover and needs to be replaced.

My 7760 only works on the lowest speed setting now — the higher speeds just cut out. Was like this right after I swapped the battery.

Higher speed settings on the 7760 pull significantly more current than low speed, and if the replacement cell has even slight internal resistance above spec, the voltage sags enough under that load to trip the tool's undervoltage cutoff. This isn't always a defective battery — it can happen if the pack was deeply discharged before installation. Run two or three full charge-and-use cycles before drawing conclusions, since Li-ion cells often need conditioning to deliver rated current. If speeds 3 and 4 still cut out after three cycles, check that the battery contacts inside the tool are clean and making full contact — oxidized or bent terminals add resistance that compounds the sag problem.