18650 Li-ion cells for the laptop battery: with protection circuit or not?
Laptop batteries are usually "smart batteries" which means they have in-built protection for the whole pack. That's why they use unprotected batteries.

Protected cells of type 18650 are also not exactly the same dimensions as unprotected batteries because of the extra PCB on one end. These are usually used in flashlights and other gear that use a spring to deal with the difference in size, but since laptop battery cells are in a close space welded together, you might find that protected cells will not fit.

From Wikipedia:


  Lithium-ion cells are made in various sizes, often assembled into packs for portable equipment. Many types are also available with an internal protection circuit to prevent over-discharge and short-circuit damage. This can increase their physical length; for example, an 18650 is around 65 mm (2.6 in) long, but may be around 68 mm (2.7 in) long with an internal protection circuit. Safe and economic recharging requires use of chargers specified for these cells. Popular applications include laptop battery packs, telephones, electronic cigarettes, flashlights and cordless power tools.