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My laptop battery died and I've took it apart to replace the cells inside. Do I have to buy new cells with protection circuit or without it?

Googling the existing cells part number LGABC11865, I've found numerous Alibaba pages where it says

top: flat | Bottom: unprotected

But I wonder does it mean cells don't have a protection circuit?

Here are photos of my existing cells: 18650 battery in the laptop pack part number label on the battery

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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.

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  • \$\begingroup\$ The circuit board on the side in the photo is probably the protection circuitry. \$\endgroup\$ – Matt B. Sep 5 '14 at 17:10
  • \$\begingroup\$ The Alibaba photo with calipers shows ~65mm long. (There's clear tape on the calipers to prevent shorting out the cell while measuring.) \$\endgroup\$ – Matt B. Sep 5 '14 at 17:14
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    \$\begingroup\$ You'll want cells with the tab welded on already, not bare cells. In your photo the tabs are spot welded, not soldered on. If you try to solder directly to the cell (rather than tab) at home you'll probably damage the cell with too much heat. \$\endgroup\$ – Matt B. Sep 5 '14 at 17:16
  • \$\begingroup\$ Adding to @Matt B.'s comment: the reason you need the "welded on" batteries is because they're hand picked and closely matched together. That's important for safety and performance reasons. \$\endgroup\$ – Guillermo Prandi Sep 5 '14 at 21:17
  • \$\begingroup\$ I'm going to build my own spot welder. It would be a simple thing made of a bunch of capacitors and one big MOSFET. \$\endgroup\$ – Volodymyr Smotesko Sep 8 '14 at 19:28
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you need matched cells of the correct capacity and you MUST spot weld. soldering any old cells in is a great way to get your laptop to 'overheat' at some point. Complete with smoke and flames. If you look at the price of the proper cells and a whole pack, you'll see there's not much difference.

Don't do it, unless you actually want to create a random stealth incendiary device

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