Laptop batteries and 18650's

Been on a power kick lately and had bought some 4-battery holders for 18650 batteries.
They are round batteries between AA and C cells(18.6mm) but with higher voltage.

How happy I was to discover that inside 2 old Dell notebook batteries I had were 6 each 18650's Li-Ion 3.7v cells. Separated the batteries to use in my plastic battery holders.

4 battery pack gives me 14v-17v. Mounted a LM7805 for 5 Volt regulation for Arm boards.

I had big plans for a redundant power board for Arm usage but decided to go minimal.

Anybody reuse notebook batteries?
I cannibalize each and every worn-out laptop battery that comes along. Usually, there's only one cell bad and the others are fine. Not like new, but with at least 50% capacity left.
I've worked on older laptop batteries and found them to have miniature internal electronics and complicated wiring. Not too difficult to replace the cells though.
This device is what caused me to buy the 18650 battery holders in the first place. I had no idea 18650's were in laptop battery packs.
This device uses the LiFePo4 variants and not Lithium-Ion.

There are lots of DANGER warnings posted about these batteries..
I see the dangers more in overcharging and heat than anything so far..
How do you test for capacity or wear?

The voltage after charge is the first indication and the voltage before charge, compared to the other cells in the pack. That shows usually which cells are still good and which aren't.

But, honestly, you never know. I've used cells that hadn't been charged in ten years and I've had to throw away cells that hadn't been charged for six months.

And I test capacity in a simple test setup: discharge with a resistor as load and stop when the voltage reaches a set point. It's not exactly a "measurement", it's a reference. I compare with data from other 18650's. And I've been doing that for a very long time.

The only problem is that it doesn't give an enumerated outcome. It's useless to others, I'm afraid.

You can also buy charger/measurement boxes for very little money. They are popular in the RC (radio control models) world.

I notice some battery packs use a thermal fuse.

There are 2 types of 18650 cells:

- The "naked" cell
- The cell with a protection circuit

The protection circuit is a small circular PCB with an integrated battery controller that cuts the power before the voltage drops too low. These incorporate a current fuse and usually also a thermal fuse. Both are auto. Sometimes, the battery pack also includes thermal fuses that have been strapped on with heat shrink. I usually cut the heat shrink and eliminate these fuses, as they won't fit standard battery holders.