If you've owned a laptop for a few years, you've no doubt encountered the problem that the battery eventually loses its ability to hold a charge. Generally what happens is that one or more of the cells inside the battery pack degrades to the point that it cannot be recharged, and the entire pack loses its effectiveness. At this point, your choices are to buy a new replacement pack, or put on your hardware hacking hat and safety glasses, and start taking things apart.

Now, before you start tearing things apart, note that the lithium cells in a laptop battery can contain a lot of power, and you can hurt yourself or set things on fire if you're not careful. Sound scary? It should! And if so, then perhaps you should content yourself with just reading through this posting, rather than messing around with things yourself. Either way, on we go.

As you can see in the image on the right above, a laptop battery pack consists of a number of individual cylindrical cells. These cells look a lot like a standard AA alkaline battery, but they're actually a bit longer and wider than a AA battery. These are lithium cells which go by the part number "18650" (this is usually printed on the side of the cell, in fact).

The cells are wired up in the battery pack in a combination of parallel and serial circuits in order to provide the voltage by the laptop to operate properly. As mentioned earlier, if one or more of the cells goes bad, then the entire pack cannot supply the requisite voltage, and the pack is effectively dead. At that point, you can buy yourself a new battery from the manufacturer (safe and easy!), or you can tear down the pack and replace the individual cells yourself (risky and difficult!). I'm not going to go into the details of rebuilding a laptop battery pack in this posting, but if you're curious, check out the articles here and here, and or take a look at the following YouTube video:

Rather than rebuild a battery pack, I'm going to discuss a couple of options for re-using the individual 18650 cells. I recently disassembled a dead pack and set about determining which cells were salvageable. "Good" 18650 cells present a voltage between 2.7V and 4.1V; bad cells measure less than 2.7V. Using a digital multimeter, it was simple to identify the cells which were still in decent shape. For this particular battery pack, about half of the cells were still good.

Once I identified the functional cells, I charged them up using an external charger designed specifically for 18650s:

Once again, I'll mention that 18650s are NOT AA batteries, and much more caution is required when working with them! In particular, a damaged 18650 can easily short out, resulting in a lot of current flow, heat, fire, and other Bad Things. Really, don't try this at home.

So, once I had the cells separated and charged, what did I do with them? Well, contrary to the title of this article, I really didn't come up with 18,650 different applications. Rather, I've come up with two uses so far, the first of which is as a power source for an LED flashlight:

The flashlight shown above is designed to accept an 18650 lithium battery. This particular flashlight is "regulated" in that it includes circuitry to deactivate the light if the voltage on the 18650 drops below 2.7V. This is important, since the 18650 will be damaged if over-discharged, which is again a Bad Thing. It's worth noting that there are "protected" lithium cells out there that include a small circuit on the top to do the same thing as the flashlight circuitry– i.e. to stop current flow if the battery voltage drops too low.

I obtained my flashlight from DealExtreme, though they don't seem to carry that exact model anymore (though they have dozens of others to choose from). Similar flashlights can also be found on EBay, or in the vendor section of CandlePowerForums (CPF is "the largest and friendliest flashlight community on the internet," and those guys know their flashlights!).

By the way, a good quality LED flashlight is incredibly bright. I tried to take some pictures and video to demonstrate just how bright, but you really have to see this with your own eyes, in person to appreciate it. And the LED is incredibly power-efficient, so it runs for a very, very long time on a single charge. It's easy to see that the future of household lighting is not compact fluorescents, but LEDs.

The other application I found was as a power source for an Arduino project. This one required the purchase of an appropriate battery holder from Digikey:

The 18650s pack a punch, and make a nice power source for driving motors, as shown in the video below:

In the video, I'm actually powering the Arduino as well as the motors, and I'm surprised it works. Motors tend to create a lot of electrical noise, and I've read about many other people who ran into trouble using a common power source for their Arduino and their motors. I presume I'd start seeing trouble if I was driving a heavier load than those little Lego motors.

So there you have it: rather than simply junking your old laptop battery, there are some things you can do with it, if you are careful. I'm sure there are lots of other possibilities, so hit the comments with your ideas...