Have you ever had a laptop computer battery that didn't hold a charge for very long? This usually happens after a few years of use. There are several good reasons for you to not put that battery in the trash! If you are ready to get rid of it, be sure to dispose of it properly, as directed by your local waste management authority- but before you do that, realize that there may be some value lurking in that pack.
Laptop computer manufacturers design battery packs to make them seem like a single unit, but they are almost always made up of multiple internal cells. One of the most popular cell types is known as the "18650" cell, which is a name that comes from the approximate external dimensions of the cell (18mm in diameter and 65mm long).
Before we start getting destructive, it is important to think about some safety concerns. Batteries are energy storage devices, and it is crucial to respect them as such. If you manage to touch the positive end of a battery to the negative end of that same battery with a conductor, you may see sparks or fire, and the conductor and battery may become hot enough to start a fire. The most important safety concern is to make sure that you don't short the cells in this way. The second most likely way to hurt yourself with this project is with destroying the original plastic battery pack housing. Be patient, and when possible, look for screws or other fasteners instead of prying and bending with sharp tools. Eye protection would be a great idea.
When you are sure that your laptop battery is done with life as a laptop battery, carefully disassemble the plastic housing to look for something inside like this:
These nine cells are 18650 cells. They are attached to a charging circuit board that monitors the cells, and that will no longer be needed. The cells are usually spot-welded to tabs that are folded in half between the cells. Use a small flat screwdriver similar to the red and blue one in the picture to carefully work under the spot-welded tabs. Apply a radial twisting motion (like you would when using the screwdriver as a screw) and not a prying motion. If you must use a prying motion, be careful on the positive end of the battery, because it is possible to apply enough force to damage the plastic housing and short the positive lead to the case of the battery, which is electrically common with the negative end.
With a little practice, you can turn the above pack of cells into a pile of individual cells. From there, will likely find that some of the cells are bad, and these bad cells were what made the pack performance unsatisfactory. Check the voltage of each cell with a multimeter, and mark any with a voltage of less than 2.5 volts as "bad." Charge the remaining cells in a stand-alone 18650 charger- there are hundreds available from places like eBay, Amazon, and DealExtreme, though one of my favorites is the Liitokala from Chinese sellers on eBay. This one has the capability of recording how much energy a charging cell has taken, and also discharging the cell and recording how much energy it has returned.
Even with the least expensive chargers, it is still easy to tell which cells are not up to snuff. After the charging cycle is complete, remove the battery from the charger and check voltage. It should be 4-4.2 volts. If the cell is still in that voltage range after half an hour of sitting, mark it with a distinctive name or number so that you can tell it apart from the others, and place it in a box that you can label "my free rechargeable lithium batteries."
Once you have a few cells in that box, the possibilities are endless for putting them to work. One great use is illumination. The same sources above are great places to look for 18650-powered flashlights. Since the lithium cells have such low internal resistance, they have a very low self-discharge rate. This makes them a great option for backup flashlights. With torches like the one in the Amazon link, you can populate the glove box of several cars with high-power, well-made flashlights for a very reasonable cost.
If you find that you need illumination while working with both hands, search for some of the many headlamp options that run off of 18650 cells. I've found the headlamps to be great for everything from roadside emergencies to camping.
Now that you know what to search for, you'll be able to find all sorts of other 18650-powered devices. How about a box that will convert their output to a 5-volt USB port for charging all of those USB-powered phones and devices? How about a little portable cooling fan? These and other choices are readily available.
Perhaps the only real challenge in putting these cells to use is keeping straight which ones are charged and which ones are dead. The best solution I have come up with is to use a divided box, where each side of the box is large enough to hold all of the batteries. Label one "charged" and one "discharged," or something similar, and store the cells in the corresponding spot.
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(price as of Oct 1, 2014)