I have found that often a laptop battery pack that has been discarded as bad as a laptop power source is still usable as a power source for other projects. Most laptop battery packs are internally constructed from 18650-size lithium cells. This instructable describes using cells from a laptop battery pack to power a bicycle lighting system.
The tools and supplies needed for this instructable include: hammer, medium-sized flat-blade screwdriver, wire cutters, wire strippers, 18-gage wire, solder, soldering iron, electrical tape, and of course duct tape.
Step 1: Disassembling the Battery Pack
Use a screwdriver to gently pry the plastic case apart at the seam. Do not use too much force or you may damage the cells inside the pack. Take appropriate safety precautions so that you don't injure yourself. If you don't feel comfortable using these tools safely, then don't do it.
Step 2: Repackaging the 18650 Cells
In a typical battery pack you will find 6 x 18650 cells arranged as 3-series x 2-parallel pairs. These can be disassembled into individual pairs using wire cutters. I forgot to take a picture of the 6 cells before I separated them into pairs.
For this bicycle lighting system, I am using one of the pairs of 18650 cells. I simply added a connector (note: the connector I used is an oddball connector that I got at a local surplus store because they were cheap.) and then wrapped the pair of cells with electrical tape and then duct tape. It is not pretty, but it's be adequate for my needs.
Step 3: Charging the 18650 Cells
Laptop battery packs come in 2 flavors: 10.8V = 3 x 3.6V cells and 11.1v = 3 x 3.7V cells. Earlier Lithium batteries were rated at 3.6v and were charged all the up to 4.1v. Later as the chemistry became more stable they were rated at 3.7v and charged to 4.2v.
WARNING: Lithium cells can overheat and catch fire if overcharged or charged to quickly so you must be very careful when charging them.
For this instructable, I used a current limited bench power supply set for 4.1V and current limited to 200 mA to charge the cells (this was a bit slow, but I was in no hurry and the cells show no heating at all under this slow charging condition). Alternatively you could use a dedicated lithium battery charger which can be found on eBay (search for: USB 18650 Lithium Battery Charger with Protection).
Step 4: Modifying Your Bike Lights to Use an External Battery
For this instructable, I powered both headlight and taillight from the same external battery pack. The headlight and taillight were tested using a bench power supply to verify that they worked well over the voltage range of 3.0V to 4.1V. Before modifying your bike's lights, you should test that they work well over this voltage range. I don't have many details on this step, but to modify your lights you just need to open your lights, remove the batteries, solder wires to the battery terminals and run the wires back to your external battery pack.
Safety Note: An inline fuse is needed at the battery connection point. This is because a short between + and - anywhere in the system can cause the battery to overheat (possibly catastrophically). The fuse value will depend on the current draw of your lighting system. A rule of thumb would be 1.5x the typical current usage. For example, if your lighting system uses 300mA, then choose a 450mA fuse.