Recycling a Laptop Battery Pack for Bicycle Lighting





Introduction: Recycling a Laptop Battery Pack for Bicycle Lighting

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.



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    12 Discussions

    While a good idea, i think that this is a slightly poor choice of battery source because you get rid of your laptop batteries because they don't hold a charge anymore due to the breakdown of the chemicals inside.

    4 replies

    In a typical laptop battery pack there are 6 x 18650 cells arranged as 3-series x 2-parallel pairs. After disassembling the packs I have found that it is often only a few cells out of the 6 that won't hold charge anymore (which disables the entire pack). The rest of the "good" cells can be re-purposed for bicycle lighting. Point taken that even the "good" used cells won't last for as many recharge cycles as brand new cells, but hey.. they're free :)


    Appreciate the explanation. Any safety tips on re-purposing the batteries apart from over current protection when charging?

    Over current while discharging is another danger. A short between + and - anywhere in the system will cause the battery (and probably also the wire) to overheat (possibly catastrophically). In hindsight, this lighting system needs an inline fuse. I will update this instructable accordingly. Thanks :)

    Another thing to be careful of is accidentally shorting the battery + to the - of the battery case when soldering a wire to the battery +. I had this happen once and the battery quickly became hot and I had to scramble to clear the short.

    Another safety tip would be to wear rubber gloves when working on the battery pack. I wore lightweight latex rubber rubber gloves when soldering the wires to the battery terminals. For this battery pack, the voltage level is low and dry skin resistance is high so I think the shock risk is low, but better to be safe.

    I had a friend once tell me that whenever he works on high voltage, he keeps one hand behind his back so there is never a chance of accidentally creating a circuit from one hand (through his heart) to the other hand. Good tip.

    Okay awesome. Thanks for the tips.

    I've got a question for scd. I saw in one of your earlier projects when you made a controller for an electric bicycle hub. Is it possible to make a controller that will control multiple hubs simultaneously?

    1 reply

    The answer is 2-part ..

    1) For brushed DC motors if the controller's output current capacity is sufficient, then you can just use one controller to control multiple motors (just wire them in parallel). This assumes that the motors are reasonably matched (they spin at the same RPM for a given voltage input.

    2) For brushless DC motors it is possible, but it requires a more complex controller, since the controller would need to separately commutate each motor. (Note: We know it can be done since the Tesla Model S P85D has 2 electric motors)


    18659 batteries can be used invaping mech mods. So it would be a cheaper way to get those

    Cool, digging in closet for old laptops even as we speak... probably too old... (NiMH or NiCAD batteries.) Crossing fingers.

    Nice bench power supply you have there.

    If you search for "LiPO charger" here on Instructables you can find alternative methods of charging, possibly safer or faster. (One OR the other, maybe even BOTH. :-)


    2 years ago

    Thanks, Appreciate your post!

    Hi, sounds great. I have 2 laptop battery to use. I will try to do it the next weekend.