Introduction: DIY Battery Pack for FPV (recycling 18650 Cells From a Laptop)

By The Way Things Work
About: I'm a highschooler who is interested in technology, science, and engineering. In my spare time I work on projects that allow me to learn new skills and concepts.

Old laptop cells can hold a charge long after your laptop goes kaput. Here's how to recycle those cells to make a general-purpose battery pack!

This battery pack is good for things like RC planes, quadcopters, FPV, and more (if your cells have the correct C rating)! It has a nominal voltage of 11.1v (equivalent of 3 cell), and has a capacity of 2700mAh. The weight is 143 grams, and flight-time on my 250 quadcopter is 4-7 minutes. The cells of the battery pack will determine the capacity and C rating of your battery pack; so you can use the techniques in this instructable to build better battery packs by using better cells.

This battery pack is also very safe; the balance connector makes sure that the cells do not get overcharged, and the shrink-wrap makes sure that the battery cells are not short circuited.

Step 1: Materials

Here's a list of all of the materials:

  • 3x 18650 Lithium-Ion cells
  • 29.5mm heat-shrink tubing
  • 55mm heat-shrink tubing
  • 1mm heat-shrink tubing
  • Electrical Tape
  • JST type connector (or connector of your choice)
  • JST-XH 4 pin connector
  • Solder
  • 5mm Tin Wire

Here's all the tools I used:

  • Soldering Iron
  • Heat-gun
  • Hot-glue gun
  • Flux Pen

Obtaining 18650 Cells:

18650 cells are very commonly found within old laptop batteries. Laptop batteries that contain 18650 cells are commonly 19mm or thicker, and/or have circular ridges which correspond to the shape of the 18650 cells. You can Google the part number of your battery pack to figure out if it contains 18650 cells or not.

If you do not have an old laptop battery lying around, you can always buy 18650 cells online. Here's an eBay search link. Beware of fake cells! Here's another instructable which goes over fake 18650 cells. Basically, if there is no weight of the cell in the description, the seller is not genuine!

Step 2: Prepare the Lithium-Ion Cells

Before actually building the pack, you want to make sure that your cells are clean and well insulated. 18650 type cells have a negative outer jacket and a positive tip. It's important to insulate these cells from each other to avoid a short circuit.

The insulation on my cells was a little bit too thin for my liking, so I decided to insulate them even more. I cut some 29.5mm diameter heat-shrink tubing to the size of the cell, adding about a centimeter extra. I used a heat-gun to shrink the wrapping over each of the three cells.

Step 3: Join the Cells Together

Align the cells like in the picture above, with the center cell being backwards. This alignment is necessary to connect the cells together. To keep the cells together, I wrapped the cells several times with electrical tape, then I hot glued the cells together. At this point, I also marked where the positive and negative ends of the battery pack would be.

Step 4: Connect the Cells in Series

Once your cells are physically held together, it's time to form the electrical connections between them. Cut two, 20mm strips of 5mm tin wire. These two bits of wire are going to connect the cells in the diagram above. Use a flux pen to clean the two tips of each strip of wire, then use your soldering iron to tin the two bits of wire.

Do the same thing with all of the contacts of the cells (clean with flux, then tin). Then, using pair of needle-nosed pliers, solder the two wires according to the diagram above. These wires connect the cells in series, that is, plus to minus, and minus to plus. If you measure the difference in voltage from one end of the chain to the other, you should get somewhere near 11.1 volts.

Step 5: Solder the JST Connector

In order to connect the battery pack to different things, you need to solder a connector of some sort to the battery pack. Use a soldering iron to solder a connector according to the diagram above. If the leads of your connector are not long enough, you may need to extend them by soldering another piece of wire to the connector, then using some small heat-shrink tubing to protect the extension.

Step 6: Solder the 4-PIN JST-XH Balance Connector

Charging the battery pack through the JST connector is possible, but it is not recommended. Over time, the voltage in each cell deviates, which may cause a non-balance charger to overcharge a cell. A much better way to charge a battery pack is through a JST-XH connector. A four pin connector is required: one for each cell + a connection to ground. Solder the connector to the battery pack according to the diagram above.

Step 7: Add Tape and Shrink-wrap

I wrapped the leads of the connectors in electrical tape in order to secure them to the battery pack. I also cut some 55mm heat-shrink tubing and shrunk it over the battery pack. This is going to protect the battery pack from short-circuiting due to contact with other metallic objects. Add some velcro, and you are done!

Step 8: Charge the Battery Pack

In order to charge the battery pack, you will need to know the max charging current. If you got your cells from a laptop battery, you may be able to Google the datasheet for your cells. I found my datasheet from the manufacturer: Panasonic. My datasheet suggested a max charging current of 1925 mAh. If you cannot find the datasheet for your cells, a conservative charging rate is: capacity / 2, where capacity if the capacity of your cells.

There are two main types of chargers, "dumb" chargers, and "smart" chargers. Dumb chargers charge at a constant max current. You cannot change the max current on a dumb chargers. Smart chargers have an adjustable current limit. Whether you have a smart or a dumb charger, you must not charge your cells above the max charging current for your cells.

To charge your battery pack, just connect the balance lead to your charger, and if necessary, connect the main power connector as well. Set the voltage to 3 cell (11.1v), then set the max charging current (if available) and begin charging.

Step 9: Use the Battery Pack!

To use this battery pack, just plug in the JST connector to anything that needs power! I use this battery pack for my 250 size quadcopter, and FPV station. I do not know the C rating or internal resistance of the battery, but I can fly my quadcopter without them getting too hot. Use your battery pack at your own risk!

So here's how to build a recycled 18650 battery pack! If you liked this instructable, check out my electric longboard!