Almost inevitably, every hacker will start to collect a few batteries from old laptops. Even though some of these batteries start to get old and have reduced ability to store charge, they are still useful for other projects.

One of the main mechanism lithium ion battery degrades is the internal impedance increases over time. The battery usually can still take the rated charge, but can't be charged or discharged quite as easily. For a high current load such as a laptop, the battery will start to fail as the charge level suddenly drop from 30% to 0%. Even though these old batteries can't power a laptop, there are still plenty of low power applications that are useful.

A laptop battery pack integrates a battery management system(BMS) that ensures that the battery is kept safe even when the outside environment is unfriendly to the battery. The laptop BMS typically will protect the battery from short circuiting, under charge, over charge and over heat conditions. The BMS also ensures that the batteries are well balanced. Thus, wouldn't it be nice to have a way to use the battery directly without extracting the 18650 cells that are inside the pack?

Fortunately, there is an easy way to reuse the packs directly. Let's get started!

Words of warning when dealing with lithium batteries: even though the BMS circuit for the laptop battery pack is very good at protecting the cells inside, care must be taken when working with lithium batteries. These are the guidelines to follow:

Work on a slightly charged battery: Keep batteries charged at less than 20% as they hold less energy, if something goes wrong, it has less energy for combustion
Do not over charge the battery: General rule is 4.1V per cell
Do not overheat the battery: If it feels hot, it is too hot; lithium cells do not heat up under normal usage
Do not short circuit the battery: Keep an eye on metal objects when working with batteries
Do not charge the battery if the temperature is below freezing; discharge is ok, just don't charge them
Do not drop, puncher or crush batteries: If the juice starts to leak, get away from it

Step 1: Make a Test Probe

The first step is to find out which connector is the positive and which is the negative terminal.

For cell configuration, you can figure this out by looking at the voltage rating of the battery. If the pack says it is 10.8V, this means it is configured with 3 cells in series (3S). If the pack says it is 14.2V, it is configured with 4 cells in series(4S).

Most laptop battery packs are either 3S or 4S. For small laptops, they sometimes will have batteries that are configured in 2S, but those are rare.

For a battery pack with 3S configuration, the voltage range is 10.8V to 12.3V. The suggested charging voltage is 12V.

For a battery pack with 4S configuration, the voltage range is 14.4V to 16.4V. The suggested charging voltage is 16V.

Step 2: Measure the Terminal

Measure the terminals on the battery pack until you see anything over 9V. Charge the battery slightly before you do this so the BMS does not shut down the battery due to under charge condition.

For most battery packs, the power terminals are the most exterior (Far left and far right) terminals on the connector.

Once the terminals are identified, record which terminals so they can be identified later. Personally, I like to write the terminal identification directly on the battery for easy reference.

Step 3: Check If You Can Charge the Battery

If you have access to a power supply, set the power supply to 12V/1A for 3S pack and 16V/1A for 4S pack. Check to see if the battery starts to draw current. Some battery pack will take a few seconds after the power supply is connected to start drawing current.

There are battery packs out there that have a safety switch that won't engage unless 5V is applied to the SMB bus connector. These are rare, so hopefully you don't have one of these kinds of battery packs. To get around this, you can wire the SMB (System Management Bus, see next step for more information) connector to the positive terminal of the battery with a 100K ohm resistor. Consider using this type of battery only if you know what you are doing. If you wire it incorrectly, the SMB transceiver in the battery pack can be damaged.

Step 4: Optional, Find Out Which Terminal Is the Data/sensor Output

Laptop batteries always have two connectors for communication with the laptop for charge information. These two pins are known as the System Management Bus (SMB). Using the resistance setting on the multimeter, measure the resistance to ground. The data/clock line is usually 1Mohm to ground. The laptop can send commands to the battery pack to ask for the status of the cells inside the battery pack. There are number of projects out on the net describing how to build a reader for the SMB. I suggest you look into this website (https://github.com/PowerCartel/PackProbe) if you are interested in hacking the SMB.

There is always one connector for the temperature sensor. The sensor is usually anywhere from 10K ohm to 100K ohm at room temperature. This connector will not be used in this project.

Step 5: Making the Cable for the Battery Pack

Time to start making a cable for the battery pack.

Start by cutting a piece of copper tape. Size of the tape is about 8x8mm. Any copper tape will do, they are available in most hardware stores.

Step 6: Fold the Copper Tape

Fold the tape in half without removing the release liner paper

Step 7: Solder Wire to the Folded Copper Tape

Solder wires to the folded copper tapes. Add a connector to the other side of the wire.

I like to use a 5mm barrel connector, as they are a pretty commonly used power connector.

Step 8: Attach the Finished Cable to the Battery Pack

Push the folded copper tape into the connector slot that has been previously identified as the positive and negative power connection.

Tape the wire so it doesn't move around. Tape the terminal end so it won't accidentally short circuit.

Step 9: Time to Put the Battery Pack Into Use

The most useful laptop battery pack is the one with 3S cell configuration.

With this configuration, the output voltage is from 10.8V to 12.3V. This is a good voltage to power all sorts of electronics that require 12V input.

One common use for this battery pack is to power LED lights.

Charging can be done with any variable power supply that can be current limited. You can also use a LiPo battery chargers such as the iMax B6 battery charger that's available in most hobby shops. Do not use a battery charger designed for car batteries. Those chargers have a set voltage that's too high for the laptop batteries.

The battery pack is also well suited for storage of solar cell power. Make your own power wall at home. I'll write that up in the future instructable.

Step 10: Another Idea, Use It for USB Charging

A 12V to 5V converter with USB connector is an easy addition to the battery pack to make it even more useful as a phone charger!

Simply add wires and a 5mm barrel connector to the converter board will expend the usefulness of the battery pack! Wrap the finished board in tape of shrink wrap tube to protect it from the element.