The Objective of this project is to test batteries to find out if they hold a significant charge and are still usable. I recently had a laptop battery fail and after taking it apart I realized I needed a quick way to test each individual 3.7 volt lithium ion cell. As a result this is what I came up with, its really simple and can be put together in minutes with no soldering required. It can be used to test and charge any battery as long as you have the correct charging voltage / current and use the correct load for the situation. This charger is not maintenance free and should not be left unattended. Its important not to overcharge or discharge a battery past its specified rating. It is also important to monitor the temperature of the battery, if it should heat up excessively at any time its probably a bad battery.

Step 1: The Parts and Build

The following Parts are used for 18650 3.7 Volt Lithium Ion Batteries

2_ free with coupon harbor freight multimeters

1_ SPDT Miniature slide switch

1_ 4.7 Ohm 5 watt Ceramic Resistor (Load can be changed based on battery)

1_ Power Adapter 4.5 volt 1amp (voltage and amperage can be changed based on battery)

You will also need around two feet of wire, alligator clips and a small hobby box.

As far as the build goes its really a matter of connecting the wires as shown in the circuit diagram. The small hobby box is really optional, it cleans up the wires a bit and gives you a place to mount the switch. I also found a small plastic (non conductive) clamp and two small metal strips came in handy when the battery had no tabs the clips could grap.

Step 2: Operation

In the top picture the battery is being charged and its charge rate in amps is displayed along with its voltage. The next picture shows the battery being discharged and you can see the discharge rate in amps with the negative sign on the meter along with the voltage. Since you are trying to find bad cells you have to do one cell at a time. The switch is used to change from charging to discharging its important to keep an eye on the temperature of the cell as mentioned before. Once your battery is up to 4.2 volts I recommend a 5 minute discharge cycle to see if the battery doesn't drop in voltage too much. I found all the good batteries I had maintained at least 3.9 volts or higher with a 720 mah load for 5 min. Its recommended for a standard 3.7 volt lithium ion battery to charge up to 4.2 volts max and not to discharge past 3.0 volts. If you change to input voltage / amperage and change the load resistor to match your battery you can do this test on virtually any rechargeable battery. Its important to look up the specifications for the type of battery and adjust this accordingly.