Sunday, August 24, 2014
A 5c lithium ion battery charger
My step-daughter lost the battery charger for her camera (for the second time in 2 yrs). It takes a few weeks for a new one to arrive from DealExtreme, and she was hoping to use the camera over the weekend. So I decided to hack something together.
As various sites explain, lithium-ion rechargeable batteries should be charged to 4.2 volts. USB ports provide 5V, so all I needed was a way to drop 5V down to 4.2 or less. Standard diodes have a voltage drop of 0.6 to 1.0 volts, so I pulled up the datasheet for a 1n4148, and looked at the I-V curve:
Connecting to the battery was the hardest part of the problem. I tried making some contacts out of 24Awg copper wire, but that didn't work. I though of bending a couple 90-degree header pins to fit the battery contact spacing, but I couldn't find my prototyping board to solder it into. I ended up tack-sodering a couple 26Awg wires to plug into a breadboard.
For a charge status indicator, I used a couple LEDs I had on hand. A 3V green LED in series with a 1.7V red LED start to glow visibly at 4V, and are moderately bright by 4.2V. The few mA of current bleed from the LEDs over 4V would ensure enough current through the diode to keep the forward voltage above 0.8V, and therefore keeping the charge voltage from going over 4.2V.
The results were quite satisfactory. After a few hours of charging, the voltage plateaued at 4.21V. I removed the wires I tack soldered to the tabs, and the battery was ready to be used. The same technique could be used with higher capacity batteries by using a different diode - a 1N4004 for example has a voltage drop of 1.0V at around 2A.