since I really like using LiFePO4 AA and AAA batteries in some of my projects , I finally gave in and built a dedicated charger for them.
Previously I used a lab power supply to mimic the constant current/constant voltage charging curve, which worked also fine. But after seeing Patrick Van Oosterwijck nifty LiFePO4wered/USB™ , I thought it would be a bit more convenient to charge these batteries using USB.
Since Patricks PCB is OSHW , I made a few quick modifications to fit my needs:
* Changed the Micro-USB connector to a Mini one, because I happen to have these connectors in my parts bin, and also because I have more Mini-cables lying around.
* Replaced the solder bridging jumper pads by a regular jumper connector so I can switch between high and low charging current depending on whether I charge an AA or AAA battery.
* Changed the resistors which select the charge current to values I had readily available. (Jumper open: 185 mA for AAA batteries, jumper closed: 525 mA for AA batteries.)
* Got rid of the long PCB extension where the battery holder is soldered to reduce the PCB cost ($2.90 for three boards from OSH Park).
* Changed the USB-breakout header to contain VUSB (5V) instead of VBAT.
Soldering the 3x3 mm DFN-10 package was done by first tinning the pads, placing the chip, and then heating the board from the bottom with a heat gun. The remaining SMD parts were soldered conventionally with a regular iron.
Here is a picture of it charging an AA battery:
Please find my modified EAGLE files attached.