I've been using A123 batteries for flying for a while, and like many other, I love these cells. I've been using a Lab DC regulated power supply to charge these cells using CC & CV method. But I can't bring that heavy lab power supply to the field or with my Heli on vacation so I have to do something. I thought a portable & cheap charger for these cells would have some interests, so I hope this project is useful for anyone wants a quick and easy to do device.

I saw the the Li-Po Adapter http://www.slkelectronics.com/lipodapter/index.htm from SlkElectronics. And being a DIY guy, I want to build one myself. I have small Li-Po charger that can charge 14 Ni-Cd cells, so that would be perfect for my A123 cells.

It took a few hours to convert a software that I've written for another project to make it an A123 charger. I have 2 different packs on my Swift 16 Heli, 5cell, 4 cells to combine it to become either 9 or 10 cell pack to fly. So I need the auto cell count function. The current limit function is provided by the Ni-Cd charger. In fact, these A123 cells can charge as high as 10A so current limit function is not really needed. The auto cell count & voltage monitor to decide when to cut off the charge are done with my circuit & software. That's pretty much it for a functional A123 Ni-Cd adapter charger, I guess. So here it is:

The circuit:


It has a PIC that does ADC and detects the number of A123 cell in the pack. The software can detect up to 10 cells. The voltage reference of the PIC's ADC comes from the Vcc of this chip, so I use a 0.5% LDO regulator. There is an opto to isolate the high voltage battery pack from the PIC. A FET acts like a switch to charge and cut power to the pack. When the battery is first plugged to the circuit, the cell count LED will blink the number of cells it detects. If It sees the cell < 0 and > 10 it will report the error on the error LED. The "charging" LED turns on when the circuit finishes detecting the cell count and turns on the circuit to charge. When the charge is done (3.7volts per cell,) the current will be cut and the full charge LED will turn on.

The PCB of the PIC, ADC and monitoring LEDs:


The finished charger:


Charging a pack:


Here is the Software
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