How to use AREF

Forum 2005-2010 (read only) Hardware Interfacing
1

I have a very nice RC battery charger that I wish to collect voltage/amp data from. The people who make this charger actually use an ATMEGA chip as the processor, but of course they wont share the source code.

I found a .01 Ohm precision resistor inside the charger that I assume is used for current measurement, but I'm not sure from what to what I need to be hooking my pins up to to measure amps and voltage. The working range of the charger is 0-10 amps and 0-14v.

Here is a picture of the inside and the resistor:

2

Ohm's law tells you that at the maximum rated current of 10 amps, the voltage across the 0.01 ohm resistor will be 10 * 0.01 = 100mV.

Suggest you use the internal 1.1V analogue reference, and even then, you might want to think about some amplification.

3

http://www.seattlerobotics.org/encoder/200210/lithiumion.htm
This is AVR Battery Charger circuit that use Op amp to amplify the current measurement resistor.

The reference voltage is 3.67V with 3.58mV / step
Read more info in thjis Doc.
AVR450: Battery Charger for SLA, NiCd, NiMH and Li-Ion Batteries