Following up from my talk at the July 2010 meeting, here is a link to the slides and v1.0 schematics, etc.
http://www.fveaa.org/fb/charge-logger-v1.0_259.zipi'm currently making changes for v1.1 including:
- adding an micro-SD card to log the data to
- adding a battery backed real time clock for time stamping all logged data
- changing to an all 3.3v design (from 5.0) since the SD card needs it anyway
- change to an all SMD (surface mount device) board (no longer need to drill holes :-) and will be more manufacture-able.
- changing current sensor from 50amp, bi-directional to 30amp uni-directional. Charging is always one way and this will increase the accuracy/resolution of the analog to digital conversion. The original sensor ACS756 is being replaced by a ACS715.
- using higher bit ADC to measure the voltage. v1.0 uses the atmega328p built-in 10-bit adc, but i'm going to be using a separate
-bit adc. Measuring a high voltage like our EVs have (100-200vdc) requires a voltage divider. The v1.0 divider is approx. 32:1 so it is already losing 5 bits. my goal is to measure pack voltage accurately to a single digit precision (eg, 120.2v).
- using a differential adc instead of a single-ended adc to measure the voltage. v1.0 currently is tied to the battery pack negative (in order to use the m328 single-ended 10-bit adc). It is bad practice to connect the low-voltage digital side of the circuit to the high voltage battery pack side (common ground in v1.0), and this change will solve that. The current measurement isn't an issue because the ACS (hall-effect) already provides this decoupling/isolation.
V1.1 schematics coming soon.
