Voltage and Current data logger

[pasken]

Hi everyone,
am currently working on a project that involves the use of an arduino board and data logging shield. Please I need help on how to use it to measure and store voltage and current from a 12V car battery.
Thanks.

[floresta]

Have you looked at the "Light/Temp Logger" example (http://www.ladyada.net/make/logshield/lighttemp.html)?

Don

[pasken]

Thanks floresta, yep I have looked at it. That one is for logging light & temperature.
I want to log voltage and current, I think the major difference would be in the choice of the sensors but I dont know where to go from here.

[JamesC4S]

Maybe you missed the connection, but most sensors provide a voltage. So the Light-Temp example is effectively how you create a Voltage data logger.

As for current, you'll need to do some research on ways to measure current. The method of measurement will depend on how much current you expect to be flowing.

[pasken]

James my fear about using it to log voltage is, can I connect 12V directly to it, or do I have to scale it down? No specifications were given as per the maximum voltage it can take, or the terminals to use.
Same goes for the current. The expected current to be measured should not exceed 5Amps.

[philba]

Ohms law is your friend. commit it to memory - not just a good idea, its the law!

time to learn about voltage dividers http://en.wikipedia.org/wiki/Voltage_divider aka resistors.

current is easy as there are many ways to turn current into voltage. If it's DC (which I am guessing it is), then you use a current sense resistor. AC, a current transformer.

[JamesC4S]

The data logger is based on an Arduino, which uses an ATmega328. The max input for any of the I/O pins is slightly higher than VCC. A Uno uses 5V so you can't put much more than 5.5V into a pin without causing damage. (A 3V3 Arduino is I/O limited to around 3.7V).

The most common way to measure a 12V battery is using a voltage divider.

Since current must be measured in series, you actually measure a voltage across a shut resistor. 5A is pretty high and would require a very large power resistor. Another approach is to find a hall-effect sensor like an, Allegro ACS714, which will be more efficient.

[pasken]

Thanks philba & james. I will work on your suggestions and get back to you.

[lizhenry]

Awesome, this looks like exactly what I want to do with my mobility scooter. It has 2 12-volt batteries and I'd like to have a log of voltage and current on them both!