monitor car battery current draw

I want to measure the total current consumption in my car, I want to add additional electrical items and worried I will have a flat battery. I also want to log the current, so thinking ardunio would be ideal to show the data with rtc to provide time stamp.

The question is how to measure the current, I have been looking at shunts and hall effect sensors. Both seem to do the job, but when the vehicle current drops to 30mA at key off this is a very small current to measure and so many factors could affect it such as cable length to the sensor. The range will be 30amp's to 10mA with +- 1mA accuracy.

I have been looking at amplifiers, but there is so much information about monitoring temperature with the shunt and protecting the hall effect sensor from external factors which could be present in the engine bay.

I think either option would be hard to build as the accuracy of measurement at low currents would cause a very small signal which would be subject to noise and the ADC would need a large range to provide a very small resolution. I am thinking of using a teensy board with a 16-bit adc and is very small, so I can mount everything on the battery cable.

Any help would be appreciated, I seem to go around in circles to understand if this is possible.

A Hall effect sensor with the amplifier at the sensor end of the cable should work. Carry the amplified voltage back to your Arduino so any interference picked up along the way is minimal compared to the voltage you are measuring. I would be very cautious of using anything in line with the battery cable, consider the cranking current when the engine is started. Even on my modest 1.4 litre engine it peaks at around 125 Amps.

Brian.

Car consumes too high current from its battery.Therefore a "Clamp Ammeter" should be used.This ammeter measures the flowing current by clamping the main supply cable so there isn't any circuit that interrupts the circuit.It uses magnetic field which occurs while a current is flowing.
There should be some data logger models.

There often is a separate smaller wire from the car battery that carries all the current for the rest of the car's electrics including the generator current, with the exception of the starter current which is carried by the large cable. That smaller wire is the one you want to monitor.
But of course that will not indicate any battery discharge from the high starter current.

Affordable hall effect sensors don't have that low offset and offset drift to measure with 1 mA accuracy, I fear. It should be also noted that maximum battery current (not considering starter current anyway) can be considerably higher than 30 A. The fan current draw might be already above 30 A.

I guess that a shunt with a "zero" offset amplifier would be the most economic way.

Thanks for the quick response, could anyone recommend a hall effect sensor which could be suitable for this project?

I think you are going around in circles because you attached a secondary conflicting objective.

Do you want to measure 'maximum' current draw?

or

Do you want to measure quiescent current?

If you are trying to determine if the battery can handle more devices under present maximum load, then no need for the quiescent current measurement/tracking.

You are trying to find one device to measure microns and light-years. Different tools for different things.

As mentioned a hall effect tranducer with an amplifier at the front end. What about having the amplifier having a switched gain, controlled by the micro?, this way you could maintain the same accuracy across several decades of current. My VW Passat has two 12V circuits, one from the battery and another one when the alternator is running. This means that you can either measure the current in/out of the battery or what the car is using from the alternator.
Frank