Author Topic: Car battery measure current draw  (Read 3631 times)

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Offline brendonshaw

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Car battery measure current draw
« on: January 17, 2015, 04:47:22 pm »
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. 
 

Offline IanB

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Re: Car battery measure current draw
« Reply #1 on: January 17, 2015, 05:03:18 pm »
I think you don't have enough clarity about what you need to measure.

The battery is not the power source in a car, it is simply a reservoir. The power source is the alternator or generator system.

When you consider the maximum electrical load you can add to the vehicle the generator is the constraining factor. Since the current produced by the generator is tens of amps, measuring down to 10 mA for this is not necessary or useful.

On the other hand if you have standby items that draw power even when the car is switched off, like clocks, alarms and such things, then small currents like milliamps do become important. In this case you will get a flat battery if the leave the car parked for an extensive period of time without applying any fresh charge to the battery.

You can improve your chances of success if you can be more precise and focused about your particular goal.
I'm not an EE--what am I doing here?
 

Offline firehopper

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Re: Car battery measure current draw
« Reply #2 on: January 17, 2015, 05:23:01 pm »
keep in mind when car is started it will pull upwards of 600 amps from the battery to start.
 

Offline LaurentR

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Re: Car battery measure current draw
« Reply #3 on: January 17, 2015, 05:36:41 pm »
If the main thing you're worried about is overdraining your battery, I am not sure why you're trying to figure out the total car consumption. You should be mostly interested in what the devices you're adding would contribute.

Are your devices high-current and only work when the car is turned on (e.g. big amplifier) or are they low-power devices and you're worried about slowly draining the battery when the car is off (or both)?

If they are high-current, some kind of hall-effect / DC clamp is by far the safest solution to avoid putting one of your own devices in the very high current path of the battery.

If they are low-current, then you should measure them, which should be easy. If you're curious about the power of other systems when the car is off, putting a shunt in the path on the various circuits in the fuse box is probably easiest . This way you know what the max Amps you'll see is.

Regardless, a small logging DMM should be able to do either the DC-clamp version straight on the battery and inline on your device or fuse box and with auto ranging should be able to cover a wide range of current. Something like the Fluke 289 comes to mind as a good candidate, but I know there are a few less expensive meters like the Uni-T that might fit the bill for < $200.
 

Offline Wytnucls

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Re: Car battery measure current draw
« Reply #4 on: January 17, 2015, 05:50:22 pm »
If your testing is done with the engine turned off, I would use a shunt hooked to a logging multimeter with a large internal memory and which has enough resolution and accuracy to cover the whole current range. Something like a UNI-T UT71D comes to mind, for an affordable solution.
 

Offline brendonshaw

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Re: Car battery measure current draw
« Reply #5 on: January 17, 2015, 05:55:36 pm »
Thanks for the quick response, I should have added more about what I want to do. I want to measure the total vehicle current when the vehicle is not modified and switch off & locked. I can then see the current draw and as I add kit to the car, work out the current draw with the additional current with the new kit. Ideally I would like to monitor the current if it's really high for a certain period of time, I can trigger a delay to switch off the additional equipment. I am planning to monitor the battery voltage, but I need it's more important to measure & record the current.

It would be nice to use a hall effect sensor, really easy to fit onto the battery cable rather than have to mount a shunt some how in the engine bay.
 


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