State_of_charge_meter ( was: How to increase accuracy...)

Projects General Guidance
1

Hi all,
I have a EMCO Novum Electric Scooter with a crappy voltmeter in the cockpit to show 'state of charge '.
I want to use a arduino and a bidirectional current sensor ( http://www.ebay.de/itm/110894073072?var=410115820128&ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 ) to caculate a real state of charge by integrating the in- and outgoing current. Then scaling the output to match the existing voltmeter in the cockpit ( showing full at 56V and empty at ~40V )

My question is: how can I increase the ADC accuracy while the battery is charged with low current or while the scooter is in 'idle' condition with low powerconsumtion?

Is it possible to use two currentsensor in series? One for highcurrent, the other for lets say max. 10A?
How do these hallsensors react when a 10A sensor has to handle 50A-100A frequently?

THX
Olaf

PS: decided to change the title, since I posted in projects area

2

It will depend on your set-up / "accuracy". A few possibilities:

  1. some current sensors use an external resistor to set gains. You can switch in a higher value resistors to increase the gain in the low current mode.
  2. you can use a variable gain opamp to increase the gain in the low current mode.
  3. you can use a higher resolution adc.
  4. you can lower the AVref in the low current mode.
    ...

Some of the suggestions above are actually for increased "resolution", not "accuracy", which I think may be what you were asking for actually.

The best way to increase accuracy is actually to use a better adc + reference.

3

Resolution is exactly what I meant.

I dont want to use external components if possible. The current sensor delivers 10mV/A. Doesn't seem to be scalable.
The only way to improve accuracy then, is to change Aref by building a voltagedivider and use two or more outputpins to change the ratio. ( add more and more resistors to the upperside of the divider )
Or could I use a PWM pin to charge a capacitor parallel to the upper resistor? That way I'd have lots of possible 'dividers'.

I have to read the Aref section of the ATMega manual to find out whether the voltage can be changed 'on the fly'...

4

Made a drawing of the two possible Aref divider options.

Any comments?

adc_Aref.jpg
adc_Aref.jpg1024×768 30.9 KB

5

What happens to your reference if the supply voltage dips?

6

AWOL:
What happens to your reference if the supply voltage dips?

The scooter has a 12V/500W dc-dc converter to power the arduino. The arduino will only do his state of charge thing, no blinking LEDs or other stuff. So I believe, 5V will be pretty stable and Aref too.
I'm also planning to use a supercap across 5V to be able to store the last SOC when 'ignition' is switched off.
Unfortunately, I have no clue how to detect that ignition off situation.