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There is far more to testing a car battery than if it has more or less than 12volts.
In fact a car battery (I am assuming a real car, an automobile with a 12volt accumulator) will be well discharged if it only has 12 volts available.
Do you know how comparators work? They can simplify the circuit quite a bit. As others have pointed out though, this will just test the voltage not the total storage capacity of the battery. If that's good enough then once you have looked at comparators you will have a much better idea how to do the voltage testing with the comparators.
For example, you would set up one comparator to detect anything under 12v (or maybe higher like 12.2v) and another one to detect say 11.5v, and another to detect 10.5v. These are just some rough guidelines. When one trips maybe it turns an LED on, so you know how bad the situation is.
I would suspect that a final year project for car battery condition would involve a carbon pile or equivalent load with a couple comparators to assess the voltage drop. Essentially the starting capacity of a car battery is related to its internal resistance. The capacity to run loads is more related to its state of charge and Ah capacity.
It is possible to devise a rapid pulse test of internal resistance using a microcontroller and precision resistors. However, the carbon pile loading is better. I have met a 12.9V battery with 11 milliohms resistance that failed a carbon pile test cuz it had almost no AH capacity left.
From the resistance test derive CCA = 3200/ milliohms. HCA = 5000/milliohms
I dont see it as vague at all. Other people have brought up ideas for more thorough testing, but taken at face value the "objective" is a very simple one: turn on LED's based on the voltage. If the OP wants to add more to this objective, then of course it becomes more complicated.
Interesting topic being we have both a fancy hand held digital battery tester at work and a old style heavy resistance type.
Last week I was doing a 750 watt inverter install on our service truck , 2012 Ford F350 diesel, and figured I would do a battery test being we had had a few issues with the batteries running down way sooner than expected for the given load they had when on site.
Interesting thing was the digital unit gave both batteries a good rating of 90% plus, based on what I don't know being the test takes about half a second using 14 ga leads, whereas the old heavy high current one requires a load test of some 20 seconds at about 200 or so amps to get a reading.
According to that both batteries barely passed as Okay falling right on the bottom line at 20 seconds and failed at 25.
The general consensus of the shop guys no one trusts the new digital unit being it has let too many questionable batteries go out to site where they failed a short time later yet the old high load long duration tester has yet to miss diagnosis a weak battery.
BTW the truck battery issue came down to weak batteries and one battery likely not being connected at all due to considerable terminal and cable corrosion. This week the guys say it never gave them a bit of trouble and seems to start better now as well even though the batteries are at the bottom end of their service life.
Yes, the tester i have puts a huge load on the battery and you take a reading after a number of seconds.
On a new battery it tests very good, and on an old battery it tests right at the lowest possible cold crank amps, so i know it is at least somewhat accurate.
Also when testing don't attach to the alloy bolts; there is resistance when checking the load. It's ok to remove the terminals on an upright; theres plenty to attach the clamps.
My battery has the side mount screw on terminals; I made the mistake of leaving them on when I took it to be tested at Pepboys. Then they told me that the battery was sold at O'Reilly's; Pepboys said it was bad; so O'Reilly's re-tests the battery, only this time they removed the mounting screw that were not lead and screwed in some that were and ge-whis it's still a good battery.
That's very good advice. My batteries are all standard top terminals and i test them with no other connections to the battery. I suppose i might test one with the heavy duty top terminals still on, but if i saw a problem i would disconnect the battery and try again.
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