battery state of charge

Ok guys, how accurate is using voltage to determine battery state of charge?
I found this table online a whike ago for AGM batteries
100% 12.8v
75% 12.6v
50% 12.4v
25% 12.0v
0% 11.8v

Also my battery normally has around 13v once charged (rested, off charger for a few hours)
Is this bad??

Surface voltage - which is what you're reading when you attach a multimeter - is a vague guide at best.

Put a small load on the battery and then measure it (1A - or a 10W globe like you might find in a number plate light).

Under higher loads, voltage can drop in proportion to the load on the battery. This is the exact reason why in my tub, I invert the power so that when my battery charger is charging the battery, the Engel fridge has switched over to the 240V supply taking the load off the battery. The (smart) battery charger will see the rate of voltage increase in response to the amount of current going in and will know the state of charge.

The chart you have isn't completely accurate for lead acid batteries but it's not a bad guide. Dead flat is actually considered to be more like 10.75V at which point you can probably get one decent crank out of a starter battery and hope there's enough to power the ECU to get the engine going. Your chart isn't bad but you'd had to unload the battery (or only have a small load on it) to know for sure.

When cranking the car, you'll find your starter battery - even a fully charged, near new battery - can fall below 11V "surface". In reality it's a fully charged battery that's lost only a small amount of power. Let's work it out - 3 seconds of cranking at 550A is about 550*3/3600 = 0.458 amp hours. If it's a 70Ah battery (like the batteries that come with our car) it's still at 99% capacity yet it's showing under 11V while cranking - what I'm demonstrating here is that you have to remove large loads before measuring the battery in order to get a better idea of how the battery is performing.

Great Info mate, that all makes perfect sense.
Ive got a wizz bang meter for my battery box (you may remember me talking about it in the camping solar panels thread) which I still need to calibrate properly. As such the % of charge & amp hours remaining function isnt working correctly, so ive been using voltage to get a rough idea.
Ideally ill get an auto elec to calibrate it with a know load, but i haven't had the time yet

This works for me

To save a new thread, my aux battery when charging while the truck is running is showing a reading of 12v while it is receiving 14v+ from the alternator. Is this normal? I would have expected it to show a reading of 14v also?

Cheers
Tim

Depends on the cable size, Timmo. The aux battery is drawing power from the cable to charge, and cables change their resistance when power passes through them. This change results in a drop in voltage. Thicker cables don't suffer as badly.

Best bet is to get an accurate multimeter (not a needle, a digital) and measure the following voltages:

1) BEFORE you start the car, and after the aux has been uncharged/unused for about an hour: remove the cables from the battery and measure its voltage. This will tell us roughly how full/empty the battery is.

2) BEFORE you reconnect the cables, start the car and measure the voltage at that end of the cable.

3) BEFORE you reconnect the cables after starting the car, measure the voltage on the main battery

4) Connect the aux battery and measure the voltage on the aux battery's terminals again.

There should be next to no difference between 2) & 3), but if 1) is significantly lower than 12.8V, then 4) will be quite revealing about the quality of the cables going to the rear. 4) should drop a little but not significantly, because the voltage should be held up while the current increases (by virtue of the alternator's regulator). With decent cables I'd expect somewhere between a 0.3 to 0.5V drop from front to rear of the car, so with 14.4V up front I'd expect 13.9V at the rear.

Thanks Tony, ill give the above a go. For the record the batteries are connected with 4ga cable and only under the bonnet.

Timmo_821 said:

Thanks Tony, ill give the above a go. For the record the batteries are connected with 4ga cable and only under the bonnet.

Click to expand...

Have you got an isolator separating the batteries? I'm only asking because I have had a couple of the solenoid type isolators stop letting power through from the main battery to the aux battery... I'm not sure whether the actual control box has died, or the contacts have become corroded inside the actual solenoid, but caused the same issue. If the aux side of the isolator is the same voltage as the aux battery then that's what your problem is...

4Ga cable over such a short distance should provide next to no voltage drop at all, so the issue with voltage loss may just be due to the alternator not providing enough amps while you're measuring it (because the aux is discharged enough to want a large number of amps).

In this case, you'll find it will provide different results if someone held the engine at 2000rpm while you took your readings. At that RPM, the alternator will provide enough amps to keep the surface voltage up - it may be being held down artificially by the large current draw.

Old.Tony said:

voltage loss may just be due to the alternator not providing enough amps while you're measuring

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As I understand it, alternator output is related to how high the motor is reving.

So if it is just idling, you will not get much. Should get more on snow switch and full about 3-4,000 revs.

terryc said:

As I understand it, alternator output is related to how high the motor is reving.

So if it is just idling, you will not get much. Should get more on snow switch and full about 3-4,000 revs.

Click to expand...

And pretty much that was the point

Full song on these alternators is at stupid RPM levels. We'd never rev 'em that hard and expect them to continue doing it for long. I mean, who's going to sit in 3rd gear at 110km/h from Port Augusta to Erldunda?