^^ I'm not so sure about that one bud... my battery was sitting at 11.9 volts for over a year and still worked faultlessly, even being left for 2-3 weeks without being used.. it was the boot full of water and the drowned DSP amp that finished it off  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

The test has nothing to do whether a battery at 11.9V can start a vehicle, I would hope it would!
That response shows that you don't understand what the test does or I haven't explained what the test does very well.
As with anything, please go and do your own research, search out battery testing, there are plenty of resources out there which will probably explain it better than I can.

Just testing the voltage of a battery without knowing anything else will only give an indication of the current State of Charge at that time, it won't tell you anything about it's Capacity. Capacity is the main indicator of remaining battery life.
The testing I've described is how you can easily roughly check the capacity of a lead acid battery to know whether it is at the point of needing replacing (you really need to check the Specific Gravity but you can't on sealed batteries so voltage can roughly equate to the SP but will vary with temperature).
It is testing a battery voltage after it has been FULLY charged up and left to sit for a short while to allow for any surface charge to dissipate.
A brand new SLI battery should be able to produce 12.6V or more after being fully charged.
If it is less than 12.6V after a full charge then it is showing some signs of battery aging (reduced capacity)
If it shows 12.3V or less then the capacity is reduced to the point where it is likely to fail completely within 6 to 12 months.

As an analogy, the capacity test is equivalent of checking how much fuel you can put in to completely fill an unknown fuel tank. You don't know how much the maximum capacity is until you fill it completely to the top. Just looking at the fuel gauge (or voltage check) will only tell how much fuel is in there at that time not how much fuel you can put in there.
Unlike with a fuel tank though, a batteries maximum capacity reduces over time (like putting rocks in that fuel tank). If you do the capacity test then you can see how much capacity it has remaining compared with a new battery and judge whether it needs replacing or not.
To do a proper capacity test you really need to charge the battery fully and then fully discharge it while measuring the Ampere/hour output but that is not really practical and fully discharging an SLI battery is not good for it. There are also other reasons than reduced capacity why batteries can give poor performance and they are covered by different tests. For example you can test the voltage drop on cranking (load test) but that is slightly more involved or requires other test gear. Testing internal resistance is another indicator but again it needs other test gear.

As an example of the capacity test:
The battery in a Transit I checked the other week was at 11.8V and it still started but it was slightly slow. Did that mean the battery needed replacing? Not necessarily, 11.8V was just an indicator of the current State of Charge, it might just need a recharge. On doing the full charge capacity test the voltage was only at 11.9V so that meant it was seriously down on capacity and did need replacing, a replacement was ordered.
The brand new battery turned up and that was only at 12.3V, did that mean that the new battery was down on capacity? No, 12.3V was just it's current State of Charge and didn't indicate it's capacity. Once fully charged up it was at 13.0V, around the capacity you would expect for a new battery.
You also can't go on the actual age of the battery although obviously a newer battery would hopefully be better. The above battery that was replaced was 12 years old. It was on a twin battery set-up and the other battery was 8 years old and sitting at 12.3V SoC. I did a capacity test on that and it came out at 12.8V, perfectly fine and no need to replace it.

All I'm trying to do here is point people to a simple established test that they can do themselves and will give a very good indication if an aging battery needs replacing or whether the problem might be elsewhere. You just need an accurate meter and a battery charger.

I feel wounded

Before you load the gun again........

If you want to test a battery properly then it needs to be fully charged and then drop tested.... what the voltage is at has no direct indication on the state of the plates and the batteries ability to hold charge. Yes if it's at 10.5 volts then there is a strong chance that there is a dead cell and it is deceased But saying that a battery that's sitting at 11.9 volts is a dud is incorrect, yes it was old, yes it did die eventually...... but it took 14 months to die Also putting your faith in a meter is not to be advised unless it has been calibrated and certified by a laboratory.. there is so much cheap electronics on the market, that any meter should be used as an indication not actual.

It rattles my cage a wee bit when it's just accepted that a battery anything below 12.5 volts is on it's way out, cos it just isn't true.

You say to charge the battery fully and then check if the voltage is xx after a time.... it would be better to take 2 readings, immediately after charging and then after your waiting time, that will tell you how much the battery has lost... not everyone has the cetc charger... some of us live in the real world and rely on battery chargers that are 10 a penny.

The best thing to do is fully charge the battery and then leave it, overnight, and see if it will start the car, with out struggling, if it does great... if it don't then it's replacement time.... if you want instant results then a drop tester is the only way of telling the condition of the battery... but then they cost a oner

At the end of the day, maybe the start of day, if the engine starts with out any bother... don't go looking for problems.. if it ain't broke don't fix it  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!