Thanks for the help guys. I wasn't aware a battery can be fully charged, show 12.6v+ and still not be able to provide any cranking amps. I'll try a known working battery and report back.
Just in case you're interested, and I'll try to keep this simple, your typical lead acid battery has lead plates - quite thin because that lets more plates be used and so more surface area and more surface area = more electricity available (capacity) for any given size of casing. They are arranged in cells with each cell producing just over 2 volts (around 2.2 actually which is why a fully charged battery - which has 6 cells for a 12 volt battery will read more than 12 volts! The "old" 6 volt batteries had, surprise surprise, 3 cells!) This might interest you:
https://www.power-sonic.com/blog/a-guide-to-sealed-lead-acid-battery-construction/
The cells are flooded with a sulphuric acid liquid (electrolyte) which is trying all the time to form Sulphate on the surface of the lead plates. As the battery discharges the sulphate builds up on the surface of the plates but, as long as it's not given time to crystalize it will dissolve back into the electrolyte when the battery is recharged. The trouble comes if you leave the battery in a discharged state which is when the sulphate crystalizes and then won't dissolve when you try to charge it. This blocks the electrolyte's ability to react with the plate's surface so, in effect, with some of the plate's surface now blocked by the crystals, you now have a smaller battery because only some of the plate area is now reactive. Go far enough down this road and you'll end up with a big looking battery (physically it's size will be unchanged) but with the capacity of a tiddler and that's not going to hack it when you've an engine to turn over. This is most likely the problem you have. To a large extent you can slow this process to a minimum by keeping the battery well charged on a "smart" maintenance charger as sulphation happens slowest when the battery is kept well charged. Unfortunately you can't entirely stop it though so all lead acid batteries will eventually "die". Perhaps you can now see how a heavily sulphated battery with a lot of the surface area of it's plates covered with the solidified crystals can still charge up to show a good state of charge because each cell still has a small area of plate still reactive. When you try to pull current from a battery in this state each individual cell will discharge rapidly because it has no surface area to work with.
Interestingly what can also happen is that, especially as a battery ages, some of the plate material can flake off of the plate surface and build up a sludgy residue in the bottom of the cell. This sludge is somewhat electrically conductive and can cause a slow discharge of the cell. Typically with a battery suffering from this malady you will find it works quite well through your working day, starting the car even after it's stood for some time, but overnight the cells have plenty of time to discharge for longer and you find the car won't start in the morning. So, perhaps you get a jump start and it all works fine for that day, but then next morning it won't start again. Of course the same problem can happen if you've left something on like the interior light or maybe the glovebox or boot light isn't going out when you shut the lid. A few years ago, when batteries had removable plugs in the top of their cells, you could buy "battery reconditioning tablets" which you dropped into the cells of your aging battery to give it a fresh life. What these did - the ones that weren't "fizzy" fakes that is, was to chemically loosen some of the crystaline deposit from the plates so they could react again, and sometimes they were actually quite successful. The trouble was, of course that this loosened rubbish fell into the bottom of the cell and added to the shorting out problem!
So keeping your battery well charged is the way to go. Using it in a frequently run vehicle and not doing too many very short journeys is also good. If you have to lay it up then keep it well charged, probably on a maintenance charger but don't over charge it because you'll then loose electrolyte and any part of the plate not submerged will sulphate rapidly and become unusable. A modern "smart" charger is the way to go. and, as you alternator is specifically designed to stop short of overcharging your battery it's actually good for it to give it an overnight once or twice a year on your smart charger, even if you think the battery is absolutely fine, just to take it up to a full state of charge. I find on our cars the charger will usually charge a "good" battery in this way for about an hour before notching back to it's maintenance level.
If you'd like to punish/bore yourself further here's some not too terribly technical reading for you:
https://www.lifewire.com/car-batter... a Car,the plates through excessive sulfation.
and:
https://www.crownbattery.com/news/sulfation-and-battery-maintenance
Well I hope someone found that useful. A jiggered battery is such a bummer to deal with and it always seems to happen at just the worst possible time! Oh, and don't forget, most cars - but not absolutely all - with stop/start are going to have a battery condition monitor (sensor), most often a small black plastic thingy on the negative battery terminal, which may well need to be reprogrammed when a battery has been allowed to get especially flat or when a new battery is fitted. My Ibiza is like that but I'm not so sure about my older boy's 2012 Punto Easy which does have one but I think? is happy to sort itself out - we will be fitting a new battery to it in the next couple of days so I'm about to find out!
And a wee PS. Regarding that battery condition monitoring sensor? If you've got one, and if you've got stop/start you almost certainly have, then your battery charger's negative connection must not go to the battery negative but connect it to a chassis earth point - engine block works well - or the sensor can't "see" what's going on with the battery. I believe in some cases damage can actually be caused to it. Some manufacturers have very specific instructions in their manuals and you would be well advised to have a read and follow their advice - Electronic units are disproportionately expensive! Oh, and I'd only use a Smart charger now a days. The old style transformer type have a far too coarsely regulated output and can't protect against mains spikes etc so is only to be used if you "bench" charge with the vehicle leads completely isolated from the battery. Jump starting? a whole other "can of worms" probably best discussed another day. Suffice to say, DON'T unless you know exactly what you are doing.
