On older cars without the battery conditioning module, 14.4 volts on charge is OK. 12.7 volts looks about right for the battery conditioned modules with the engine off. I had an Alternator regulator pack go on the Freelander 2 which read 16.5 volts across the battery terminals.

Its good to hear batteries can survive large discharges, I suspected they could.

How did you diagnose the regulator pack if the voltage was okay? or did the 16v give you the clue?

I was thinking of once I have completed my few days of not using the car and checking the battery voltage to confirm no parasitic drain I would see what current the alternator puts into the battery after start up.

If I connect the clamp and then open the car I will get the direction of discharge (as the clamp shows either negative or positive depending on the orientation of the clamp on the wire).
I can then start the car and the charge current will be the opposite direction (and hence sign).

Does anyone know how much current to expect, I can guess at least 50 or 60 amps?

Rog L322 4,4 TDV8
Freelander 2 sold
Honda Transalp
Sunbeam S7 deluxe (in bits)
Sunbeam S8 (restoring)

It depends where you measure it, the alternator output is connected to the starter stud, which connects to the battery - so measuring it around the starter cable will give you the alternator output in total - up to 200A depending upon exact type of alternator fitted.
Assuming a measurement after five minutes of 60A (delay due to inrush to the battery due to cranking and inrush to resistive circuits in the vehicle) and then put the clamp around the supply to the BJB (battery junction box) and that is drawing 40A, the battery is taking a charge of 20A and the vehicle is consuming 40A. To be honest, you're better off just reading off the alternator duty cycle %age using diagnostics, it's a lot quicker and together with an accurate measurement of battery terminal voltage, there's a table you can refer to that will give you a yes/investigate/no answer as to the correct charge rate.

In the old days, one turned all the lights and accesories on, including the air con if fitted, then checked the charging voltage. Make sure the engine is running first of course. The voltage will read on a diognistics through the OBD port. Everthing on, 13.2 is good. I found a ripple current in the battery to alternator lead, hence 16.5 volts, AC is no good for ECU's. On my Discovery 2's the warning lights go mad if the cranking voltage goes below 9 volts. The Fatty stutters a bit at 11.8 cranking, so one needs a good battery reserve. I believe the battery conditioning modules only allow an 80% charge. 200amp alternators only give that after start-up, and modulate downwards as the battery charges. I must admit that 200amps into a battery is hefty charge rate, but normal on a bank of traction batteries. All the information from a battery manufacturer would be helpful fo people, don't you think?

The bulk of the 200A output capacity of the alternator is to run the vehicle systems and accessories, not shove it all into the battery, there are various rate-limiting strategies, not least, the rpm of the alternator. It'd be very rare to see 180/200A from the alternator, and then only briefly, the control strategy and physics ensures that the output is 'just enough', but I'm sure you know all that.

I was trying to find some public-domain stuff that wasn't too mathematical on charge rates, mainly because I'm unable to post copyrighted stuff from work, this is probably the most accurate. https://workshoppist.com/agm-battery-charging-rates/ there are related pages that may be of interest.

Interesting article Phoenix as you say it may be a 100A alternator but rarely outputs that and most of the charge goes to running the car not charging battery.

So last day of the testing for now, the battery is now showing 12.67 volts so is losing around 0.01/0.02 volts per day, current drain still showing less than 0.04 Amps. After starting the current to battery is initially around 30 amps but quickly drops back to less than 10, (assuming smart charger controlling this). The charge voltage is 14.75 whatever the revs.

My conclusion is I have no idea what happened but everything seems to be working as it should, so going onto a keeping an eye on things stage and see what happens.

Just when I thought all was well my rear caliper started overheating, new one on order and a job for the weekend !

Thanks for all your comments and suggestions as always

Roger L322 4,4 TDV8
Freelander 2 sold
Honda Transalp
Sunbeam S7 deluxe (in bits)
Sunbeam S8 (restoring)

Part of the reason for a high output alternator is that they can produce a useful amount of current at tickover - the rating is usually max output @ 1850rpm, obviously it's much lower at 750-800 but needs to be able to supply enough to keep the terminal voltage above 12.3VDC.

From your measurements, it does sound like the battery disconnect/reconnect has provoked everything to behave itself - and for your starter battery to be holding >12.6VDC indicates to me that the battery itself isn't the causal part, neither is the alternator - that would be pulling your battery down very quickly.