My Car Breaks Alternators! Please help

[geoberni]

Repeat your test with one headlamp disconnected then swap sides and see if it happens with left only, right only or only with both connected.

You could also try with the cars lights off temporarily connect two head lamp bulbs directly to the battery and see if it creates the same issue
This may help determine if it's a headlamp wiring issue as suggested by others. Or is it just a load issue causing the alternator to stop charging.

I agree, applying the same load without involving any of the associated wiring would be a useful indication.
Not forgetting the 2 lamps need to be connected in parallel, directly from the battery positive to earth. I'll be very surprised if the charging system doesn't behave normally.

https://www.youtube.com/watch?v=IEylpJ8selc

I recharged my battery and reinstalled it. In this video you can clearly see the problem. First I start the car with the headlights off and the voltage jumps straight up to 14.4V. Then I turn on the lights and it immediately drops down to 12V (not charging). I have experimented and found that this is not affected by high or dipped beam (happens either way). I turn the headlights off and the voltage does not recover to charging levels. I restart the car with the headlights off again and this time it does not recover to charging levels. If I leave the car for a while (a few hours in this case), the alternator will charge again so long as the lights are off.

That video is quite perplexing, viewed in isolation, without knowing that the battery actually isn't charging and leaving you stranded, I'd be thinking there was something wrong with the voltmeter.

This part of your initial post is relevant:

This is a pattern which I can now consistently predict will happen every 9 months or so, and I only average 2.4K miles a year. What I would do is then charge the battery, and replace it with a Chinese made Lucas-type alternator ordered from a reputable well known Morris parts supplier. I have been through at least five of these in 12k miles of ownership. Eventually instead of replacing the whole unit I would just replace the regulator, which would temporarily fix the problem for a few months (back up to 14V) and then fail.

In that the car is clearly, somehow, damaging the Regulator of the Alternator...
I don't understand how it is causing a transient fault that recovers after a few hours, before eventually becoming a permanent fault.
It can only be a massive current draw, greater than the rating of the Alternator, but I don't understand why the wiring hasn't caught fire...
In my career as an aircraft electrician, I encountered such puzzling faults which would be traced to cable damage; the difference being that every circuit had it's own circuit breaker or fuse, which helped immensely.
I would be going over the headlamp wiring in forensic detail...

[BenDixon2002]

Disconnected the left headlight, started the car, jumped up to 14V turned the right headlight on with the switch, idled for 5 minutes and it stayed at 14.3V.

Swapped the headlights over at the connector block, with the headlights off started the car and got a loud bang (sounded like a backfire) and stuck at 12V without me turning the headlight on. First time it’s not charged with the headlights off.

I don’t think the bang was related to it not charging, it was an old car occasional startup bang. Alternator was spinning nicely, not slipping, and not warm. My theory is that the single headlight being on for five minutes was perhaps tiring the alternator out, but not as instantaneously than when I’ve got two headlights on. So when I started the car up it was too worn out or whatever to give out a charge.

I’ll try again tomorrow with the other headlight on its own.

I imagine I’ll end up having to replace the headlight wiring, but no I’ve definitely had no fires. I keep a fire extinguisher in the car at all times but I’m not planning on having to use it

[Bill_qaz]

Where is the connector block you refer to?
Where are you measuring your voltage readings and what with?
Your alternator earths via its mounting is it clean and tight?

[svenedin]

I'm inclined to think that your loud bang was not a coincidence......I wonder did a headlight bulb explode? Maybe water is getting in to the headlight connections in the headlight bowl. The headlights have a separate little sub-loom that goes through the inner wing near the front of the car. The connectors are bullet connectors and often corroded. The earth, which is a little earth plate bolted to the inner wing is particularly prone to a bad connection. You can buy the headlight sub-looms ("pigtails") from ESM. Ensure you replace the grommet where the headlight wires go through the inner wing (often absent) and if corroded, replace the female bullet connectors. That is of course assuming that the problems is there and not somewhere else!

Stephen