over charging 65 jaguar s type

[Avo Mk8]

both

So the voltage across the battery posts actually gets to 18/19 volts, and we can't blame a corroded connection somewhere?
The multimeter is on the DC range, not AC (sorry I'm sure you know what you're doing - just looking for an explanation of the impossible!)
I cannot see how the battery is OK. Know anyone with a high load battery tester? Or can you take it along to a garage for them to test?

interesting article here suggesting that a battery might be 'conditioned' over time to take up an 18V characteristic voltage, don't know if this applies only to AGM batteries.

How does a 12 Volt Alternator charge a battery to read 18+ Volts

Hi All, The last few days I have had a riddle to solve and haven't. My Son owns a 2011 Kia Soul and had a very nice battery I could use for my Healey. When checking the battery, it red close to 20 volts if the car was running or not. Surprised by this condition, I placed my charger on the...

www.britishcarforum.com

Might still be easier to try another battery (briefly) to check if this 18V continues?

 

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neg earth not the only downside, alternator pumps out 45 amps instead of 30 amps and that not good for old wiring or electrical parts that were made to cope with 30 amps.

Having a fault like you have where a 12 volt lamp having 19 volts across it is far worse.

The alternator feed cable to the battery would be the only one to carry any more current.

Another alternative would be to fit an Electronic Dynamo Regulator