Richard, many thanks for providing your acknowledged superior knowledge on matters electrical and much else from which I have learned a lot.
I guess my attempt was to reply to Rob`s thinking on the charge requirements of an AGM and it`s differences to that of old style lead acid batteries, for an AGM to operate efficiently.
I was quoted circa £500 to replace my 10 year old AGM battery thus I suspect any Morgan running an old style lead acid would have to investigate carefully as to the possible advantages or disadvantages which might exist, when thinking of replacing an old style lead acid battery with a similar capacity AGM battery.
I suspect if attached to an old style electrical system an AGM may not reach a fully charged state, whether that might affect it`s performance long term connected to such a system, I have no idea.
As for longevity, given my AGM would seem to be original to the car from it`s date stamp, and has lasted 10 years and circa 42k miles over that time, seems to indicate it may have spent most of it`s life in a situation of irregular use, and which might also seem to suggest that perhaps an AGM may well operate well enough in a seldom used Morgan.... again I have no idea...
However as to whether in those 10 years my AGM may have run flat or whether it lived much of the time on a float charger of some sort would seem to be anybody`s guess..?
Genuinely surprised at the £500 you were quoted for an AGM battery George. I checked on AGM replacement for my Roadsters 075 series 60 amp hour battery and equivalent same brand, size and capacity came out at less than double the cost at £108. Even then I suspect the hike was down to supply/demand given conventional will suffice. I did notice a jump to 680 CCA for AGM from 540 CCA though.
As already mentioned they employ same basic materials as conventional wet lead/acid with the justification for a premium price derived from the advanced construction maybe.
Because AGM are so similar, charge profiles will be as well compared to conventional. The 10 years life you got from your AGM tending to support this. Yes they can take a tad more charge current in the same way they can release more given construction, but understanding that (theoretically) the lowest powered charger will eventually fully charge a lead/acid battery give enough time, that wouldn't be an issue.
All lead/acid batteries benefit in terms of longevity from not being charged at a current above a certain amount, 10% of amp hour capacity in the case of conventional, abbreviated to 0.1C. In reality when you have the choice a rate as high as 0.3C is a reasonable trade off when charge times are limited, this being more applicable to boat service battery banks but useful to highlight lower charge currents can be beneficial.
