I have 3 cars where I am monitoring charge rates at the moment. Similar issues, where the battery is too weak to start the engine, beyond a few cranks. 4-7 yr old batteries which are always charged with my C-tek chargers.
Or when after a 20 min drive, park, switch off the engine, which 5 min later wont start.
They show 13,6 after charging. Checked the level of dist water in the only one with lids, and it was ok.
Earth wires are much thicker than stock and the regulators are quite new.
Is it simply a question of accepting that batteries these days dont last very long despite constant charging?
Some of the cars have older batteries which show no problems.
Peder
Is it calcium batteries? 13.6 is low for them.
I’ve been through all this with my Audi TT:
First, check that the battery is charging when the engine is running.
Second, check if the battery goes flat when it’s disconnected from the car
In my case, it turned out the battery was charging fine, and holding its charge when disconnected, but there is a common fault with Audi TT’s (and presumably other Audi’s) that causes battery drain when not in use.
Solution: sell the car.
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The batteries are old…
4 years and they basically are done, yes some last a bit longer, but it seems 4 years and its time for a new one.
GTJOEY1314
Remember the sears DIEHARD, They used to last 10 years! But thats before computer draws and all the rest.
7 years is really pushing it.
Get new batteries.
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I’ve always considered they are living on borrowed time after 5 years, although the one I replaced last year in the E was 12. But it never had a real hard life and much of it was in cool climates. On my daily drivers, I routinely change them at 5 or 6, simply to avoid having to deal with a dead battery when I least suspect it. The cost isn’t worth the hassle
Sad development but the battery manufacturers make more money.
The Tudor battery in the S-type, which I installed in 1985 upon completion of the resto, lasted about 15 years, and that car was often parked without a charger. I topped it up with dist water every year.
Lead acid batteries die faster if not kept above 1/2 charge. To fully charge, this means a terminal voltage (while charging) of about 14.7V at 25C. There are lots of smart chargers on the market. They take the battery up to >14.7V and then back off to a lower float charge level, typically 13.8V. The better chargers are temperature compensated as the charge and float voltages vary with temperature.
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If you’re checking right after charging you may want to apply a brief load (e.g. turn on the headlamps for 15 seconds) before measuring, otherwise you can get fooled by a surface charge.
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just replaced my original BMW battery after 8 years and 22K mi.
Took the 22 yr old battery for the Jag off the shelf put it on a 2 amp charge for 20 min and to my amazement it lit my bug lite so I used it to test the continuity in the bonnet harness. Thought I would have to buy a lantern battery to do such testing of the continuity of the harness. ha ha. It even lit a headlight. Of course, I am sure it doesn’t have much power but still amazed.
Don’t think I could manage to only put 22k miles on a BMW over 8 years. I probably have close to that in 8 months on mine. 
Any auto parts store will have a tester that can tell you the condition of any battery, for no charge. Sounds to me like yours are dead.
Factory batteries may be unfair comparisons. I know the Panasonic that came in my wife’s Toyota lasted 12 years / 80,000 miles with total neglect. Probably replaced it too soon.
I just replaced the battery in the FHC, it was almost 5 years old. The engine seemed to crank OK but the car was hard to start. It has a Pertonix distributor. Bought a new battery and the car now starts from cold on the first couple of engine revolutions while cranking. I never had a battery last over about 5 years when I had the 2+2. On the flip side the huge battery in the trunk of our 03 Mercedes lasted 15 years.
I once had a rubber/tartop original battery in late 60s Ford last 12 yrs.Failed dead with zero warning. Next best was a German made Varta in a 90s Volvo last 11 1/2 yrs, then a Mercedes AGM battery last 11 yrs.
As for the other side, defective out of the box more times than I care to remember and many others about 5 yrs.
The Optima Red Top in my E now is dated from April 2009. It still does fine but I live in a warmer climate. I have a “Batteryminder” charger that I usually put on it for a few hours every week. If the car will sit for more than a few days I disconnect at the negative post. My main motivation for going with the Optima is do away with acid and venting in my engine bay. They are a bit more expensive but so far I have had good longevity with them in my E and 3 other cars too.
David
68 E-type FHC
David I agree
Optima on old pre computer cars can go a long long time
On a tender constantly
Also no acid , smell or powder
Agreed - I have Optima redtops in all of my cars as well.
Where the heck am I gonna go? I am always in the garage trying to reassemble a Jaguar, whose parts arrive by UPS.
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OK, that’s a solid point that actually hits a little close to home for me.
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Mine is still going strong, 03 too!
Yes, 6 years might be pushing it, maybe not. maybe, again, the charge voltage is a bit too low for these modern ones.