Redd

Batteries are funny things, the various makers and charger makers have taken to contradicting each other about the fine points a lot these days. And the ones with really sophisticated (allegedly(G) chargers will just mumble about proprietary algorithms.

For instance, one of the Big Three in the US recommends pulse width DC charging, not pure DC. Because with PWM you can hit the battery harder, but it cools off between pulses, enough to prevent some microbubbling in the electrolyte.

And one of the charge controller companies (as well as that Big Three battery company) says that traditional "3 stage" charging is an obsolete joke, because you want to pump all the amperage you can, at a (proprietary) small amount just over the battery voltage. Then every couple of minutes, you adjust voltage to keep it just above the ever changing battery voltage. Which might well explain why Audi's voltage keeps moving around so much.

In the last car, I had a secondary battery in the trunk. (Special needs.) Last time I changed it I said "Gee, that didn't last very long" and went back to the distributor to enquire. He pulled the label down and showed me the date code, turns out I'd gotten 8 years out of it.(G)

With a real good battery (like a 100Ah AGM) costing $200 discount these days, if I can get 8 years instead of 3-4, I'll go for it. I'm one of the folks who just kinda stretched into an Audi, I don't keep three or four to match my outfits, or swap 'em with the weather.(G)

ex-quattro PETE

Finally got an email from VARTA. In response to my question about recommended max charging voltage, they sent me this 5-page document, of course in German. For their AGM batteries, Google translate says:
Kind of a wide spread.

Redd

It sounds like they are more concerned with charging amperage (not to exceed 25% C at any time, or 10%C for float charging) than with voltage. And from what I have been told elsewhere, the amperage is more important, since it is responsible for boiling the electrolyte, even if that means just microbubbles.

So as long as you control the charging current, the wider voltage range would be tolerable. I'm surprised that they don't indicate an "optimum" charging & float voltage, and that they don't indicate temperatures either.

Maybe 14.1-14.8 really means 14.45 @ 20C and it all revolves around that. Which would, after all, be very similar to the 14.4 that Delco used (uses?) for so many years.

Since they mention Optima batteries, and Optima is/was owned by Johnson Controls Inc. (JCI) here in the US of A, maybe you could get flat plate AGM information from JCI and rashly assume that would also apply well enough.

ex-quattro PETE

Does 'C' refer to the battery's capacity? So, for a 100 Ah battery, you want your charging current not to exceed 25 A?

Redd

Yes, "C" means "Capacity" in the battery business.

While many folks say you can charge AGM's at up to a 1C rate (i.e. in one hour) the makers tend to say not to exceed 25%C. Versus 20%C for wet acid batteries.

And while Varta says to use a DC charger, JCI and others say that a PWM-DC charger is far better for the battery, and actually will be faster, because the pulsed DC causes fewer microbubbles in the electrolyte, which raise internal resistance and temperature and slow down charging. Could be that Varta consider PWM-DC to be DC...that's akin to arguing how many angels can dance on a pinhead. Or the head of a pin.(G)