Battery BEM Codes, again

[mhobson]

Two years ago I changed my A6 Allroad battery for a Varta Silver Line 110ah battery. No BEM codes were on the battery and I got help from this site in trying to invent codes which may have worked, but now just after two years I am again suffering low battery power issues.

Are the only batteries issued with BEM codes those which are supplied by Audi? And how much do they charge, because it will be a supply and fit, oiks like me keep our filthy fingers off! Is it just a big con? I suspect so.

The first issue I notice is that the electric tailgate would not open, except with the engine running, just after a longish run the battery was only showing 12.16V, but with the engine running it is 14v.

Is there no way around this problem, a new Varta battery costs around 200 Euros, and a new one every two years?

Suggestions appreciated, I often wish I had never sold my 1998 A6 estate AEL 2.5 tdi, so much more simplicity.

[Crasher]

I enter VA0 for Varta (some say it is VAO) and then the rating plus the date with spaces to make up 10 digits. The six month also Varta silver in Mrs Crasher Polo has died, modern batteries are rubbish.

[smiffypr]

12.16v (if it was with no load) would be about 60% charged. Whether that is bad depends how long your longish run lasted.
I recently found that my battery was discharging more quickly than it should, turned out to be something had been left plugged in, draining at about 100mA, unplugged and drain is down to 9mA, problem solved. But I fitted a bluetooth battery monitor and have been reading up on batteries.

A lead acid battery needs to be "bulk" charged up to 80% then "Absorption" charged to 100%. The bulk charge can be done at high current, 55amps for the 110AH battery, but the absorption phase takes time (an hour at 22 amps), and you can't hurry it. So if your battery was flat and you jump started the car, you would have to run the engine for 2.5 hours to get it to 100%. That's all theory.

In practice, starting with a 100% charged battery (charged from a mains powered charger), I start the engine and it takes a chunk of energy out of the battery, the car charges at around 14.5v and over a few minutes that gets up to 14.75v. I would guess that the car is limiting the charge rate to suit the battery, say 55A at first, then when the voltage comes up it switches to constant voltage charging for the absorption phase. Then when the car detects that the battery is 100% charged, the voltage drops to 14.65v. In this case that takes about 15 minutes. So if each time I run the engine is 15 minutes, my battery charge level will be maintained. I reality it takes me 5 minutes to drive to the supermarket, so after a couple of weeks of just short journeys, the battery is down to 70%, the short journeys mean it never gets to absorption. With the short journeys it will maintain the 70%, but it needs a long journey or a spell on the charger to get it to 100%.

My advise would be to charge your battery fully, then make a mental note of your journey times, then check your voltage again. (Note, you need to wait an hour or so after locking the car before everything goes to sleep and the battery recovers from the locking load and you get a almost no-load voltage reading). Then make a judgement as to whether your battery is failing or it's just not getting charged for long enough.