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Discussion Starter #1
I posted this on another thread on 4th May

"I had the “12v battery low, recharge by driving“ a couple of times over the last three weeks but the warning went off after about ten minutes of driving.

Yesterday I got in the car, switched on, and was met by a string of error messages concerning systems on the car and then it went dead and the alarm went off. I called VW assistance and an AA man came out. I told him I thought the 12v battery was flat. He put his booster on but we could not lock the car or take the HV charge cable out and the alarm started going again. He then charged the battery a bit with his van, enough to lock it and stop the alarm and he escalated the call to a VW engineer.

The VW engineer came today, replaced the battery, cleared all the error messages and test drove it. Pretty good I thought but this is a clear weak link if you don’t use the car much or leave it in an airport car park for two week. Like MrMoo, my car was in the garage for two weeks, mine with a camera problem. It spent a lot of time doing nothing, waiting to have the camera recalibrated. I got it back just as the lockdown started. Also, I bought it new in December. The dealer had it in stock and I don’t know how long for.

My smart charger arrives Wednesday!"

An update:

My Ctek 5 amp charger arrived on 6th May and it took about 15 hours to get to the green light, battery full. I have used the car roughly once a week since then for on average an 18 mile round trip. I started getting a few error messages, lane assist not available, drive assist not available which I have seen before. These went away fairly quickly after starting off but look like early warning of low 12v battery despite my new cigarette lighter volt meter showing 13.3v with start button pressed but no foot on brake. (14.4v on ready to go). So I charged the battery once more, took 15 hours again.

Do others have this problem or do I have something running which should not be?

If this is normal, two weeks in an airport car park or three weeks for a long haul holiday look to be not possible.
 

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I'm wondering if your replacement battery has a bit of damage already. 5A x 15 is 75Ahr which is a lot of charge but maybe not if its a large capacity 12v battery (and Lead acid has poor colomb efficiency).

Others may be able to advise how to check the 12v battery
 

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Discussion Starter #3
It’s not easy to calculate what went in on a charge because it’s a smart charger that goes through six stages from rapid charge to trickle to check charge is holding to maintenance.
 

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Upon Golf GTE it is 44AH so no reason to expect one in Passat GTE to be any different. I don't know how many days you have to leave without using for car to go into "hibernation", but if you look in WeConnect it won't be able to access available range once in that mode. By using Halfords Smart Charger every 10 days or so seems to keep mine happy. Have to wonder whether when battery was replaced this was one that had already been removed from another vehicle and has suffered degradation. Certainly 'permanently online" GTE with on-board eSIM seem to consume a lot more juice than previous GTE without this feature.
 
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