I never looked and am too lazy to go check now but what kind of switch controls the light in the trunk? Is it a mercury switch? If yes, do you park on an incline? Maybe the that refrigerator errrr trunk light is NOT going off? Easy way to check would be to fold down the RR seat and look through the passthrough when your are parked normally. Just a WAG.

It would be interesting to see if something is coming on after leaving the vehicle . I believe there is an item or two monentarily activate after a few minutes , but after that if a Fluke 88 automotive meter/ or other recording device was connected using the 10 amp range and enabling the min/max record feature , the current could be monitered over a 12/24 hr period . I do not have information as to when or what devices are triggered after shutdown , but after the system settles down the meter could be attached and then the electrical system monitered . If the vehicle is in a secure area the meter can be left on the trunk lid so nothing in regard to the electrical system has to be touched or activated to observe the readings .

I can also remember reading (somewhere) that the TCH performs some sort of function hours after it is shut off. Something like 8 hours. I think it has to do with purging the evaporative system (not confident on that) but I have heard this process when I've been in the garage and thought the car was haunted. It sounds like a pump running but only for 10 seconds or so with my car.
Are there other functions that run periodically even when the car is off?
I would THINK this would be something powered off the main inverter and therefore not something that would draw off the 12volt battery but who knows.
If this process got "stuck" ...
??

All good suggestions. I have looked over the car at night and have not been able to find anything left on – at least nothing obvious like lights. I’ve often heard the pump running sounds and have also read that there are a number of funcitons like that. The Fluke 88 automotive meter is a great idea and I did suggest something similar to the dealer (a Halter cardiac monitor); don’t think they did it. My impression is that this information is available on the computer diagnostic system, but maybe not.

A relatively high amperage drain happening intermittently seems to be the most likely answer, but I am also beginning to wonder about the charging system. I have a relatively short commute and often do not drive the car for more than a few 10 minute city street trips in a day. The high voltage battery is always near max charge, but I’m wondering if the 12 volt battery is not being maintained at peak charge. Then when the car goes through its various “private functions” with a less than fully charged 12 volt battery, it drains it in 2-3 days.
The first two times it happened, the dealer swore that the 12 v battery was defective and replaced it twice. Could they have been "defective" because they were not being charged?

Tell them to fix the SOB! I left mine sit for 7 weeks when I was out of the country and the big battery still showed full and the other battery was far from dead as it started fine, I did not test the % of charge in the small battery. You have a major problem and let them fix it, that is what warrentee is for.

TCH-43mpg
VW TDI--48
08 LS 600 HL-20
08 LX 470-16

obrienklm — If the 12-V battery was not being charged while the car was in READY, the battery icon (lower left of dash) would remain "on." The idea of fitting a current clamp to (say) the negative lead of the 12-V battery, and recording the current flow in and out is a very good suggestion, because something is certainly draining your battery. But, even if you did this, you still have to determine the source of the current drain. However, if it's not an intermittent drain but a continuous one (of the order of 0.5 A if it runs the battery down in 3 days), it should be easy to verify with a current clamp, and then all that's left is to locate the source. I think the most likely culprits would be the battery control electronics (in the trunk just forward of the NiMH battery) or some intermittent short in the wiring harness.

Stan

I wonder if the inverter/converter is not sending the necessary current to the 12V battery to keep it charged. Has the dealer looked into that possibility?

ag4ever — In that case the battery icon would be illuminated on the dash. It goes off when the 12-V battery is charging. I have confirmed this with my ScanGaugeII.

Stan

I'd be curious if the 12V battery drains if you disconnect it before parking the car for several days.

Squint — The chances of this have got to be nil. The battery has already been replaced twice. The problem isn't the battery.

Stan