2012 Q5: Hybrid drive: system fault

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02-07-2021, 08:56 AM

#11

MP4.2+6.0

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Quote:

Originally Posted by Steppenaudi

Well, I had a journey of discovery this afternoon. I took the rear cargo area floor off my Q5 Hybrid and started hunting for the extra 12V battery you described. I finally found it (its ground lug at least) buried under the Battery Cooling Module. The battery itself is not visible from above. Up to now, I thought the 12V battery in the left cargo compartment wall was the only 12V battery, like in a Prius. So, if one wants to work on the vehicle without the "battery" connected, one has to remove the Battery Cooling Module to get to what the Audi repair manual refers to as Battery -A-. Battery -A1- is the left side battery, and Battery -A2- is the Li-Ion (Traction) battery for the hybrid.

I'm mystified now about why Audi put two 12V batteries in this vehicle. In a Prius, the 12V battery starts the electronics, which pull in the contactor for the traction battery, which starts the vehicle. The battery doesn't have to be very big. One regular 12V battery should be sufficient for both electronics start-up and engine start in my vehicle.

From the Audi shop manual I have, it seems to be relatively easy to remove the Battery Cooling Module, though it at first glance it doesn't look easy to remove. Disconnect the clip-on ducts, remove four nuts and supposedly lift the unit out and set it on top of the traction battery, The manual says to put a drape or towel across the traction battery before resting the module on it.

Component layout under Q5 Hybrid cargo floor looking forward.

Looking down along green arrow in previous picture. Neg battery terminal is slightly out of focus.

I didn't actually try it. There are freon lines and wiring bundles running to the Battery Cooling Module. They make me skeptical about how easily one can move the module aside.

Also, there's an engine starter motor. I had thought that the Q5 hybrid system electric motor (replaces the torque converter) started the engine (like on a Prius). That would help explain the existence of Battery -A-.

This is one where overanalyzing/studying probably gets in the way of just doing it.

1. Take up load floor by removing 4 tie downs and pulling hybrid high voltage disconnect.
2. Remove that fibrous looking big duct piece on left side of your picture.
3. Keep digging, starting from that side under the duct. Remove various other parts I don't recall as you keep excavating down to the battery.

It was basically some clips/tabs, maybe a few wire connector and some Torx screws as I recall. You do end up with a fair pile of parts set aside.

You definitely do NOT have to do anything with freon lines, etc.

Last edited by MP4.2+6.0; 02-07-2021 at 09:38 AM.

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02-07-2021, 09:13 AM

#12

MP4.2+6.0

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Quote:

Originally Posted by Steppenaudi

...
I'm mystified now about why Audi put two 12V batteries in this vehicle. In a Prius, the 12V battery starts the electronics, which pull in the contactor for the traction battery, which starts the vehicle. The battery doesn't have to be very big. One regular 12V battery should be sufficient for both electronics start-up and engine start in my vehicle.

...

It ain't no Prius. Good luck translating from what was German engineer speak in the old Hybrid SSP into actual English. As we used to joke, "Call 'Art' in Engineering." He'll explain it sehr gut.

My sense of what it is saying is there is a weird redundant conventional starter that takes over in some conditions. The pancake drive and high voltage system would otherwise deal with that IIRC. So then they dedicate the conventional 12V underfloor battery to that starter, and in turn the little motorcycle battery deals with remaining critical stuff, presumably momentarily. Like the basic instruments and ignition switch perhaps? A guess is all this is some cross redundancy thing if the hybrid pack, the pancake drive, or the conventional battery craps out.

Unfortunately they missed that the control electronics key module JX1 and whatever else integrates into it is still critical path--the stuff under the hood at the back of the motor and thus in the high heat and cold environment. That's the (only) one that crapped out on ours, under warranty. They literally finally had to get the part from Germany off the assembly line and air freight it. ...Closer perhaps to Rube Goldberg German over engineering, not Prius.