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Personally, for what they cost (but not VW dealer prices!), I would always change both at the same time anyway.
 

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Interesting that on my car, which had VW banded batteries in it still, the one under the seat was larger (longer) than the one in the trunk. Like the Interstate H9 specs above.
 

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thanks for that V10Touareg1, not sure what you mean by &amp 'house battery'? i put a varta din 100 AGM under the front seat. I1 varta 393x175x190h, 920CCA, 110 AH,

So now 2 or so years on the front battery is dying again? thankfully the self jump start system works. if the $440 battery only last 2 years, maybe a cheaper battery is better value?


what I can't get my head around is if the rear battery is the starter how come when the front battery is flat the engine won't crank over?


I fitted a C-Tek battery monitor to the front battery starting at 40% charge, the jump large volt ups are when I put an external charger on.
 

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As I understand it the front ancillaries battery helps the rear starting battery start the car by operating several of the start functions.

When the starting battery is dead the ancillary battery shuts down a number of the ancillary functions so it can start the car on its own.

Reading your post seems to indicate if the front battery is kaput the rear battery can't do the job on its own.

Regardless of what I think, it's all explained in the VW Touareg Self Study Programme 298.
 

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thanks Noobytoogy I believe you, But to me electricity is a mystery of smoke and mirrors.

one thing I know is, when the smoke comes out of the wires one must get an electrician to put the smoke back in...
 

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Old thread, but probably still useful for some. Replaced both batteries in V10 about 5 years ago. When I took out the battery under passenger seat, the bucket where it sits in was half full of battery acid! Now I regularly check voltage - front battery via terminals under bonnet. Question: back battery 12.9 V with engine off, 13.7 with engine running. Front 12.13/13.12. Any comments as to front battery replacement? Drag, I would especially appreciate your comments. And please no comments from anyone in replacing one or both batteries - that has been covered extensively on the forum. Cheers, Werner
 

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The 12.13 is showing the battery is reaching the end. The gold standard is a replacement.

If you have a battery minder and about a week, you could put it on the desulfation mode and see if you regain capacity. I do this when I have the time. It is very gentle on the battery.

If the front is a flooded cell and you have a CTEK, you could try the recondition mode. CTEK is the gold standard of battery chargers.
 

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Old thread, but probably still useful for some. Replaced both batteries in V10 about 5 years ago. When I took out the battery under passenger seat, the bucket where it sits in was half full of battery acid! Now I regularly check voltage - front battery via terminals under bonnet. Question: back battery 12.9 V with engine off, 13.7 with engine running. Front 12.13/13.12. Any comments as to front battery replacement? Drag, I would especially appreciate your comments. And please no comments from anyone in replacing one or both batteries - that has been covered extensively on the forum. Cheers, Werner
Hi Werner
Love conversations about V10's interesting stuff with the smart twin battery setup but not sure what your question is.
As a some what related comment know a R50 that was having low charging issues that had the alternator drive pulley changed18 months ago.
Turned out to be the cable from the alternator to the jump start connections under the bonnet couldn't carry the current needed any more.

regards
Drag
 

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I read it as a 5 year old front battery that has a resting voltage of 12.13. The rear battery is being charged from the same alternator and has a resting voltage of 12.9 volts.

The rear battery is being fully charged, so I doubt we have an alternator charging problem.

There is the small possibility that there is a high resistance connection somewhere between where the wiring splits from the front and rear battery. In that case, we only really have the small amount of wiring from the distribution block under the front seat to the positive side of the battery and the small amount of wiring that is specific to the negative battery cable.

Since wannabe stated that the original front battery tray got flooded with acid, there is a fair chance that the battery cable and the battery terminals inside the battery box could be damaged.

Still, change front battery. At 5 years old, it is due. Also, inspect those battery cables and look for any signs of damage. If they pass visual, put the vehicle back in service.

If problems continue, change both the positive and negative cable that go inside that front battery box.
 

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Cheers for that - will let you know how it went
 
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