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On 11/5/2016 at 2:04 PM, angusc said:



I just replaced the main battery under the seat and while looking through the hand book found pictures of a battery in the trunk. When u removed the Bose unit there it was. I will replace it anyway as it is from 2006 so probably the original unit. But can anyone let me know the functionality of the two batteries, is there any user interaction required to switch from one to the other, which is the "main" battery?

Thank you in advance.

Sent from my SM-G935F using Tapatalk

I thought you either had the bose or the second battery not both?  I' have bose and have assumed I dont have the other battery.  My car is a 2009 GTS.  Anyone confirm before I rip the bose out?

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  • 2 months later...
Hi again,


I eventually ordered a service key (plastic) with no electronics and headed to where my Cayenne was parked with flat battery. I tried to start the car with the service key but it did not work and the key was held in the ignition lock due to no power.


So it didn't seem to switch over batteries for cranking......unless both the rear and front battery were both discharged.


After jump starting and later fully recharging the car on a trickle charger, I tried the service key again. But I only see a message about the immobilizer  being active on the dash display and the key can't crank the engine. Is there something else that I would need to do to get the car to crank from the front (under drivers seat) battery?


Thanx for any support.

@bigbuzuki bumping this one for your advise about the immobilizer message that I get when trying to start the car with the service key......I'm going back out to the car next week and for sure the main battery will be flat when I get there.


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I believe there is an easy way to to disconnect the negative terminal without removing the seat, so you can disconnect the batteries and lock it manually before leaving the car parked for long periods of time. You may clamp the solar chargers directly to batteries as well, so you'll have decent charge when you get back.


Also, I think the main thing you need to troubleshoot is whether the emergency start mode is operating at all, and whether it's putting the load on the aux battery when needed. 


Quick google search says you can get the car into "emergency start mode" by cycling the key 1-2 times all the way to left then right and it should start the car by getting a helping hand from the aux battery. If the emergency mode works and it turns on a relay/switch or some sort of solenoid, then it should be putting a load into a rear battery. 


I guess you just need to study the wiring diagram of the optional aux battery system, and test components using a multimeter to see if everything working.

Some scenarios to test:


Disconnect the front battery, try to start, and see what happens.

Disconnect the rear battery, try to start, observe the behavior and any errors that might pop up

Have both connected, but see if it is putting a load to rear battery like in the video below.






Edited by Nurbek I
  • Thanks 1
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@Nurbek I

Thanx very much for posting this information and the video! I will try this next week to see if the second auxiliary battery will kick in by double switching the ignition on and off in quick succession. It also gave me some ideas about testing the batteries and those solenoid relays.


I already purchased 2 battery isolator switches but yet to fit them, so I may look in to that while I have the batteries exposed.


My solar charger is hooked up to both batteries, the only trouble is that I park in a multi story car park and face the car south side, but in the winter in the northern hemisphere the sun is never enough in this situation. It works better during the summer months as to be expected.


I'm also going to text for any parasitic battery drain. When I got the car it had an immobilizer installed but I couldn't re-activate that one so had a new one installed but the guy said he couldn't find the original one so I guess that means I have two immobilizer installed and pulling power now.


Thanx again for your very useful post ?


EDIT:  I did a bit of searching and found this useful thread on Rennlist: 


Cayenne 955-957 2003-2010 - 2nd Battery Question - Since both batteries equally drain at the same time can you disconnect the 2nd battery and use to start truck in the bizarre event the main one runs down? Taking in account...


it reads:


Porsche doesn't supply a manual to techs that explain vehicle systems. Short of going to the actual shop manuals, there is no concise explanation of how things work. Volkswagen does however provide a Self-study Program. In talking with our techs, who have been on training for both Cayenne and Touareg, the electrical systems of both vehicles are identical. The following info is taken from the "Touareg Electrical System Design And Function" portion of the "Self Study Program Course #89J303." (updated link here: http://www.volkspage.net/technik/ssp/ssp/SSP_298_d1.pdf )

In this electrical system, one battery, the starter battery, ahs the role of supplying the starter with power and, if neccessary, also the electrical consumers required for the starting procedure (start-up consumers). The second battery, the on-board power supply battery, provides the rest of the electrical consumers with power.

The batteries are connected in parallel.

To prevent the starter battery from becoming discharged by too many electrical consumers, the consumers are split into two categories: start-up consumers & on-board consumers.

The start-up consumers and the remaining electrical consumers are supplied from the on-board power supply battery. If neccessary, these consumers can be supplied from the starter battery via the Power Supply Relay 1. Consumers that require a large amount of energy are always supplied from the starter battery.

In addition, both batteries can be connected via the Second Battery Charge Relay to charge the starter battery.

The actuation of the relays comes from the Vehicle Electrical Systems Control Module. It monitors the voltage of both batteries whenever the ignition is on and can thus detect when the starter battery needs recharging.


The system is on standby when the Vehicle Electrical System Control Module is in sleep mode. If on standby, Power Supply Relay 1 and Second Battery Charge Relay are open. Power Supply Relay 2 is closed.

Starting Procedures

When the ignition is turned on, the Vehicle Electrical System Control Module is activated (wake-up mode) and evaluates the state of charge of the batteries. If the voltage reading of the on-board power supply battery is below 10.5 volts, it is deemed to be discharged.

The starter battery is considered discharged if the voltage reading is below 11.5 volts.

There are four different conditions that can be detected before the engine is started depending on the state of charge of the batteries:
-On-board power supply and starter battery both charged
-On-board power supply battery discharged, starter battery charged
-On-board power supply battery charged, starter battery discharged
-on-board power supply battery and starter battery discharged.

Over the next few pages, it describes the starting procedure for each of the four above conditions. Long story short, the Power Supply Relays 1 &2, Second Battery Charge Relay, Access/Start Control Module, and Vehicle Electrical System Control Module are constantly measuring voltage in the two batteries and are switching and transferring power as needed.

Now for charging:
Two-Battery System Charging


The on-board power supply derives its power from both batteries which are connected in parallel and protected against overload by Power Supply Relay 1.

Charging of the on-board power supply battery
The on-board power supply battery is charged continuously.

Charging of the starter battery
Charging of the starter battery is controlled by the Second Battery Charge Relay which is activated by the Vehicle Electrical System Control Module. The normal charging time is 20 minutes. After this period the Second Battery Charge Relay will open.

if the starter battery voltage drops below 12.8 volts, a new charging cycle of 20 minutes maximum is started.

Not sure if that answers any of your questions, but it may help in understanding how the system works.

One of my customers with a dual-battery Cayenne S left his vehicle for 3 weeks while on vacation, and upon hs return, it started with no problems. I have no idea how long it would take to completely drain the system.

Edited by angusc
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  • 2 weeks later...

Some great info in this thread.


So if I want to connect an aftermarket audio amplifier, is either battery an option since they are both full size, both being charged by the alternator, and wired in parallel?  Kinda nice to not have to run a 20' length of power cable from the engine bay to the trunk, as is usually required.

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