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Dr Pepper

2003 Cayenne S - Dual battery - Desulphation & Charging

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After standing for a week or two, my Cayenne’s battery power drops - the engine starts but the cranking is slow, it’s creating temporary dash error messages & the comfort settings reset to factory – not good.

 

I don’t know the age of the current batteries – there’s no record of them ever being changed – so I was about to bite the bullet & buy 2 new batteries at a cost of approx. GBP 300.00. Then I got to read a number of accounts saying that sulphation is the principal cause of power loss in lead-acid batteries & that this effect can be reversed – seemingly dead or dying  batteries can often be recovered. So, rather than buying two new batteries, I’ve first ordered a CTEK charger which runs desulphation cycles.

 

CTEK state that their chargers are safe for the Cayenne electrical / computer system – whether in desulphation, full charge or maintenance mode - so if I end up having to buy 2 new batteries anyhow, then at least I have a decent charger in my workshop.

 

Q1 - When I connect my CTEK to the charging terminals in the engine compartment, will I be connecting with both batteries?

 

Q2 - If I connect my CTEK to any of the 12V auxiliary / cigarette sockets (for low amp maintenance), will this connect with both batteries?

 

Q3 - My understanding is that the battery under the seat provides the cranking power & the (full size) battery under the spare wheel provides power for everything else. To me this is the logical option but I’ve also read accounts stating the exact opposite. Which is right?

 

Any advice on how to maintain a dual battery option & how it works as opposed to the single battery option would be much appreciated.

Edited by Dr Pepper

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From the Cayenne Technical Service Book...

"If two batteries are fitted to the vehicle (main battery and backup battery), these are connected or disconnected to the bus bar in the power distributor through a relay. The switching of the two batteries is controlled independent of the load management (function is integrated in the vehicle electrical system control unit).

The starter relay of the 2nd battery is actuated parallel to the starter relay. That means for every start the starter is supplied from both batteries. The backup battery is disconnected from the vehicle electrical system when the engine is switched off and then supplies just one consumer, the vehicle electrical system control unit."
 
"On vehicles with two batteries (vehicle electrical system and starter battery), the electronics of the vehicle electrical system control unit are supplied by both batteries. The vehicle electrical system control unit evaluates the exact vehicle electrical system situation by comparing the sensed voltage with the permissible minimum voltage. On vehicles with two batteries, the load management always takes the voltage of the vehicle battery as reference.
 
If the vehicle electrical system control unit detects a critical battery condition, the consumption is reduced. For this the CAN bus requests some of the high voltage comfort consumers to reduce or switch off their power demand. By reducing the comfort functions with the “engine running”, the battery can be charged more. With the “engine off”, discharging of the battery can be reduced through the load management. 
 
Currently the following consumers are integrated in the load management:
• Heated rear window, heated outside mirrors
• Heated seats, front and rear
• Heated steering wheel
• Heater blower (3-stage)
• Interior light, reading lamps, footwell lighting
• 12 V sockets"

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Thanks for posting this extract Loren - it does fill some gaps in my knowledge but I'm still not clear which, in their words, is the main battery and the backup (2nd) battery - but unless I'm told otherwise, I'll assume for now that the backup (2nd) battery is the one under the spare wheel.

 

This point is interesting: The backup battery is disconnected from the vehicle electrical system when the engine is switched off

Based on this statement, it sounds that unless the battery terminals are physically accessed, it's not possible to charge the the backup (2nd) battery except by running the engine - which means my idea of being able to keep both batteries fully charged using the CTEK in maintenance mode is scuppered.

 

Plan B then:

For the main battery, I should still be able to desulphate / charge / maintain via the charging terminals in the engine compartment.

For the backup (2nd) battery, I'll disconnect it (to be safe), desulphate, punch as much charge in it as the CTEK will allow & then see if it'll hold a higher charge for longer periods.

 

I'll report on how this works out in a few weeks.

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The backup battery is the one in the rear of the vehicle.

The main (starting) battery is under the left seat and closest to the engine/starter.

 

As I understand it the heavy load of starting is when the relay kicks in and uses the backup battery to assist starting. And remember the starting circuit is much different than the system power bus - starting uses huge current draws.

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Here’s an update from my original posting

 

I have determined that charging via the under-bonnet terminals or via any 12V Cigarette / Aux socket will only charge the battery located under the seat – it will not charge the battery under the spare wheel.

 

It appears that the battery located under the spare wheel can only be charged via the alternator – so only when the car is running.

 

I have run the CTEK desulphation & recondition cycles & the condition of the battery located under the seat appears to have improved – on the button strong cranking with no error codes thrown up. It holds it charge better – holding 12.2V after a week – this was down around the 11V area previously.

 

I also exposed the battery under the spare wheel & performed a number of tests which showed it only charges with the engine running – this battery was also only holding a charge around the 11V area.

 

To gain quick & easy electrical access to the battery under the spare wheel, I hard wired to it, a 12V Cigarette / Aux socket - a modified male / female 12V extension lead. This was wired directly onto the battery terminals, feeding the wires through one of the existing rubber grommets so not to compromise the battery box itself. The wire runs alongside existing already protected wires & the socket rests alongside the bottle of Porsche tyre sealant – so safe, out of sight but easy to access. Oh yes, I fitted an in-line fuse right next to the battery +ve terminal too.

 

Having gained easy electrical access to the battery under the spare wheel, I again ran the CTEK desulphation & recondition cycles & the condition of this battery also appears to have improved – now holding 12.5V after a week.

 

Early days I agree – but I’m happy with the results seen so far & now, if I know the car’s going to be idle for a while, I can trickle-charge both batteries (not at the same time) via a lead I can have running out under the closed tailgate (there’s enough ‘give’ in the runner seal not to crush it) to the CTEK located in my porch.

 

As I side issue, I’ve also used my CTEK to desulphate & recondition an 8 year old Silver Calcium battery fitted in a Ford Diesel – again, the cranking is now noticeably stronger. With a seldom used Mk1 Cortina in my garage too, I'm reckoning on this CTEK getting a lot of use.

Edited by Dr Pepper

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Okay, so I am bringing a zombie thread back to life but I do need to thank Dr Pepper for this. I am a noob owner of a Cayenne and am struggling with random codes being thrown up for misfires and the like. New plugs and coil packs. New MAF. New flipping everything or so it feels. Anyway, I had a flash of inspiration yesterday and wondered about the battery. Having seen this thread I am now waiting for a CTEK MXS 5.0 to arrive and see what the state of the batteries are.

 

I know about the aux battery in the boot but wondered the same 3 things as Dr Pepper.

 

So, big thanks to him and to the other contributors. This charger should arrive by 8pm. I have to drive at 9am so am hoping that 13 hours will be enough to recondition the main battery. Not going anywhere from about lunchtime Wednesday to Friday morning so I hope to check and recondition the aux battery.

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On 10/7/2015 at 11:22 AM, Dr Pepper said:

Thanks for posting this extract Loren - it does fill some gaps in my knowledge but I'm still not clear which, in their words, is the main battery and the backup (2nd) battery - but unless I'm told otherwise, I'll assume for now that the backup (2nd) battery is the one under the spare wheel.

 

This point is interesting: The backup battery is disconnected from the vehicle electrical system when the engine is switched off

Based on this statement, it sounds that unless the battery terminals are physically accessed, it's not possible to charge the the backup (2nd) battery except by running the engine - which means my idea of being able to keep both batteries fully charged using the CTEK in maintenance mode is scuppered.

 

Plan B then:

For the main battery, I should still be able to desulphate / charge / maintain via the charging terminals in the engine compartment.

For the backup (2nd) battery, I'll disconnect it (to be safe), desulphate, punch as much charge in it as the CTEK will allow & then see if it'll hold a higher charge for longer periods.

 

I'll report on how this works out in a few weeks.

CTEK sell an extension cable that allows you to plug your CTEK charger directly to that rear battery whenever you want to charge it. I have added one to my 2003 CTT

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if the battery is over the warranty period just replace , and it is not a good idea to charge in the vehicle these chargers will cause the battery to produce hydrogen, as gassing is the way of cleaning the plates.  Out and on a bench is always best, prevents explosions and any AC getting into the ECUs'. trickle charging is ok in the vehicle. but in normal use a battery should never need  external charging. it seems the Cayenne is very power hungry.

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