dawktah

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 that the main battery is no different than the trucks being delivered without second battery.

Also any idea of how long it will take for it to drain from non use? Would reattaching it for a weekend a month (provided its driven) be overkill or sufficient to keep it fully charged?

Is this new territory?

Difference Engine

My understanding is that the second battery isn't used until the main battery dies. If the main battery is drained and the vehicle won't start, remove the key, reinsert it, and that trips a relay to switch to the second battery. Once the vehicle is running it switches back to the main battery. The second battery should never go dead, and a Cayenne with two batteries should never have a starting issue related to the batteries.

RAKLAW

Read my prior post regarding the porsche chargeomat. The red light took 2 days to go out which shows the amount of draw on the battery in the cold weather. For the little inconvenience of plugging and unplugging the charger at every use, I have comfort in now knowing I am always at full charge,

dawktah

Can you locate this info again for me? I'd greatly appreciate it.

Difference Engine

Okay, I was a little off.

Now for some reason, 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."



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.

Standby

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

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 bythe 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 questons, 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.

mudman2

Great info Diff we appreciate the time you took to dig this out.

Thanks

dawktah

THANX, this is going to get copied to my folder for reference.

Matt C

Wow, brilliant.
Thanks for digging.

Difference Engine

No problem guys. Glad I could help.

dawktah

Thank God for the second battery! Apparently my main battery is dead and will not hold a charge. This was indirectly determined by my PCM and CD changer freaking out. The service tech inserted the real key versus the plastic key and it reset the system and showed that it wasn't providing the power.

So now I have Porsche Entry & _____, because I have to use the real key until I get another battery and the system is tested to see if something else could be faulty.

CayenneS2008

I am looking to install a 600-1000w inverter in my CS 2008 S

Firstly does this have two batteries?

I am hoping I can install it of the second battery in the trunk to power my air to water machine the little dipper a 420w requirement

https://www.facebook.com/Droughtmasters

would be nice to have free water generated for camping, and other small draw electrical devices

Can any of you techy CS owners help me install a inverter without damaging my CS electrical system.

Divot

I think I'd just bring some water with me, but I'm crazy like that and live in an area that has water flowing out of taps. You'll waste a dollar of gas for every nickle of water you generate. I'd only use an inverter for stuff like recharging cell phones, etc.

My PS hates having any battery draw without the engine running.

Stringbag

Stringbag

Jaeger

Does anyone know what general automotive size battery will fit for the trunk auxiliary battery, or is you have pictures and dimensions, rated CCA that would be helpful too. And are all 2003-2009 cayennes wired for the aux. battery in the trunk?

Well now I know why when if the battery was weak in the winter cold, and I shut off and removed the key after a first start, and then tried again the engine would then start! Auxiliary battery!