Porsche Cayenne Project: Replacing The Main 12-Volt Battery

2022-05-14 06:54:43 By : Mr. David Wong

A dying battery can cause electrical faults before the starter loses too much power and struggles to turn over that twin-turbo V8.

When I bought a 2006 Porsche Cayenne Turbo with 115,000 miles on the clock, my list of planned maintenance included fluid changes, checking suspension components, and little details like new struts for the rear hatch and glass. But one little niggle kept popping up in my mind every time I started the Cayenne's twin-turbocharged 4.5-liter V8, which seemed a little hesitant to crank over. The seller claimed that after almost a solid year without driving, the engine fired right up, but I began wondering whether either the starter or battery (and hopefully not both) might be on the way out.

Using a multimeter, I tested the battery with the engine off, during a cold start, and while running—at 12.29 volts average when cold, dipping to 10.2 while cranking, the battery seemed a little low. Well over 14 volts while running signaled that the Cayenne's heavy-duty water-cooled alternator still works great.

Luckily, the truck came with plenty of records, so I dug in and found a receipt dating back to the original owner replacing the battery in 2014. Even with (or perhaps because of) a year off, that seemed pretty old, plus a new battery would cost about half what a new starter runs. But in classic Cayenne form, replacing the battery ended up requiring much more work than expected.

First, I needed to figure out whether my Cayenne uses one or two batteries. Turns out, because mine came with the upgraded Bose sound system, a subwoofer under the spare tire in the trunk takes up the space where a second battery lives on some Cayennes. That extra power source can serve as a backup when the main battery dies—not so for my truck.

Without that second battery, I plugged a CTEK battery tender onto the terminals under the hood to keep a bit of power in the system while removing the main battery.

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Now I need to admit that the lead photo for this article with the battery resting in the engine bay serves as something of a fake-out. In reality, the 955-generation Cayenne houses the main battery under the driver's seat, which makes this job a bit more annoying than on most cars. The tools required include a 10-millimeter triple-square drive, a 10-millimeter socket, and a T20 Torx drive, plus a flathead screwdriver (that latter not 100% necessary but definitely helpful).

Begin by scooting the driver's seat as far forward as possible to make room to, you guessed it, access the battery compartment cover from the backseat footwell. One of the most absurd aspects of this job is that it would be impossible if the battery is completely dead —those electronic controls would not function with a dead battery. Brilliant!

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With the driver's seat all the way forward, remove the plastic trim covering the seat rails. The first on my Cayenne came off very easily but the other required popping off the center-top portion before jimmying a flathead between the plastic and the metal railing. With the trim off, the carpet slides out from under a foot-warming air duct quite easily.

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The whole reason for peeling back the footwell's carpet is simply to unclip the battery compartment lit retainers seen above. Yes, Porsche engineers could easily have made this much more accessible—as you'll see later in the job. Some wiring next to the lower clamps prevented my finger from being able to pop the clips up, so I used a flathead here. Smaller fingers may get the job done more easily.

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​​​​​​​The next step involves scooting the driver's seat rearward—again, hope your battery isn't fully dead. Move the driver's seat cushion as far up and back as it will go, then tilt the seatback all the way forward. Unclip the floor mats and pop out the two plastic trim pieces at the base of the seat where it meets the floor. Again, the flathead helps here.

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Porsche engineers apparently love the triple-square drive for bolts and screws—also known as XZN, they're neither a Torx nor a square drive—so every Cayenne buyer should make sure to keep a set handy. In this case, use a 10-millimeter triple-square to loosen the seat retaining screws visible below the carpet layer. They should come out quite easily.

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With both triple-square screws removed, tilt the driver's seat as far back as possible. Wiggle the controls (again, using electricity that won't be possible if the battery is dead) can help make even more room to work. And you'll want the space, since the battery hiding under there will feel quite heavy with very little leverage possible.

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To remove the battery compartment cover, unscrew the T20 Torx holding what appears like a completely useless piece of plastic trim. I considered throwing this piece away but decided to trust Porsche's engineers in this case, even though no potential purpose appeared throughout the job. Next, unclip two more clamps at the front (while wondering why the design doesn't use hinges or slats at the rear, which would remove the entire set of steps required for access from the backseat).

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Pull off the cover and take a second to check out the battery hidden beneath. Note the positive and negative terminals, any loose space at the ends or sides, and especially the vent hose, which you can unplug from the battery. Next, use your 10-millimeter socket to loosen the negative lead and put it to the side. I wrapped the negative wire in a paper towel, then a tight piece of plastic to prevent any accidental discharges. Do the same with the positive lead—being able to move the thick wires will allow better access to the brackets holding the battery itself in place. The same 10-millimeter socket should unscrew the two brackets.

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Flip up both the battery's handles then carefully scoot the heavy unit as far to the rear of the Cayenne as possible to clear the built-in front bracket. Using as much leverage as possible, lift the battery up and out, being careful not to crunch any of the other wiring or relays beneath the seat.

Before I even pulled the Interstate battery out, I noticed it had plenty of room at the rear of the compartment, an H8 designation, and a November of 2019 date sticker affixed on top. It looks like a Costco purchase that perhaps the seller forgot to include a receipt for in the three-ring binder of record that came with this Cayenne. At that age, the battery should have been fine and the Interstate H8 rating for 900 cold-cranking amps should be plenty. But after sitting for a year, maybe the charge depleted enough that a reconditioning is in order.

I spent a fair amount of time researching batteries before beginning this job. Most car part dealers and websites will suggest an H8 battery with the correct CCA rating, but the 955 Cayenne's battery compartment is designed for a longer H9 battery, which will typically provide even more cold-cranking power. I bought a Super Start Platinum from O'Reilly because their H9 puts out 950 CCA with a three-year warranty—plus, same-day pickup and an end-of-the-year 20% discount helped. With all the complex electronics, air suspension, and aftermarket sound system in this truck, I figured a bigger battery sounded better.

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I carefully slid the big, new H9 into place. It weighs 65 pounds, so forearm strength once again comes in handy here to keep from damaging any electronics. Reinstall the brackets and leads (positive, then negative), plug in the vent hose, and replace the battery cover. Fold the seat back to level and, before continuing the rest of the reinstallation, go ahead and fire your Cayenne up to make sure the battery works!

Mine cranked much more happily than before, though still a bit slower than I'd hoped. Maybe the low H8 put enough strain on the starter to require that job next; maybe that big V8 just takes a bit more grunt to coax into life. I'll need to listen to a few other Cayennes to figure out whether I need a new starter, something I'd like to avoid because replacement does require removing the intake manifold.

If your battery gets the engine humming no problem, then go ahead and tighten down the triple-square retaining screws, replace the trim up front, and scoot the seat forward again. Don't forget to clamp down the rear cover clips and replace that trim. Hopefully, this H9 battery's 950-CCA rating will serve better on cold mornings during a few upcoming ski trips this winter. In the meantime, in the name of science, I've got the H8 on a tender to see whether it will accept some reconditioning.

Sources: rennlist.com, ctek.com, breakingfreemediation.com, costco.com, and oreillyauto.com.

Michael Van Runkle grew up surrounded by Los Angeles car culture, going to small enthusiast meets and enormous industry shows. He learned to drive stick shift in a 1948 Chevy pickup with no first gear and currently dailies his 1998 Mitsubishi Montero while daydreaming about one day finishing up that Porsche 914 project. He's written in various media since graduating from UC Berkeley in 2010 and started at HotCars in February 2018.