Tom M

I replaced the original battery on my 2011 S the other day with and H8 Autocraft Platinum AGM from Advance Auto Parts. Things went pretty smoothly and all seemed well other than needing to clear the codes that come from such a swap. My old battery was routinely charging @ 14.7V too/from work (not in Sport mode) and with the cold weather was reading anywhere from 11.6V - 12.1V after just turning the ignition to the On position along with slightly extended cranking times. The low value is what prompted the battery change as I didn't want to get stranded.

After the installation I hooked up the CTEK to make sure things were fully charged and on the ensuing drives that day it charged at 13.6V so all seemed grand. The next morning the initial voltage was 12.3. The drive to/from work the following day saw the return of the 14.7V charge cycle. This morning the initial voltage was 11.7 but it fired right up so I'm really not sure what's going on.

I'm planning on putting the CTEK on it again tonight to see if that impacts the charging in the morning. I've got no new codes since clearing them after the swap with the iCarScan but is there anything else I should be checking? I haven't checked to see if the Stop/Start function is working again as I had it turned off since it wouldn't function with the old battery so maybe I'll try that on the way home tonight.

hotrod2448

I'm seeing similar numbers, was told by Porsche the battery was weak and needed to be replaced. Numbers with the new battery look about the same but, it does seem to crank easier and not dip quite as low in the cold. On the old battery I saw 10.7- 10.9 a few times leaving work. Now it is closer to 11.2. You have to take that voltage display with a grain of salt though because it doesn't appear to respond fast enough to get a good read on cranking. With my voltmeter and POR V2.0 I see in the 9s when actually cranking.

Tom M

The battery I swapped out actually showed 12.53v with no load on it but not really a fair test. The MFD voltage reading does take a while to settle down as it sometimes starts at 11.2v but then will rise up so I don't worry about the initial reading and always give it a few seconds. If I read it right after shutting down it's about 12.3v currently. I just wish there was a quick way to tell the health of the battery w/o having to disconnect it. Last time I tried to have the original battery read at the posts in the engine compartment they couldn't get it done so I pulled the battery and took it in where it tested good. That was 3 years ago.

hotrod2448

Yeah, static voltage is not a good representation of battery health. I didn't think about till just now but, the other thing I've got going on and why it's hard for me to get a good read on voltage prior to cranking is the glow plugs which you don't have.

kilster

I use to test my battery in 991.1 C2S. My charging voltage went up to 14.5-14.8V over this winter, but the tester says that the battery is in good health. My initial voltage with key on is 11.9-12.1V, but with the tester, it reads 12,5-12.6V, which can be 0.1-0.2V higher after the CTEK light is green.

Tom M

I've got another data point for what's going on and a possible theory.

When I drove home tonight I turned on the Stop/Start functionality and it didn't work for the first 15 minutes or so and the charge rate was ~14.6V. However then it started to work as the battery continued to charge up. However the voltage dropped rapidly and then hovered at 12.1V for about 20 second before dropping to 12.0V. The light changed so the car started back up when I took my foot off the brake and off we went. It also successfully stopped when I reached an interim destination. On my final leg of the drive home the charge rate magically dropped to 13.6V. When I got home tonight I put the CTEK on it again so we'll see how it behaves on the way into work tomorrow.

The theory I'm working on now is that the manufacturing date on the battery was 11/18 which means it could have been sitting for 1.5 to 2.5 months so the cold weather along with the limited amount of driving is barely giving it a chance to top up the charge. My initial charge with the CTEK explains the behavior on the first drive and the drive home tonight points to it being fully charged as well when it finally dropped to 13.6V for the last few miles. The morning drive after charging overnight will probably tell the story.

CAVU

If your ctek does not have a labeled agm mode, charge it using winter/snowflake mode. AGMs need a higher charging voltage.

Kris Murphy

Not sure if this is what is happening to you, but I had an AGM from Advanced die, as in DEAD 2 weeks after install.

deilenberger

There is a story around - that claims that ANY battery should be fully charged BEFORE being installed and put into service. That's unlikely to happen if you simply grab one from the parts store and install it. I usually leave the battery on my smart-charger overnight and check in the morning that the charger has gone into float/monitor mode. If it hasn't - I'll give it more time. If it never goes into that mode the new battery is likely not good - and should be exchanged.

Tom M

As noted I did at least fully charge it once installed prior to starting up the car. The CTEK did swap from amber to green several times so it was happy that it was charged. At least at some point. I guess the next couple of days will tell.

garrett376

Hey Tom,
Have someone or a shop with a Midtronics battery tester check your battery; that's how you can test it without disconnecting it as it has an "in vehicle test" routine. But in general, with these 958s, if your vehicle cranks to a start quickly 2 to 3 times in a row after shutting off each time, especially in cold weather, your battery is doing fine.
The Cayenne is not like a normal "old fashioned" car when it comes to power management: it will cut off power to certain things if the voltage drops to protect itself and to permit starting when the battery starts to fail; it will recuperate with a high charge when coasting with the brake applied; and it will charge higher in very cold temps. The voltage provided to the display is not necessarily reflective of the actual battery voltage: hook up a voltmeter and stand it on your windshield and compare what the display reads: you'll see it varies by a lot at times.

Tom M

Charged the battery overnight with the CTEK again and this morning the charge rate was as low as 13.0V on the way in to work and Stop/Start worked as expected. On the way home after sitting for 8 hours in the cold the charge rate started at 14.5V and dropped quickly to 13.5V so all seems good for now. Will probably turn Stop/Start off again now that things are OK and no longer need it as a test tool as I find it more of a nuisance then help during my normal commute

PorscheTech788

New batteries need to be coded to the car so it knows the capacity of the new battery and can maintain it accordingly. Not saying this is causing your issue but its something to consider.

Tom M

This thread from renntech a couple of years ago seems to say otherwise and that all entering the battery info does is log it for the purposes of record keeping.

https://www.renntech.org/forums/topi...ry-replacement

TXCOMT

Here's my battery story; after much reading on this forum about how critical electricity is to Cayennes (and modern vehicles in general), I bought a Bosch AGM from Pep Boys with the intention of installing it in the wife's '14 S myself. After getting it unloaded, I promptly put it on a new charger with an AGM setting...it sat that way in my garage for several months! Eventually, I figured my trusted indy shop could do this admittedly easy task much quicker than I could, so they did that and an oil change, too. I have no idea if they "coded" it, but I do know that I've had no issues with the Cayenne since the swap. My rationale for taking to a P-car shop was I'd have a receipt for PCNA if any issues arose since it's a CPO vehicle.

TXCOMT