Replacing 997 Carrera S battery with an AGM

[joelk]

Many people on forums (as well as battery suppliers) are recommending AGM batteries as replacement batteries not only for cars that had them as original equipment, but also for older cars that were originally equipped with standard wet cells.  I'm wondering if that's always the best choice.

 

I've read that AGMs charge differently than flooded batteries.  In particular, that the bulk charge voltage is typically 14.7v for AGMs vs. 14.2v for flooded (source: https://www.power-sonic.com/blog/flooded-batteries-vs-agm-batteries-understanding-the-differences/).  I believe all 997s were originally equipped with flooded batteries, so presumably their charging system doesn't provide the higher voltage necessary to quickly charge an AGM.  There are probably other differences in how the 997 charging system is regulated that was not designed to optimally charge AGM batteries.

 

If all of this is accurate, then particularly for cars that are generally driven short distances, it seems that an AGM battery would be less likely to get fully charged during normal operation, and would therefore be dependent on being connected to a battery tender (e.g., CTEK) more often and for longer periods than a flooded battery.

 

 

This seems to be an important consideration for people who often park in places where it's impractical or even impossible to use a battery tender, and might outweigh the supposed advantages of an AGM battery.

 

I'm not an engineer, so I'm only basing this on my own interpretation of what I read.  Wondering what the experts here have to say about this.

[Loren]

AGM will be fine with or without a battery tender. The newer (since the late 1990's) battery tenders adjust for the battery type.

[joelk]

Thanks, but the thing that sent me down this rabbithole is that my 1 year old Deka Intimidator (AGM) is dead as a doornail and apparently unchargeable after I neglected to plug in the CTEK & left it parked for about 4 weeks with only one short drive during that time.  I have to replace it & was wondering if I'd be better off with a wet cell.

[Loren]

New batteries still die whether AGM or wet cell. I have seen good 8 year old batteries and I have seen bad 3 month old batteries.

You likely just got a bad/weak one - you never know how long a battery sat on the shelf before you purchase it.

[JFP in PA]

AGM's tend to be longer lived and more forgiving than flooded cell batteries.

[joelk]

Of course it's possible that I got a bad battery.  I'm just trying to understand in more technical terms what (if anything) makes AGMs preferable in 'older' cars such as a 997.

 

Some of the commonly stated advantages of an AGM battery - that it's better able to withstand vibration, that it can handle many more charge/discharge cycles, that it has a lower self-discharge rate - are irrelevant in this application.  The car isn't going off-road and it doesn't have start/stop technology.  And the car's electronic systems are a constant drain even when parked so the battery is going to run down long before self-discharging is an issue.  AGMs theoretically can be charged faster than wet cells, but only when the charging system is designed for an AGM, so this benefit seems inapplicable here as well.

 

The one thing that I found that seems relevant is that AGMs tolerate a deeper depth of discharge than a flooded battery.  If I understand that correctly, an AGM can be drained down as low as 80% without seriously shortening its life, vs. about 50% for flooded, and so maybe this allows the car to sit idle for a longer length of time.

 

It would be interesting to know how long other 997 owners are typically able to leave their cars parked and NOT connected to a CTEK, and how much this varies by type of battery.

[theblooms]

This is the real deal with car batteries, be it AGM or not. In America, there are only 3 battery manufacturers that dominate 95% of the car battery market: Johnson Controls Inc, Exide, and East Penn. And of those 3, JCI has about a 75% market share all by themselves. 

What that means is, when you walk into a store to buy a car battery, no matter what the sticker on the side of the battery says, be in Duralast, Duracell, Energizer, Interstate, Deka, SuperStart, whatever, they are all made by pretty much the same company, coming off of the same assembly line. 

What this means to you as a consumer is, buy whatever battery has the best price to warranty ratio. For me? I buy EverStart MAXX batteries from Walmart.

"But Walmart you say? For my Porsche? I'd never do that!"

Do you really think Walmart makes their own batteries? Of course not. They get them from the same place everyone else gets them, one of the Big 3, just with a different sticker and a lower price.

As far as AGM vs standard? I've used AGM in the past, and I honestly didn't see a difference in performance long term, except in the price. They weigh the same, so you're not gaining there, they generally have *less* cold cranking amps for a given group size, but the certainty cost more. So for me, I just get the longest warranty standard type battery that fits. For my 997.2, an H8 fits in the tray just fine, so that's what I use. Walmart has a 4 year 900 CCA AGM for $180, or a 3 year 950 CCA lead acid battery for $160. Is the extra year worth the extra $20? That's for you to decide.