You would need to measure at 1500 RPM which is about where an alternator comes to full output.

13.8 would be considered somewhat low in many brands. Acceptable range is usually up to 14.8 or so for a Denso or Bosch regulator. Those are the two most common regulators used in imports today.

Use AGM if you like. Especially in high vibration applications where they have clear advantages.

But, a properly sized, well maintained conventional lead acid battery is more than adequate for most automobiles. In a Jaguar it leads a very easy life when mounted in the trunk away from engine heat. 6-10 years of useful life would not be a surprise in those conditions.

A sealed battery is like a sealed transmission. Not good for maintenance.

 

Plumsauce,

I agree. We can all reach our own decisions about this. It's the old YMMV.

My own take is that AGM batteries are fully compatible with auto/MC charging systems, and have these (and other) advantages:
- much slower self-discharge
- no venting of explosive gas or leakage of corrosives
- longer service life
- truly zero maintenannce
Disadvantages:
- cost
- more sensitive to overcharging, when using an external charger.

It's becoming more and more the standard technology for "premium" batteries. Same lead-acid chemistry as a wet cell, superior physical design. I did not know until yesterday though that there was one available for our cars (group 49).


Question for all: has anyone measured over 14.2V from their XK8/R (or some other vehcle) charging system?
~13.8 at idle, max (14.0 to 14.2) achieved at ~ 1500 RPM is what I'm used to seeing.

 

The AGM's can handle any charge voltage but if it is too high or not ramped correctly, it will shorten the life of the battery. AGM's do not a like a straight charge rate like a car system provides, they need a smart charger for max performance, one that initiates, charges and idles at the correct rate, buy continually measuing the battery condition. A top of the line lead acid battery should last about 6-8 years in a car with no power system problems, probably not justifying the $200 or so cost of AGM to last another year or so. In an engineering economy sense, the AGM is an experiement in a car, the lead acid is an established quantity. By the way, most of my cars charge from between 14.2 and 14.4 v at 1500 rpm. AGM won't fail at these constant rates, they just won't perform as advertised.

 

OOps, I left out some basic critical info to make this all clear....AGM does not like float rate charge over 13.3 v. ... this is the time when the battery is fully charged. The constant 14.2 v of the car charger will create a problem unless it is shut down during float times. Single stage 14.4 v chargers in cars must be disconnected during float....an inconvenince for most of us.

 

Oldmots,

Thanks for the input on the alternator output. So maybe 14.5 is more like the practical upper bound that 14.2. Live and learn.

After reading your post, I did some further (quick ans dirty) reading on charging profiles. Some of this seems to take on the flavor of a motor oil discussion if you know what I mean (no limit to the onion layers). Lots of opinions on how important this detail or that is. But I may not be reading the right stuff. Do you have a reference(s) you like on this topic?

Our cars of course can't implement any particular charging profile tuned to any particular battery type ... they just put out their regulated voltage, and most wet cell batteries seem to do fine with that. I don't see (so far) a reason this would be less true of AGMs than of wet cells. The chemistry is the same after all. (I can certainly see that an external charger could (and they do) have different profiles available for different battery types.)

One big piece of evidence for me is how AGMs are taking over the premium auto/MC battery space. If the makers thought they would be dying early in these environments they certainly wouldn't be doing that. It would get awful expensive. There are at least a few years of AGM experience in these environments. Has anyone seen anything indicating problems?

(I do have different view than yours on the "handle any charge voltage" thing. If that happens on a wet cell, water boils out and can be replaced. With an AGM if the internal pressure hits the point where valves open and it out-gasses, that water is lost and can't be replaced. Score one for the wet cell.)

 

Speaking of new batteries and venting, does anyone have pictures on the XK's battery venting system? It would be nice to see it as I don't think mine has it in place.

 

A daily driver spends most of it's time on the alternator, not an external charger. After starting, the battery should charge up fully, and then it is on float charge. Since the alternator is doing the float charging, it will be at the usual alternator output voltage which is higher than desirable for AGM because the system is tailored for conventional lead-acid.

The battery functions as a reservoir and buffer. Therefore, after the recovery charge for starting drain, it is on float. And, for AGM, it will be on float at a non-recommended voltage.

It is only evidence that some people like red and yellow battery tops. They get sold at boom-boom sound system places. They are popular with the boom-boom crowd because they need the power reserve when they outstrip the output of the alternator. The battery is acting like a capacitor. The car itself does not need that much current, it's the add on amps.



If you accept that the float charge is too high, then the AGM *will* gas. And the AGM is sealed. Doesn't sound like a good combination for long life.

 

I replaced the battery in my 2005 XKR last year with an Interstate Mag-Tron, 85 month battery and it has worked excellent. The cost was right around $180. I have not noted and corrosion or odor issues in the trunk or passenger compartment.

 

This stuff is just engineering. It is not unusual for products to be marketed that are not exactly suited to the need. I just wanted to expose the engineering reality that is under the hype of AGM batteries...they are not suited to cars as well at lead acid, great for boats with the right chargers. I have spent a lot of time learning about this subject as cruising boats rely on battery power. If you want specifics go to google for "AGM battery charging" there will be charts and descriptions out the wazoo. The charging of these batteries is very complex and takes a computer to do right compared to lead acid.

 

Yikes! This topic swamps any motor oil discussion for lack of convergence. Pick a topic: float voltages, charging procedures, longevity ... and you can find on-line any opinion imaginable; sometimes with at least plausible support, sometimes not so much.

But the more I look into it, the more convinced I become that AGMs will soon replace the traditional wet cell in most Auto/MC applications. The only real barrier is cost, and that is getting to be less so.

The "nutshell" reasons for this are that concerns about charging programs, float voltages and the like do not seem to hold up well under close scrutiny, the advantages of the sealed construction are considerable, and there just doesnt seem to be any evidence for these batteries underperforming in auto/MC applications. (Maybe someone else has seen some.)

Some results from poking into this today:

Use of AGMs in cars goes back ... 20 years! The lowly Mazda Miata: every last one has shipped with an AGM battery since about 1990. So how do they perform? The user groups indicate average life of 6 years, and some guys report getting 10+ years. Not too shabby. OK, but is there something special about the charging system in these cars? Nope. A nominal voltage regulator range of 13.8 to 14.4 was quoted in user groups. Bottom line: performance at least as good as wet cells.

Has anyone seen evidence to the contrary, i.e. AGMs not performing well in Auto/MCs ?

(In what follows, it's worth noting that vendors would not be motivated to BS us about the technical points. Anything that decreases battery life would work against their financial interests.)


This from the Optima.com website, a high-end AGM battery vendor. (Some very informative videos there, even if you don't want an AGM!)
- 13.5 to 15.0 alternator voltage is OK (some have claimed 14.5+ is trouble)
- float voltage is the same as for a traditional wet cell (this makes perfect sense to me; I think it should depend only on the cell chemistry, which is identical to a wet cell). Consequently, traditional trickle chargers can be used.
- traditional charger is OK too, provided it does not go beyond ~ 15 volts. The one thing an AGM can't do without damage is accept high charging voltage. (Also, a traditional charger is not smart enough to recover a deeply discharged AGM, but they have a trick for that, see website)

And This from talking with a tech at Dekabatteries.com (who makes a $180 AGM for our cars, as well as wet-cell batteries)
Q. Do I have to worry about my car's voltage regulator setting?
A. Nope. If you want to get fancy, and the voltage level is above ~14.5 volts, you could roll it back a little, but it probably won't matter much
Q. Can I use my curreent chargers?
A. (After discussion, yes)
Q. Warranty?
A. 6 years

Then, says he, you've been on the internet, huh? He believes perceived problems with AGMs orginated with what he called "the boat guys" who sometimes used chargers which exceeded what these batteries can handle. Most of the AGM worries do seem to appear on boating websites.

He tells me (I did not check) that AGMs are increasingly being put into new cars. No charging system modifications required.


Just my particular take-away, of course, but the objections to this technology don't seem to hold up very well ... at least I can't find evidence supporting them. The advantages of a battery that emits nothing explosive or corrosive are pretty clear.

As always, YMMV.

 

Just aside from what I see is a pretty solid discussion, can someone post a picture of what the venting system looks like? Maybe it is there and I just didn't do what I was supposed to.

 

Does this do it?
http://www.bimmerboard.com/forums/posts/39512

 

Let me compare it to the misc items laying around around the battery and I'll get back to you.

 

Here are some pictures showing the adapter with an Advance Auto Parts AutoCraft battery. My original Jaguar tube was missing, so I just used some vinyl tube from the hardware store. The Jag part number for the tube is DAC77753 and it lists for all of 85 cents at the dealer.

 

Some AGM batteries are vented to a tube. I found this very interesting quote from an official at Optima on another website:

"Our group 27, 51, 78, 34C, and 31 batteries all have ports for connecting a vent hose. Although people do it anyway, we would never recommend installing an unvented battery in any enclosed space, because there’s a legitimate, albeit unlikely, safety risk involved.

For example, IF there is a voltage regulator failure, and IF the battery is severely overcharged, and IF this goes unnoticed, and IF the battery vents because the internal pressure exceeds the release pressure of the vents, the gasses are both flammable and toxic. This may sound like a lot of “ifs,” but attorneys and engineers get paid to plan for every worst-case scenario. Although our batteries can be mounted in a variety of orientations, we do not recommend mounting them upside down. If anyone has any questions about our batteries, I’ll do my best to answer them."

Jim McIlvaine
eCare Manager, OPTIMA Batteries, Inc.

Attached is a picture of a vented Optima AGM.

 

MB and BMW now provide AGM batteries as original equipment on some (mostly higher end) cars. MB supplies their AGM battery as a replacement for group 49 batteries (the size we use). Not sure, but I think they no longer offer a wet cell in that size.

From scanning Mercedes and BMW boards, it seems like we may be a little behind the times here. The questions we're dealing with on this topic were "asked and answered" a while back.

 

You may think this hard to believe but my wife's '93 Miata has an AGM Mazda branded battery installed in '96 and still going strong. That's 15 years! The reason that it was replaced after 3 years was that the dealer service department left the door ajar over night and ran the battery down. Rather than leaving us waiting they replaced the battery under warranty.

 

The Lou Gehrig of batteries. Please don't tell your wife I described the Miata as "lowly", OK?

The Deka batteries plant is only an hour or so from here in Pa. I think this brand has a good reputation. It turns out they sell seconds (case blemishes mostly) of our group 49 batteries, AGM, for less than $100 there. That would be one great deal.

If anyone near here (zip = 07853) has an interest, please let me know. Maybe this summer when I find myself over that way I could pick up a few.

 

For what it's worth, I bought a Bosch battery for $115 at Pep Boys about 6 months ago. It works fine, but the construction of it seems inferior to the battery that I removed. It has a cheap nylon strap for lifting it and the cover over the + terminal doesn't close properly with the cable hooked to the terminal. It IS vented, however.

 

Hello Jim,
I just bought a new battery for my 2000 Jaguar XKR and noticed 2 vent holes on it like the ones in your photo. Do both of them need to be vented or only one?
Yours, Ken