Do you also have a ctek or the like? (It would cover up a failure to fully charge.)

 

Any car will do an excellent job of charging an AGM, it charges faster than standard battery. The argument posed by some is that if you frequently drive very long periods of time and are not using anything in the car that is consuming power- you might shorten the battery life, as the AGM requires lower voltage to maintain it after its fully charged (thus the need for special outboard ctek chargers- original point of post). However, the more realistic odds in a car like the XK is that you are only making short trips and or using a few things that consume power.

 

It has been posted that the AGM needs a higher voltage than the car delivers (other than for a short time), leading to inadequate charging. Was that post wrong?

 

If someone posted that, they are wrong. The AGM wants a LOWER float charge than a flooded battery. Most automobile charging systems are set up to deliver a higher voltage as required by a flooded battery. That is the problem with using an AGM in a car not designed for the AGM on a long drive. For short drives this is probably not an issue.

If a flooded battery is used in a charging system designed for an AGM it will NOT be fully charged because the float voltage is set lower for the AGM. From a practical standpoint this is probably not an issue.

 

That was right and wrong. Best explained by the manufacturers words. I got this from the optima battery. Highlighting the part that answers your question directly.

Here are a couple of things to keep in mind when maintaining your Absorbent Glass Mat (AGM) battery, including an OPTIMA® battery.
Many newer battery chargers, like the OPTIMA Chargers Digital 1200 12V Performance Battery Charger and Maintainer, have microprocessors that collect information from the battery and adjust the current and voltage accordingly. Some have different settings for charging flooded, gel and AGM batteries.
All lead-acid batteries can experience sulfation—the formation of lead sulfate crystals upon discharge. Look for a charger with a desulfation mode to help condition your battery and keep it performing at its best.
Low and slow is best. A low amp charger (one to 12 amps) is generally the best choice for charging any lead-acid battery. It's quicker to charge at higher amperage, but it also can generate a lot of heat, which reduces the life of a battery, just like the heat of summer.
Alternators are NOT chargers. Don't rely on your alternator to do the work of a charger. If your battery is discharged to the point where it cannot start your vehicle, use a charger as soon as possible to make sure your battery gets fully charged.
An alternator is meant to maintain a battery, not charge it.
Batteries eventually die. Batteries are a consumable product. No battery will last forever. The goal is to consistently maintain your battery to get the most life out of it.
Different chargers. Different capabilities. Under normal conditions most 12-volt automatic battery chargers will work on an AGM battery. Many newer battery chargers have settings specifically for AGM batteries; some even have separate settings for OPTIMA REDTOP and YELLOWTOP batteries, like the OPTIMA Chargers Digital 1200 12V Performance Battery Charger and Maintainer.
AGM and gel technology differ. Remember that the technology of an AGM battery is not the same as a gel battery, which has its own charging requirements. If your charger offers different modes, select the correct one for your battery. If you use the gel setting to charge an AGM battery, it won't fully charge and, over time, it could actually damage your AGM battery.
Under normal starting conditions, an OPTIMA battery should never experience at rest voltages below 12. Most 12-volt chargers and alternators have no problem recharging an OPTIMA if it has an at rest voltage of 10.5 or greater.
The magic threshold – 10.5 volts. The charging scenario for a deeply discharged AGM battery, one that's dipped below 10.5 volts, may be slightly different. The OPTIMA Digital 1200 can recover batteries that have been discharged as low as 1.25 volts. For other charger tips for recovering a deeply discharged AGM battery can be found in OPTIMA's tech tip, How to Resuscitate a Deeply Discharged AGM Battery.

 

That sentence is priceless in understanding these battery debates and chargers and if you want to use one or not.
Naturally, many, most folks can go the lifetime of a car and not use a external charger, the alternator is ample to maintain it.
So there are no absolutes and everybody is right. However, the manufacturer has a brilliant point, how does your car know when to bulk charge, absorb charge and trickle? when does it know to desulfate.

I can add just one thing to the pie, I make Lifepo4 battery modules, its near impossible to reliably tell the state of charge. (where the battery is in relationship to charge requirements)

Here is a chart of how non-linear charging is on AGM.

 

Thanks both!

Though the posting was about whether an alternator - intended for a traditional type lead acid, and not intended for an AGM battery - would keep an AGM adequately full on an on-going basis. It wasn't about a charger as such or about a rather flat battery (unless such a traditional alternator would cause a fairly flat AGM).

Sorry for the questions/confusion, anyway!

 

I am confused then because, unless I am miss-reading it, the manual for my CTEK says that the charge rate for the AGM battery setting is 14.7V and 14.4V for normal batteries.

 

You are Correct.

It requires and can take higher current in the CHARGE mode
In TRICKLE mode it needs lower- but rarely if ever your car is going to be in trickle mode

In all reality, if we are not academic about the whole thing, SLA and AGM are perfectly interchangeable, as demonstrated by folks who have used them for 3 years

 

My charger is like the Ctek, I also have the 14,7v charge mode. In trickle mode mine is on 13. some volts!

 

Aha. The (USA 2003-on) S-Type mostly provides more like 13 (13.4 or whatever) and only a higher voltage for a short time (a few mins). So judging by the above it isn't going to charge a flattish AGM.

(I wasn't going to fit one.)

 

Does anyone communicate the battery change to the cars computer?

 

The dealer does this for the 5.0 cars. It is not required for the 4.2 cars. There is a different charging system .