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Originally Posted by X5 Meister
You're right about the recoding voltage, BMW specifies a minimum in all their related SIB's.
As far as the old wives tale thing, I believe what you heard, I just don't know if it's completely accurate. All the chargers seem to have their own proprietary formula for optimal charging. Unlike what you heard it's my understanding that AGM batteries apparently prefer higher voltate charge so an option like the Battery Tender Plus which BMW itself sells, and outputs at 12 volts/1.25A, really isn't such a great choice compared to a CTEK charger that outputs 14.7 volts/3.3A in its specific setting for AGM batteries. And hogwash or not, it's interesting to note that the Porsche rebranded CTEK charger does not have a AGM setting since Porsche didn't use AGM batteries and so they didn't want someone to charge at 14.7 volts (I spoke to CTEK about this). Now will both get the job done? Probably. But unless someone did a head-to-head test over the lifespan of the batteries I think it would be impossible to know for sure if one is better than the other. In BMW's SIB on charging they specifically call for the Deutronic charger to be set at 14.8 volts constant voltage while they don't specify the output current. So that's interesting to note as well.
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My point was the differences between the two are inconsequential. You can use either the CTEK or any other quality charger and the performance of the battery will be the exact same. The key here being quality, so long as that exists you absolutely DO NOT need a special charger for an AGM battery. Would it be nice to have, sure...but its not necessary in any capacity.
WRT Porsche, BMW AFAIK does not use AGM batteries either. At least none that I've personally laid eyes on. The 1.25A tender they sell is a joke, as the amperage really isnt sufficient to prevent Sulphation. So far as I know, 2A is the minimum to reverse the sulphation process. Also the Deutronic chargers are designed to maintain a relatively high specific voltage, and vary amperage as needed for coding/diag. Black and Decker also makes units that do this. Does that mean they're any better for simply charging/maintaining a battery on a car that sits in storage? No, not really.