It gets worse. There is a lot of contradicting advice on the internet... Don't connect a charger direct to the battery, it warns you specifically not to do that in the handbook. If you buy the CTEK then they typically fit directly to the battery/terminal post, which on a 911 means you need to open your bonnet and move the battery cover each time you charge the battery, (you can also buy another type of lead that you leave in the car connected to the battery). Another optional CTEK 12v cigarette port lead, means you can just open your car door and plug the charger directly into the 12v socket inside the car. (I think on the 991.2 you need the smoking package, but there might have been an additional 12v socket in the passenger footwell)? The Porsche Tequipment version has the 12v socket fitted as standard.
I went through this last year with my 991.2, did loads of research and am rapidly reaching the same conclusion, that the best way to look after your car is to buy the Porsche charger. Let me highlight why it works for me:
Porsche Charger (Tequipment Charge-o-mat Pro) Pros:
No warranty issues.
It will be the right charger for your car as recommended by Porsche.
Has cigarette lighter connections as standard, (but you need a 12v soccer inside the car).
Is actually a CTEK charger.
Plug in and forget.
Cons:
It is a CTEK charger but only a very simple one.
Costs slightly more than a CTEK and has less features.
CTEK Charger (5.0 or XS) Pros:
Has lots of extra features that allows greater control/recovery of a battery.
Is less expensive than the Porsche version, (but not by much).
Cons:
Not recommend by Porsche, if you have any battery or electrical issues it "could" affect your warranty, (very unlikely but worth noting).
It does more for the battery, but is it needed?
Extra leads are required to make the charging simpler, (12v cigarette or leave in the car lead). Not really a con but one to be aware of.
Its extra features mean it requires a bit more input that just plug and forget.
CTEK customer service is not great. I wrote to them yesterday to ask which charger to use for my 992 and they recommended the wrong one...
I was looking at buying the CTEK but on reflection I have ordered yet another Porsche charger for my 992.
Tom