Where to start...

So a few months ago my car had a bad crank sensor, and after the engine was warm, it would crank and crank, but not fire up. I replaced the sensor, and all is well again..

Then lately, when i go to start up the car, it is as if the starter motor is suddenly not strong enough to start the engine (or the battery is too weak). Sometimes it struggles at first and then will suddenly speed up and start the car. Other times it struggles, i have to turn the key over a couple of times, and then it starts up as if there is no problem .. I have a Scangauge 2 in the car, and battery voltage shows about 12.5 before the engine starts, and right at 14-14.2 with the engine running. Could this battery be on its way out?

To make things worse, now my biggest problem is that i just did a complete detail on my car, and although she looks beautiful, when i lock the doors, it takes about 20 seconds, and my alarm (siren and lights) goes off for no reason.

I know there is a battery in the front that runs just the starter, and when i connect my CTEK charger to that battery, after a few hours it says that it is fully charged .
When i don't drive my car for extended times, i always connect the rear battery to my CTEK , and it usually just shows the half-charged symbol when i return days later. I haven't gotten any "Consumer Functions Offline" messages, and all my consumer functions work as they should..

Another thing i should point out, is that when i connect my CTEK to the rear battery, and lock the doors, the alarm doesn't go off. It is as if the CTEK gives the rear battery enough power, and the alarm doesn't sound.

So i really don't know where to begin..

-It seems as if my front battery is too weak to operate the starter fully, or maybe i have crank sensor problems again, just with different symptoms..
-It seems my consumer battery is weak, and somehow triggering the alarm, even though i haven't gotten the battery message on the cluster


-Or maybe i got water into something that wasn't supposed to get wet when i was washing the car..I took off the fender vent/bezel and set up a fan to blow inside the fender for a complete day and it hasn't helped.

I have done some research and it seems others have had this problem, and were advised to replace the batteries. I have also read that the Walmart MAX48 batteries are exactly identical and only cost 80.00 with a 3 year warrantee. Is this true?
I have also read that the alarm siren hidden under the drivers side fender contains its own battery, and that sometimes these will go bad, and you can replace the siren assembly for 100.00 from parts.com and it will solve the issue.

I really don't want to just throw money at the car right now to try and solve my issues.. I have the CSC, but if the problem is with batteries, i am on my own anyway..
So i would really like some input..
Thanks
Nick

 

Here was the problem..
And solution

https://mbworld.org/forums/sl-class-r230/366382-alarm-siren-issues-goes-off-no-reason-kills-battery-fix-diy.html