So I purchased this car 2nd hand with 45k miles. Its 2007 s550 4matic and now at 50k miles. I had a lot of starting problems (i.e. the car would not turn over at all).

After hooking it up to the Pep Boys battery tester, they said it was at ~200cca, so basically dead. Unfortunately, they didn't carry any direct replacement. I headed down to the dealership and bought and installed an OEM battery.

Now starting was quick and worked great! We left on a short trip and all was working fine until the next day.

I went to start the engine and it would begin to turn over (make the sound of turning over) and proceed to fall back to 0rpm. It went likes this 3 times in a row. All the lights were turned on on the dash. (I think they should be to make sure all the bulbs work correct?)

Tried to start one more time and it worked. I've started it fine after that over 10 times and all is working fine. No CELs.

Any ideas on what it could be?

PS: I had the 50k miles service at ~48k and got oil change, transmission fluid change, and new spark plugs. I replaced both the cabin and engine air filters as well.

Which battery did you have changed? When I purchased mine, I had to get both batteries replaced and everything worked fine. Depending on how long the car has been sitting and how old/used the batteries are, you may have to have both replaced.

The front one (engine) is just used for starting. The rear one (behind the rear seat in the trunk) powers everything else. It may be worth it to have both tested.

I had the starter battery replaced. I will try and get the main trunk battery tested as well. But would a bad main battery cause something like this?

Crankshaft Position Sensor (CPS)

wait....these cars have 2 batteries? yikes...lol

Yeah I read a little about the CPS. Apparently, it's a relatively easy replacement. But I have no CELs..

Yes, the battery you used under the hood solely is used to start the car. I can attest to this because once removed, you can still lock/unlock the car.

The trunk battery powers everything else. It's not an MB unless it's overly complicated right?

Very rare to have CEL.

In my opinion it's worth the gamble as it is relatively inexpensive. Under $75.

Yup, when you start having small inconsequential electrical issues, check the batteries. The one under the hood is the starter battery. The bigger mamma-jamma in the trunk powers everything else. Either one will give you screwy electrical gremlins.