800CCA's is enough to start the car with no problems.
The issue you describe is that the battery is not being charged...and if it is not charging while the car runs, it will only have enough juice to start the car a few times.
You should look for a problem with your charging system, specifically your alternator and generator. You could also put a voltmeter on your battery terminals while the car is running, and see if enough current is coming back to the battery to recharge it.
I was going to suggest you search the archives for voltage regulator or brush pack, as there is a lot of information in the archives, but since the great benzworld facelift apparently the search feature has been dumbed down in an apparent effort to make it appear faster. I couldn't get anywhere with it. Tons of posts, but hard to differentiate one from another.........
I'll bet the brushes are worn in the Voltage Regulator. Very common condition. If so, not enough voltage will be supplied to the battery. Should be 13.8v - 14.2v engine running.... off idle.
2 screws, wicked easy, inexpensive too!
I changed the voltage regulator in the alternator in my vehicle. I did it with the unit still in place. It can be done, but it is much easier to do with it out of the car. One of the screws is a nightmare to get out of the alternator when it is still in the vehicle. You have to be a contorsionist to get at it, and if you are able to get it out it is just as difficult to get it back in place.
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