Mercedes 250 cdi c class engine start up

[jugsy]

Hi I'm a newbie on here, my problem is when I go to start my car it won't start, sometimes it will catch and will drive, I've had the alternator tested and it also has a new battery , yet there's no clue on diagnostic of why it will run for a week without issue but then will suddenly not start up without a jump and this could also last a week then vice versa, garage has checked all relays and there okay
On bad days the engine is trying and the feeling I get is the battery is not charged yet its new and when tested was fully charged my alternator has been tested as working as well . Has anyone on here had a similar experience? Any info will be a big help as garage saying process of elimination to find what the issue is. Thanks

 

[Blobcat]

Hello and welcome,

Are the battery terminals tight?
Is the Earth strap tight?

 

[jugsy]

Hello and welcome,

Are the battery terminals tight?
Is the Earth strap tight?

Hi, thanks for reply yes all have been checked it's really annoying as it's no longer reliable, the serpentine belt snapped in march and the car.ran normal for roughly 3 weeks then this problem started I took it back to.the garage and they checked the alternator and battery etc., plus the belt so am still confused