OK, I am little embarrased to ask this question. I know this has been discussed by I keep getting conflicting answers.
My car has two of them, one large behind the rear seat and one small in the engine compartment. Both look new-ish (I bought car used). Few weeks ago I noticed that when starting up the starter mottor appears to "struggle" for one or two engine rotations, then it rotates normally and engine starts. When to a car battery shop, they tested both batteires (loading with 50% Amps rating) and concluded that both are good. The technician insists that the large one in trunk powers the starter, but most other sources (internet based mostly) vote for a small one near the engine.
Several months ago I left my headlights overnight and drained (one or the other?) battery. I managed to start a car by adding external battery to the one in trunk and jump starting.
So, few questions:
1. Which one is starter battery?
2. Can a test miss the failing battery?
3. If the battery is OK, why it appears that there is no strength when cranking the car. (warm or cold).

Thanks a lot

 

there was a cross over

early cars get a battery under the bonnet that would suit a 1.6 litre 4cyl car with few toys - its sits the opposite side to the brake fluid reservoir above the heater blower motor
these cars gets a massive 25kg hidden consumer toy battery in the boot (behind the rear seat, accessed from the back of the boot)

if instead under the bonnet you have no space around a very chunky battery, that's half hidden as its so crammed in - you won't get the big battery in the boot

 

the only way to test a battery well, is with a very expensive load testing device after its been off the car and on charge for 24 hours - ones worth using start >$500

my original front battery still works - but I swapped it out a year ago after 15 years for good measure

the massive joke in the back can't cope unless the car does 50 miles of driving every week - yes it will do 2 1/2 weeks just sitting there but it will deplete to 50% of what it should have and be at the threshold of throwing low volt teddies and if you never look at full diagnostics 3 1/2 weeks on the car will still pretend its happy - it just won't have been

other merc's throw user messages it hit low volts - and starts with kit disabled (like non essential stuff - AKA screen demist fan and windscreen washers pumps !!! ), they come back to life when 2 to 5 miles have topped it up a bit - the w221 tends not to bother...

the harder it works the faster they age - so regular use and topping them up helps - NEVER believe the lies about sealed for life batteries - its only ever meant please don't top it up we sell more !

the rear is an AGM one designed for abuse - and can be topped up and charged on the bench and the original will likely give as good a service as any premium replacement that's past 18 month old - they have learnt to make them less well and also add less moisture on modern batteries and thus they start to age heavily from 20 months - where the originals would always do 9 years min

 

My S600 starter battery is under the hood and the big consumer battery behind the trunk powers everything else Your tech is worng. The starter battery in 2007 is under the hood. I replace my starter battery every 4 years. I replace the big consumer battery every ten years. You should also check your alternator for proper output. If your alternator is failing you can kill your batteries with low voltage. On this site is a post on how to read digital volts on your instrument panel using steering wheel push buttons. Or use a scan tool and tell us resting volts and then tell us volts after a 10 minute drive. Good luck.