I have found by personal experience that an original battery fitted to new vehicles generally seems to have a longer lifespan than the subsequent replacements.
Admittedly our climate here in NZ is not so cold in winter so the loads are less than in some countries but I have always made sure never to run my battery flat if the car is stubbonly refusing to start.
If it wont go, I look for and solve the issue before it is dead flat.
Examples of previously owned new vehicles and original battery life achieved before they eventially died or became unreliable:
1984 Corolla 1.6. 9 years.
2005 Daihatsu Sirion. 8 years (replaced this year wifes car- still owned)
1996 Rav4 11 years.

My current 2002 V40 has a 600cca Volvo battery fitted April 2007 (Japan.)
This battery is still performing reliably at 6.5 years.
I have had this vehicle for 18 months with no battery issues and have even used it to jump start other vehicles via leads.

 

Thought I would give an alternative possibility that happened to me.

The car went dead out of no where. I put a charger on it. Went dead again. The battery was 1 yr old battery (Bosch). At one point I had way under 12 volts. Into the parts house for a battery test because I just KNEW the battery was done. Tested fine. Left the battery out of the car, held a charge to 12.x volts. Put it back in, dead battery the next day. Looked for parasitic battery drain. Nothing to be found. The car would run if I jump started it. even if the battery was dead. I was getting just under 13V with the engine running but against my better judgement I bought a new alternator, not rebuilt ($145 at RockAuto). I found it rotated kinda firmly, which I thought was odd. I decided to compare it to the old unit. Spun the old unit with a good twist and it rotated cleanly, a little too cleanly. I grabbed the pully to stop it. The pully stopped nicely in my hand while the armature kept going for a second or 2. After I replaced the alternator, problem gone. I also noted the car idled better.

So... Sounds like you may have the same problem? Easy to tell if it is.

-Remove the serpentine belt.
-Spin the alternator to get up a good spin.
-Grab he pulley so it stops.
-If the armature is still spinning, replace the alternator. Your getting some charge from the alternator but not enough when it loads up.

I'm guessig there's some safety arrangement in there to prevent seizing of the alternator from locking up the belt. It kind of makes sense. RSPI, you may want to do the same check if you are only getting 12.8V.

 

I'm having a 99 Volvo S40 MK 1 2.0L Petrol sedan.
Currently I'm having a 12V 540CCA 72ah Battery and its dead.

My owners manual says 520 A(SAE) battery is the one needed.
So when I was looking for some batteries I found out a Bosch S4-005 gives 540A and its pretty cheap too.

However its width is 30mm shorter than the current battery, which would leave away space at battery mount tray.

Since my battery mount is bottom-locked type. Which also means the battery locked to tray by two ends only. Is it OK to buy a smaller in size battery?

 

I have had good success with that. I don't think I ever had an experience where having a battery unsecured actually damaged it.