Just replaced the battery today.
Old one was toast and could only (barely) crank over 1-2x, before luckily starting. It was a gamble to always go out for groceries/gas, haha.
Old one was dated 11/13, so it lasted a while (Walmart Maxx EverStart battery from previous owner).

But, just wanted to hear your input on why a new battery would make a car run better and exactly why?

Obviously, a battery is there to start the car... but, just wanted to hear the other duties of a battery other than starting the car. Like perhaps - helping alternator slowing down after rpm peak and decel (voltage drops), voltage backup, idle, etc.?

 

Once the car is running the battery doesn't do anything, the alternator and it's built in rectifiers and voltage regulator takes over - you could remove the battery once it's running, and nothing will happen. Speaking for the S2000 specifically, not new modern cars with tons of onboard electronics, communication lines and fibre optics, where the battery also acts a capacitor to store energy when big loads are needed.

But RUN better? A new battery will not make a car run better, that's simply placebo or reading into this too much.

You should be more concerned about not destroying your starter with a weak battery, that's the biggest concern with weak batteries.

 

It is a well known fact that cars always run better after:

A car wash and especially a waxing.
An oil change,
New tires,
A new date,
Full tank of gas.
Clean windows,
and a new battery,

It is possible the old battery had a short and not only was the alternator running the car as mentioned above (if your system is up to par) but it may have had to pour out energy in an attempt to charge and refill the battery.

 

Well, if battery is weak, regulator will ask alternator to do more work to charge it back up. Usually thats still such a small amount of power compared to what engine produces you wouldn't notice. But perhaps with a low torque output and a finely tuned man machine interface it would be something you could feel.

 

Maybe after you ate a whole box of Adderall, maybe.