Will fitting a physically bigger battery reduce the working life of the alternator?

Great question! Taking a few minutes to think through this logically should come up with something. First of all, realise that a larger battery with a higher amperage (CCA) will not cause any problems on the drawing end. In other words, you're not sending too much electricity to some component or something. The higher amperage simply means that more energy is available, not that more energy is flowing through the components. I'm sure you already knew that; just making sure everyone is on the same page.

Secondly, you've got to realise that it doesn't take any more energy to charge up a bigger battery than a smaller battery. That is, provided they have had equal draws. The difference, of course, is that the bigger battery can get uncharged further, resulting in a larger draw on the alternator.

Basically, this means that your alternator will have to work harder to recharge the battery, but simply because the larger battery was able to output more juice. Since the alternator wear is usually due to age, the answer is yes, the bigger battery will wear down your alternator more.

That being said, it probably won't be significant, unless you regularly leave your radio running overnight, because your alternator is always running when the car is running; the only difference is that it will be demanding more current from the alternator to recharge the battery. Therefore, as far as I and my research are concerned, the wear is

Probably insignificant.

For most of the time, the battery doesn't provide any electrical power to the car. All the power is coming from the alternator. Think about it this way: the alternator probably has a maximum current rating of say 90 amps. For a typical "standard size" 40AH battery, that amount of current would flatten the battery completely in less than half an hour, so it's obvious that the battery is not powering the car electrics most of the time!

The main function of the battery is when starting. Arguably, a larger battery might discharge less when starting than a small one, because the higher cranking current available is likely to turn the engine over faster and start it quicker - though with a modern engine and ECU, that shouldn't be a big effect unless there is a fault somewhere in the electrical or fuel systems.

The secondary function of the battery is to provide power when the engine is idling, if the electrical load is high and the alternator isn't running fast enough to deliver all the power required. But in that situation the amount of power needed to recharge the battery only depends on the electrical power being used, not on the battery capacity.

If you have added a lot of electrical equipment to the car, that will make the alternator work harder and potentially reduce its life - not the size of the battery.

Simple answer : NO, the alternator max charge will just need more time to complete a full charge, but the lifetime of the bearings is down to the speed of rotation and the working temperatures plus the load caused by the belt driving it. This is a good exercise in tribology to work out the bearing lifetime...

In fact the larger capacity battery may, under certain circumstances, increase your alternator's life.

Many vehicles with enhanced sound systems install a larger battery to smooth out the load spikes on high-draw portions of the music. A typical electrical system upgrade path would start with a higher-capacity battery, then a higher-capacity alternator or even dual alternators, then dual high-capacity batteries.

1. A larger battery rated to the same voltage will have a lower internal (electrical) resistance.
2. Less electrical resistance -> More current will be generated (flow from the alternator to the battery) by the alternator when it turns but it will be harder to turn it due to increased mechanical resistance or torque (the relationship between electrical and mechanical resistance in this instance is inverse).
3. Thus, one might expect additional mechanical stresses and potentially heating to occur on the alternator when charging a larger battery. Depending on the design specifications of the alternator that may translate to a shorter life for it.

I would argue that a larger battery could actually increase the lifespan of the alternator. The reason is that a bigger battery will help to stabilize the voltage in the system compared to a smaller one by acting sort of like a buffer. A real world example is how the lights on a car dim when the bass hits. With all else being equal, the lights wouldn’t dim as much under sudden, brief loads like bass hitting. What DOES, however, wear out the alternator is having undersized/loose/corroded grounds and battery cables. Make sure your battery cables and grounds are clean, reasonably sized and tight, and maybe look into a “Big 3” upgrade if you want to help your electrical system perform better and last longer. Obviously, none of this would matter if the alternator suffers a mechanical failure, like bad bearings, before it electrically wears out anyway though.

Car can run without battery hense power drawn from alternater. However car can run without alternater as well is it not. Therefor it a circuit charge circukated through a parallel flow. I think

As several people have stated, no, a larger battery will not harm your alternator (or other electrical components), provided that it is putting out the correct voltage. One thing that I'd disagree with most on is the charge time. First, if you're talking about charging from the battery's discharge limit until it has reached capacity, then sure, it will take more. However, that's about where the charge time difference ends. Under normal conditions, if you change only the battery capacity and make no other system modifications, it will take the same amount of time to charge both batteries (obviously if resistance is not the same, this will affect charging, regardless of the total capacity of the battery).

When you use a larger battery, your car's electrical systems don't automatically start pulling more current simply because it's available. The current draw will remain the same. Because of this, a larger battery will not deplete faster than it's smaller counterpart. So, if your vehicle has an average current draw of 75A, it will deplete the battery the same amount. Therefore, you will have the same amount of current to be replaced with both sizes.