If every Battery is Connected in Parallel every Battery will give Current but the Voltage will be the same.
I get this and how it all works.

Yes.

This is how you get more Current and the Batteries will not Last as Long.

No. As the battery pack is now able to supply more current, it will last longer at your required current than if they were in series. For example, if you have 2 x 12V 1AH batteries in parallel, and you want 100mA, basically, each battery will supply 50mA to the load, so it will last 1AH / 50mA = 20 hours. (Or total 2AH / 100ma)

But if all the Batteries are Connected in Series the Voltage goes up but the Current will stay the same.

Not quite. The voltage goes up as will the current unless you increase the series resistance to compensate for the increased voltage.
And as your required current now comes from all the batteries in series, they will not last as long. So, again, 2 x 12V 1AH batteries at 100mA (you have added resistors to suit) will last 1AH / 100mA = 10 hours.
With the batteries in series, over half of your power is just wasted in lost heat.
And drop the 9V battery idea. They are just not suitable for this. AA cells will be a lot cheaper and last longer.

 

A 9V battery is used for low power items like a smoke detector that is almost never used.
Then I think nobody makes a battery holder for two 9V batteries in parallel.
Buy two single 9V battery holders, glue them together and connect them in parallel.

 

Why this Love for PP9 ?

 

I guess he hates AA cells.

 

Unless space is a problem, 6 AA cells will always beat 6 AAAA cells (PP3) in price and performance.

Maybe you imagine that 9V batteries are just 9V versions of other things like AA cells? They aren’t. 9V batteries (in modern alkaline form) are 6 AAAA cells—tiny, very low capacity cells—in a can. They are very expensive, and the run time will be abysmal.

If you want to use primary cells (non-rechargeable) you are far, far better off with AAs. On the other hand, 18650 Lithium chemistry cells would be even better in the long run if you can work those in.

9V batteries have a few particular uses for which they are very convenient but they are very rarely a good option.

 

If an Alkaline D Cell Battery is Rated for 8. Amp Hours.

This Tells me it can Supply 8. Amps for 1. Hour.

And Yes I know it may not do this.

But what just need 1. Amp from it how many Hours can it Supply this?

 

If an Alkaline D Cell Battery is Rated for 8. Amp Hours.

This Tells me it can Supply 8. Amps for 1. Hour.

And Yes I know it may not do this.

But what just need 1. Amp from it how many Hours can it Supply this?

8 hours. The draw in amps divided into the batteries amp-hour rating gives the hours (theoretical maximum) that the device will run. Simple algebra. So dividing 8Ah by 1 A results in 8h.

 

Why 9V batteries!? 3 x Li-Ion cells, 18650, lasts forever compared to 9V batteries and they get cheaper when you can charge them 1000+ times. Charge them individually with charger module so you don't need balancing charger.

 

The 9V batteries shown from Amazon are Chinese No-Name-Brand Lithium-ion that have two 4.2V cells in series making 8.4V. The ad claims a capacity of 600mAh but it might actually be only 200mAh.

Amazon sells a second 9V Lithium-Ion set of 4 batteries with charger with the No-Name-Brand of PowerOwl. The charger appears to be exactly the same as the LP one but the batteries are a different color. The ad says the charger has each battery separate.

If you connect two 8.4V batteries in series making 16.8V then use a resistor to drop the voltage to 12V then you are wasting 29% of the battery charge making heat in the resistor. As the 16.8V battery slowly drops then the LED strip will be slowly dimming. We do not know how long the LEDs will light before their voltage becomes too low and we do not know how long the battery charge lasts until the LEDs do not light anymore.

 

If an Alkaline D Cell Battery is Rated for 8. Amp Hours.
This Tells me it can Supply 8. Amps for 1. Hour.
And Yes I know it may not do this.
But what just need 1. Amp from it how many Hours can it Supply this?

8Ah can deliver 1A for 8 hours
It can deliver 2A for 4 hours
3A for 2 hours, 40 minutes
4A for 2 hours
5A for 1.6 hours (1 hr, 36 min)
6A for 1.3 hours (1 hr, 18 min)
7A for 1 hour 8 minutes, 34.29 seconds.
8A for 1 hour.

It can deliver MORE amperage, depending on the max amperage capability of the battery. For instance, if the battery can deliver as many as 20 amps then it can do so for (8Ah ÷ 20A =) 0.4 hours, or 24 minutes.

And yes, you're right, that kind of performance is not likely. But it's the basics of amp hour ratings. How many amps you can draw over a period of time.