I want to increase the time something will run on batterys. How is this best achieved? I know adding batterys will ether increase voltage or current depending on how I put them in circuit. So can I just get a battery pack with a higher mAh ratting? Or should I use larger batterys like instead of AA use Ds? Or will that effect current too? There is probably no easy way to do it. Thank you for any help.

 

Your approach is correct. The solution is simple.

You need to get battery packs with the same voltage but higher mAh rating. As you have stated, there are two ways to do this.
You can replace AA batteries with D cells, that is, go to higher mAh rating.

Or you can add more batteries in parallel with each other.

The current remains the same.

 

D cells with good mAH rating are expensive, the cheap ones often arent much better than AA cells, have a lower internal resistance tough.

They have fallen out of favour for Lithium technology.

The rechargeable one's may even have the same capacity as AA, and most generic kinds drop down before 100 cycles. Duracell last a long time and dont go dead within a few months.

At one time, I had a large bowl of these.

What are you powering? Motors?

 

D cells with good mAH rating are expensive, the cheap ones often arent much better than AA cells, have a lower internal resistance tough.

They have fallen out of favour for Lithium technology.

The rechargeable one's may even have the same capacity as AA, and most generic kinds drop down before 100 cycles. Duracell last a long time and dont go dead within a few months.

At one time, I had a large bowl of these.

What are you powering? Motors?

A sentry gun. Its basically a airsoft gun two servos and a arduino but the arduino is powered via usb.