You don't know how process node shrinks work. Each time you move to a smaller micron process using the same design it takes up less mm2 as the tranistors get denser and it uses less energy . So if the first DSPhat was on 90nm and the Dsi is on 65nm and since its the same exact hardware (the dsi has some more ram as you said and flash storage built in and cameras) then the dsi will use less power. 40% or so less power depending on the process node scalling.
You forget that with the newer dsi screens your getting higher brightness which will come at battery life and you also have a smaller battery
The ds came with a 850mah battery and took 4 hours to charge. The dsi came with a 1000mah battery which right there is a 18% increase in battery power.
The dsi came with a faster clocked cpu the arm 9 was clocked at 133mhz which was almost twice as fast as the arm9 in the ds /dsl it also had 16mb of ram which was 4 times the ds/dsl . The dsi had a 840mah battery.
The dsi had comparable battery life to the ds phat. This was done due to process shrinks
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Why would it be comparable to the DSL ? The DSL was a refresh of the DS . IT had a 18% larger battery and had a process node shrink which could further reduce power consumption by 40%
If i took a GBC and was able to use a lithium ion battery instead of standard double a batterys i'd see a life impovement. More to the point. If we took a GBC and put it on a modern process say 40nm you'd be able to fit all the chips in the original GBC on a single small chip along with the ram. You'd make a single ultra low power chip. Battery life would be through the roof.
http://www.anandtech.com/show/3774/welcome-to-valhalla-inside-the-new-250gb-xbox-360-slim/3
Educate yourself on how process nodes work. This is whats driving battery life and not battery tech advancement.
Xbox 360 xenon (launch unit ) 90nm 155.7w idel
xbox 360 slim valhalla 40nm/65nm 70.w idle
Gears of war 2
Xbox 360 xenon 177.1w
slim 88w
As you can see the process node shrinks have drasticly reduced power usage. Its not clear yet if the slim 360 uses 40nm or 65nm chips or a mix of the two. But one things for sure , it cuts power usage in half .
Thats major considering things like the hardrive and dvd drive wouldn't have seen power consumption drops.
I believe that the 3DS is made on 40nm so the next big drop would be 28nm and we wont see that till 2012. I suspect nintendo wont do anything until 22nm sometime in 2013.
We could see battery life increase with a larger model but then again we might not. Remember the screens will get bigger also and it will offset the larger battery size.