1. Does a Quad-Core A15 even make sense? That would lower the battery life to 1 hour... big.LITTLE wouldn't help here. Full Load is full load, it would use the A15 and not the A7.
These days, SoCs will usually let you run one core or two cores at higher clock speeds than all four cores. Yet it's still worthwhile to have more cores.
Hypothetical scenario partially/vaguely inspired by reality: say you're running DraStic on dual core Cortex-A15, where at 2GHz for same game it takes 3ms to do everything but render 3D. Now let's say 3D takes 4ms, so 7ms total.
Okay, now let's say that 3D was rendered at 4x resolution like PCSX-reARMed can now. And 3D now takes 16ms, so 19ms total. That's over the 16.67ms to be realtime speed, but 3D can be threaded, so it's now 11ms on one core and 8ms on the other core. So you run the two cores at 1.3GHz, and the second core sleeps for like 27% of the time.
Let's expand that to four cores, now it's 7ms on one core, 4ms on the other three cores. So you can run the four cores at around 840MHz, and three of them sleep for 43% of the time.
So which one uses more power? perf/W isn't linear and you're spending more time sleeping in the latter, so it's entirely possible that you use less power in the quad core scenario. But I'm not going to try to actually concoct some numbers to defend that precisely. It's also possible at that point that A7 cores could handle it too, depending on how high they clock. And there are going to be at least some games that are much more well threaded than an emulator like DraStic can be.
At any rate, you shouldn't assume all Cortex-A15 will have the same perf/W core for all cores. Look at nVidia's claims for Tegra 4 vs Tegra K1:
http://www.pcper.com/image/view/35618?return=node%2F59259
Both of these SoCs are made on TSMC 28nm processes. nVidia explains the improvement as being due to a move to using the HPM variant of the process, improvements ARM made in a newer revision of Cortex-A15 (that I can confirm OMAP5 isn't using), and improvements in nVidia's physical implementation of the CPU. From the numbers you're giving, it sounds like OMAP5's peak power consumption is a lot like Tegra 4's. I'm not saying that something like A80 isn't going to also be more like Tegra 4, but right now we don't really know.
OMAP5 may just be seeing availability now but it was supposed to be out in 2012. There's a good chance TI didn't do it for an HKMG process (like TSMC 28LP, or maybe a UMC 28nm process..); they do say it's 28nm but don't specify beyond that. So they could be at a power efficiency disadvantage.
2. Should I try to increase the battery size a bit? It could be possible and get us more capacity, but the battery won't fit anymore in the Pandora (well, except for if we made it thicker and offered a new battery cover together with it).
Is there any way a bigger battery could be optionally supported with an alternate battery cover?
The Snapdragon 800 seems to be capped to using 4W max on tablets and 2W max on smartphones.
So while it's more powerful, it's limited to not use too much power.
I doubt that there aren't tablets and phones that let it use more, perhaps substantially more than that. Those could just be recommendations based around some throttle point. I think I remember those numbers being higher anyway, like 5W for tablets.
The Intel z3770 has 2W SDP (typical use scenario).
According to Intel, that's running at low speed (1,4GHz) with normal work being done (no games, etc.)
Based on various numbers and measurements I've seen I'd estimate around 1W for 2.4GHz, which should outperform an OMAP5 A15 core at 1.7GHz while using much less power. Of course you can't run all four cores at 2.4GHz while using <= 2W, much less with the rest of the SoC involved.
2W would mean approx. 3W with all other components needed in a mobile system, and that would mean 5 hours battery life with a Pandora battery.
So approx. 5 hours with 1,4GHz normal usage.
No, not normal usage. 2W for all four cores running at 1.4GHz while the cooling meets some requirement (this isn't just a different way of specifying max performance - higher temperature reduced power efficiency, there's a non-linear feedback effect at play). Normal usage doesn't dictate running all four cores at even 1.4GHz. It can probably run one core at over 2GHz and still stay below 2W for the whole SoC.
Whatever SoC we choose - while they certainly all are better than the current Pandora in terms of Perf/W, they are all able to drain the battery within hours when being run with high-demanding tasks.
Indeed. It's all thanks to the advent of tablets and the increasing size of smartphones and phablets bringing increased battery capacity and thermal mass.
