[HTC Sensation] rhd-android:
No confusion - those guys are definitely not on the chopping block ;)

So what is that max ceiling? Well, since the answer to this question seems to be at the core of what we're doing here, it's time to grab the digital caliper and do some gosh darn real math!

Using the Anker cell as the template, here are the measurements of our Sensation / EVO 3D cells:

51.30 x 51.00 x 5.40 mm = 15.059 cubic cm., which is the same as saying 15.059 ml (1 cubic cm = 1 ml)

But, we loose ~5mm in width and height to the rounding of edges, the plastic casing, the contacts, and the protection circuitry. You also lose a tiny bit of the thickness (~-1mm) to the label and the protective wall of the cell itself. So the actual volume is closer to: 46 x 46 x 4.5 mm = 9.522 ml.

The "high end" of the theoretical maximum energy density figures for lithium ion seems to be a figure of 730 Wh/L (citation: link), which is 730 mWh / ml. Now, I'll point out that this figure is way above the generally accepted norm for lithium ions. We're talking about a Panasonic press release about fantastic new technology that they were trying to push into their most efficient 18650 cylindrical cells. But as a "pie in the sky, perfect world ceiling", we can use a somewhat fantastic figure like this one. In fact, most of the energy density charts you'll find stick the upper limit for lithium ion at somewhat lower.

But sticking with that pie-in-the-sky 730 mWh / ml figure, our maximum theoretical capacity ceiling, barring a ground breaking advance in lithium ion technology, would be 730 x 9.522 = 6,951 mWh. Divide that by the nominal discharge voltage of 3.7, and you have an mAh of about 1,880 mAh. That figure of 1,880 mAh would be the highest possible mAh, if the most ambitious energy density predictions, from the world's top lithium ion cell manufacturer, were able to make their way down into the pouch/prismatic cell packs we use for our smartphones.

That's not far from where we are!

It's interesting, when I set out to crunch the math, I expected that ceiling to be a bit further from what we're actually seeing, in reality, with cells like the Anker. I have a couple thoughts as to why we're already as close as we are to this "ceiling" figure of 1,880 calculated above:

1) LiPo packs and prismatic cells (particularly LiPo packs) should actually do a bit better on the energy density front, due to the lower overhead of structural components (cell walls, etc) required in their design. So lipo and prismatic lithium ion tech might not be as far as I thought from the ambitious density targets that Panasonic has for their new-generation of cylindrical cells.

2) This second point is probably the real big reason that we're able to squeeze a bit higher figures from our cells. We're discharging them lower than we should be. Remember how I mentioned that the protection circuitry in these lithium ions cuts off discharge at 2.5V? That's too low. The opinion of almost everything you read, suggests that cells should probably be cut off from discharge at around 2.7V, maybe 2.8V. Why does this matter?

If you discharge all the way down to a lower voltage, then your nominal voltage is going to be lower also. That means that instead of dividing 6,951 by 3.7V, it might have been more appropriate to divide by say 3.5V. The implication here, is that the "ideal" scenario above might have indicated a slightly higher ceiling (6,951 / 3.5 = ~1990 mAh) if it took into account the fact that we would be draining these cells a bit lower than is generally considered ideal.