Ok, I've got some
realistic specific energy and energy density numbers.
Why do I say realistic? Because taking the Ah figure printed on the cell x nominal voltage and dividing by the physical volume of a round cell doesn't give you a number useful at all towards knowing how much space you need for your pack, or how much useful energy you can use for a given weight etc.
First, after watching a number of cycles, and taking videos of even light discharge as the cell dips into it's ~80-90% discharged state, it becomes very clear the Ri of the cell is so high at this point, that the last bit of capacity is essentially useless for EV use at that point. Fortunately for us, it's a pretty defined point with both of these LiFePO4 cells. They seem to hover pretty flat until this point hits, and then the Ri grows so much they would not be useful to continue discharge (unless you're pulling like a 5-10amp load or something, but this is a performance EV cell test, not a flashlight battery test or something.)
For the Headways "P" cell, this point occurs right about 6.5Ah, and voltage drops from ~3.2-3.1 right down to mush on a pretty steep slope. This is also the point at which the cells get the most heating from the v-drop induced from the increased Ri. For a performance application, 6.5Ah is the point at which a pack made of headways "P" cells should be put on the charger.
For the LiFeTech/BMI cell, this point occurs right about 12.8Ah. This is the point at which it's time to stop running your performance EV and plug in your charger.
Why is this important to know? Because if you choose to drain the cells clear down to 2v, you can make them give up 8.45Ah for the headways P, and 14.8Ah for the LiTech, but once you cross 6.5Ah and 12.8Ah, the performance is done, a ton of the energy will just be going into heating the cells if you continue, and the voltage drop makes it pointless to try to continue.
This means when building your performance EV, size the pack as if the headways are each 6.5Ah and the LiFeTech is a 12.8Ah cell.
This makes the useful energy capacity of the headways P is: 21.8wh's.
The useful energy capacity of the LiFeTech 15Ah is: 41.0wh's.
The volume of a headways "P" 8Ah cell alone is: 0.145L (a useless figure)
The volume of a LiFeTech X-2 15Ah cell alone is: 0.21L (a useless figure)
The weight of the headways P cell is: 337g
The weight of the lifetech 15ah cell is: 470g
Doing the math here gives us one useful number, and one usless number.
The useful number of course being useful specific energy (excluding pack construction materials, which vary greatly on appliction, but are generally in the 5-10% range).
Useful specific energy of the naked headways P cell: 64.6wh/kg (watt-hours per kilogram)
Useful specific energy of the naked LiFeTech cell: 87.2wh/kg (watt-hours per kilogram)
The useles number would be to calculate would be volumetric energy density based on the cell's volume itself, because cylinder cells pack with gaps. Stack them in straight rows, or offset them to nest (best option for volume reduction), either way you do it, you're going to have space in your pack that isn't battery, and quite a bit of it. So, to find a useful volumetric energy density, we gotta see a model of how they stack. I was originally just going to use the pre-made LiFeTech and Headways packs as volume guides, but they would make things look worse than they could be, as they fit BMS units and in some cases a lot of dead space for the purpose of matching a standard battery size case to make direct replacements from lead-acid simple. So, I did some quicky models to get a rough handle on volumes. The models use a height that includes the top of each post (but no additional height to compensate for tab thickness.) Volume numbers do decrease a bit as packs get more length and height of cells being stacked, so if you're planning on having a giant continous block of nested cells, your volumetric density will improve by a couple percent, but never more than a few percent, which is why I think an 8cell stack is a fairly common and fair pack building block size to use.
Here is a picture showing the length and width of the smallest possible way to arange 8 cells to fit into a rectangular box:
Here is a model to give an idea of the deadspace:
The smallest possible rectangular box to fit 8 headways cells: 1.86L
The smallest possible rectangular box to fit 8 LiFeTech 15Ah cells: 2.42L
Useful volumetric energy density:
Headways 8Ah P cell: 89.5 wh/L (watt-hours per liter)
LiFeTech 15Ah cell: 135.4 wh/L (watt-hours per liter)
