When I started this project, I started by talking to some Chinese folks who are in the business of making battery packs.  I'm new to that side of the business, so I wanted to learn as much as I could.  One important thing was to figure out the type of cells available.

I have always felt like a pack made with round cells seems like it's a bit of a waste of space.  There are always empty gaps between the radii of the cells.  So I started by telling them that I was looking to make a small 2S LiFePO4 pack, and I would like them to find suitable cells.  And if possible, I would like LTO cells of a similar size , so that I could make a 2S LiFePO4 pack with similar physical dimensions as a 3S LTO pack.

I never found anyone who could help me with this.  Oh, if I would order 30000 of them, they could make a custom cell for me, but below that volume, forget it.  Unfortunately, my business isn't at that volume, yet. ;)

At that point I was like: "Oh forget about making a LiFePO4 and LTO pack of the same size, what size LiFePO4 cell do you have that would work for me?"  I was sent some options, but to be honest, options were limited.  Turns out LiFePO4 cells are more often made in larger sizes than smaller sizes, to make car batteries, golf cart and electric bike systems, things like that.  High power stuff.  Then I requested specs for the few cells they could offer me in smaller sizes, I was in for a surprise when I did some calculations.

I calculated the capacity of a pack per unit of volume (energy density) for a pack made with the pouch cells I got specs for versus one made with 18650 cells, and was surprised to find that the energy density of the pack made with 18650's was actually better than one made with pouch cells!  That was completely unexpected.  How can that be with all that wasted space?  Well, I already knew there are cells optimized for energy and others optimized for power.  I have always used the ones optimized for energy, and it turns out pretty much all LiFePO4 pouch cells are optimized for power, to be used in high power applications.  I can't get any pouch cells optimized for energy it seems.  So standard cylindrical cells are going to be the best for me it seems.

The good thing about that is that there are many factories that make them, so I won't be stuck with a single manufacturer.  They exist for both LiFePO4 and LTO.  If I want a pack that's equivalent in physical size between LiFePO4 and LTO, I make a 2S3P LiFePO4 and a 3S2P LTO (or multiples of that).  Another benefit is that I already use 18650s so it makes things easy to prototype.  And because they are pretty standard, it's easy to plan different capacity packs by using multiple cells in series and parallel.  So in some ways, ending up back with standard 18650s has simplified this a lot.