[28 Dec, 00:15] Ok. I've got a plan. With the shorter Tetrinsics, I can move the CB1 under them. Then, if I increase the screen thickness to 10mm, I have 6.7mm of space (not considering the strap):

Now to dig through AliExpress to find a--

Starts playing celebratory music.

Ok, can we take things even further?

It's going to be a tight fit, but it seems I could get back to parity with the 21700 cell. However, considering that I also need to route out the cables, I think the 4.5Ah is the best I can do.

There's actually a 5.5mm thick battery that has 4.5Ah too, so I could stick with 9mm thickness.

Then again, the shortest length of the front screen panel is only 80mm. I'd likely need to go to 93mm to fit these. If I can't manage that 13mm size increase, I'd likely be limited to 4Ah.

I did notice this 3-wire battery that was only 4mm thick. Makes me wonder why I haven't seen 8mm, 10Ah batteries:

There's an even thinner variant:

And there are some dual-pouch batteries:

I could always ask the seller if he can stick 2 together vertically.

Still though, it's looking like I'd need a longer battery pack area. But that would hinder ergonomics and aesthetic.

Or will it?

I could make the white thickness 11mm and extrude it forward by 18mm. Now the actual thickness is better hidden from this angle, saving aesthetics, and the screen appears higher up on the arm, improving ergonomics.

I wonder why this battery size is so popular, or has comparitively higher density than others. Actually, it might be because it's so popular that more recent battery chemistry has been used. At any case it totally sounds like, with the correct protection circuitry and some marginally thicker bezels, I could have an 8 or 10Ah power capacity, doubling what I could've achieved with 21700 cells. 

Worst-case seems to be 4Ah, which is decent. It should mean that the overal Timerspy dimensions are primarily display-size determined.

Thankfully, it seems that Tetrinsic 2.0 is GO.

[08:15] So far, I've asked 3 LiPo battery sellers on AliExpress and 2 said "Please DIY yourself". The first seller I found in this project log said that the total thickness will be 7.3mm for a 70x95mm. That's in my space budget! 

10Ah could actually be in the cards! I think that's a worthwhile compromise for a non-removeable battery. For reference, considering the rather large size of TimerSpy, this is what I've had in mind regarding battery capacity:

• 2.7Ah: Poor (same size as the LEM-T, the largest capacity Android smartwatch)
• 3.4Ah: Acceptable (highest capacity 18650)
• 4Ah: Decent
• 4.8Ah: Great! (also around the highest capacity 21700)
• 6Ah: Amazing! (unlikely, but if the opportunity presents itself, take it!)

I need to get this concept printed ASAP to see if a 10Ah solution could actually succeed ergonomically.

I also know that a 3.5x70x90mm cell exists if a 98mm TimerSPI is too much (but a 93mm one is fine). However, I'd have to take 8Ah (which is stll way more than I thought I could ask for) or DIY.