I wish the case on the radio was just 4mm taller so I could fit 18650 cells, but it will fit 14500 (AA size) lithium cells.

The existing NiMh cell packs are 8 cells.  I think I could fit 6 of the lithium cells in series/parallel with two banks of three in series and then the two banks connected parallel.  

My concern is safely charging them without a special balance charger.  The good news is with only using 6 cells, if I install them side by side, that leaves ample space for the windcamp charging hatch.

Will it be safe to charge two 3 cell lithium packs in parallel?

Does anyone have a source for six 14500 cells with solder tabs that would run less than just buying a new windcamp battery?

What about giant heat shrink sleeves for making battery packs?

Normally you'd create a 3S2P pack. You'd put 2 cells in parallel and then put those 2-cell packs into a 3 series configuration.

Let me also suggest looking for pouch cells. If you can find 3 flat cells that take up about the same width and length (or less) of the existing pack and stack 3 of them together you'd probably get a much much higher capacity pack over cylindrical cells. There are small battery protection boards that can be added too if you wish.

Consider looking at LiFePO4 cells as they'd probably be a better match voltage wise. They are 3.2v nominal (9.6v in a 3S) and at full charge are 3.65v (10.95v).

You might be able to find a small protection board that handles balancing for a 3S configuration.

The lithium ion cells in 3S are 11.1V nominal and 12.6V fully charged, which is perfect for the 817.  Right now it has a pouch cell pack, and I can't find a replacement for it that isn't marked up higher than I want to go.  This existing pack is 3000 mAh, which is good, but the lithium 14500s should still beat the factory Yaesu NiMh AAs, even going to 6 cells instead of 8.  


I looked on Amazon because I have a gift card, but couldn't find any 3S pouch packs with matching dimensions.  But I know such a pack should cost no more than $30 based on other 3S pouch packs with the same capacity.  Yet to get the branded version, it's nearly $100.

Is there a place where you can look up packs by their voltage, capacity and external dimensions?

Take a look here. Use the filters to narrow down what you want. Can't really search on dimension but it will show you options

https://www.digikey.com/en/products/filter/batteries-rechargeable-secondary/91


Here are some possible contenders
https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-13855/7559594
https://www.digikey.com/en/products/detail/adafruit-industries-llc/328/5054542


I would avoid lithium-polymer batteries personally. Careful as some of the li-ion batteries are mislabeled.

Thanks.  Yeah LiPo are great for RC aircraft and burning down the house.