Module with 4x INR21700M50LT 2170 Cells

You get 4x 2170 5000mah for $7.20. Just $1.80/cell for one of the best lithium ion cells on the market.

This is the best way to get one of the best cells on the market. These modules are overstock from modems. Each module has 4 INR21700M50LT cells inside.

Fully charged - 4.2v
Fully discharged - 2.8v
Max continuous discharge - 10a
Max pulse discharge - 21a

The grey stuff you see in the 1st photo on the cells is a thin layer of glue. When we opened the packs the shrink wrap on the cells stayed together which is nice.

You can pry these open at the seam to remove the cover. A good pair of channel locks will help and just start at the corners and keep prying up. Or you can carefully crack along the seam. Took about 30 seconds to get the one open for the photo.