Wow, I'd be afraid of using that battery for sure. I like the solder idea. As long as it is similar capacity, it should be fine I would imagine. Then again, all I know is that I know nothing. Best do some research. Good luck finding something similar that you can place in there!
It's a grandfathered T-Mobile plan with unlimited talk, text, data, 14GB LTE hotspot and unlimited Binge On (which means that most major video services don't count against the 14GB) that costs us $37.50/mo.
For a long time we used the phone, as is, but eventually transitioned to using PdaNet+ and Connectify with it to bypass the 14GB limit.
So we no longer have to be careful with our non-video data usage.
I signed up for a line with Visible and have been test driving it for the past 5 weeks. It's working very well, for us, and I'm trying to work up the nerve to cancel T-Mobile.
T-Mobile has been bulletproof for us, for a long time, and it's hard to pull the plug. Hard to beat the $25 price of Visible, though.
I'll keep them both for another few months before giving up a plan I could never get back.
FWIW: I throttle bandwidth in our router in such a way that we can watch 3 TV shows in SD (good enough for us) with 1 GB of data.
By keeping our bandwidth reasonable, we stay under the radar and have had zero problems using cellular for home internet.
I know Chelle can handle almost anything.... but I wouldn't recommend soldering a Li-Ion, or any rechargeable battery unless one is pretty comfortable with soldering skills. It's not a beginner-level project, and they can explode if overheated in the process.
(Reference: I've soldered plenty of NiCad, and a few Li-Ion, packs in power tools, and I have pretty extensive soldering experience.)