I've repaired/upgraded battery packs in quite a few devices before. It's not rocket science tbh, and it's dirt cheap in most cases. Wouldn't hesitate to hack into a ps4 pad if that means having a controller that can stay unplugged for 6+ times longer. :edit: though I wouldn't hack into something that is under warranty, as in if it's not broken don't fix it. But if I feel like the battery life is really dire, I'll just upgrade the thing.
It's not a necessary mod by any means, though in some cases it's a repair as batteries and battery packs start to fail or die eventually. It's more like creatures comforts, for instance I've replaced my 500 mA satnav battery pack with one that's 8000. That's driving for over eight hours without having to plug it in
Now if you want a genuine suggestion: I bought a 5 meter usb extension for my pad, so you have that much extra cable when you need it, instead of having to be right next to the console.