[AsilentG]

If I get a extended battery for my 820,will it mess up my phone (since it ain't the regular 🔋 battery),and What do you recommend?

[xandros9]

Will an extended battery inherently mess up your phone? Assuming it operates properly and isn't defective, absolutely not.

Just be careful of potentially too-cheap batteries or the potential for scams. A bad battery doesn't result in anything good.

I can't say, but there's been discussion regarding a 2500 mAh extended battery here.
Heck, I recall a fancy mod here where someone wired two batteries together and stuck the combo in his 820, requiring some modding to the cover though.