Zero Lemon makes an extended that fits inside of stock cover. Its a 3000ma. Always had great products from them, too.
Sent from my SPH-L720 using XDA Premium 4 mobile app
Sprint Samsung SPH-L720 "Galaxy S4" L720.08 - Daily Driver
Rooted, Philz Touch Recovery
Android 4.3, Baseband L720VPUEMK2
4.3 Stock(ish)v03 by rawintellect
HTC PC36100 Supersonic "Evo" 0004, HBOOT 2.16n - Retired, Used as MP3 Player and PDA
Rooted (Thanks to Rev!), AmonRa Recovery.
Android 2.3.7, Baseband 2.15.00.0808
Ubersonic RC3 Rom, Mason 15 Kernel
V6 Supercharger Script