Buy the anker brand 2 pack that comes with a charger. Oem isn't worth it.

You can check the amazon reviews or answered questions to see if there is info relevant to your model.

Edit: The product description on the Anker battery says its compatible.

http://www.amazon.com/Anker-Batteries-T-Mobile-18-Month-Warranty/dp/B00Q4K7Q34/ref=sr_1_3?s=wireless&ie=UTF8&qid=1433129490&sr=1-3&keywords=lg+g3+battery

Update: Anker website points you to Amazon to buy the whole package. It is $25.99 on there.

https://www.amazon.com/dp/B00Q4K7Q34/

Do you mean an external backup charger? Because I meant an actual lithium ion battery backup. I'll link you one sec

Anker 2 x 3000mAh Li-ion Batteries for LG G3, D855, VS985(Verizon), D851(T-Mobile), D850(AT&T), LS990(Sprint), fits BL-53YH, with Anker Travel Charger, and 18-Month Warranty https://www.amazon.com/dp/B00Q4K7Q34/ref=cm_sw_r_other_awd_yhwNwmbF0T9H9D

Something like this

This thread inspired me to pull the trigger on this. It arrives today.

Anker 2 x 3000mAh Li-ion Batteries for LG G3, D855, VS985(Verizon),... http://www.amazon.com/dp/B00Q4K7Q34/ref=cm_sw_r_udp_awd_UrFBvb131ES8P

You could buy one from Lg store online. But suggest the ones from Anker, https://www.amazon.com/Anker-3000mAh-Batteries-Verizon-T-Mobile/dp/B00Q4K7Q34

I got 2 Anker batteries on Amazon for $27, that comes with a charger that doubles as an external universal battery.

http://www.amazon.com/gp/product/B00Q4K7Q34?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00

do you recommend buying these? http://www.amazon.com/Anker-Batteries-T-Mobile-18-Month-Warranty/dp/B00Q4K7Q34/ref=sr_1_3?ie=UTF8&qid=1443215595&sr=8-3&keywords=lg+g3+batteries

This is a fantastic deal but I just wanted to throw this out there if you may also be interested in a charging cradle for your battery.

http://www.amazon.com/gp/aw/d/B00Q4K7Q34/ref=mp_s_a_1_2?qid=1460600635&sr=8-2&pi=SY200_QL40&keywords=anker+lg+g3&dpPl=1&dpID=418jAdUVBdL&ref=plSrch

For $10 more you get the cradle and a second battery. I can confirm Anker makes batteries just as good as the manufacturers.

With the cradle you will never need to plug your phone into a charger again. It's a total game changer for me and makes me dread the day I might not be able to get a flagship phone with a removable battery.