[G4 Q&A, Help & Troubleshooting] feels1:
I have been comparing phone charging for my Samsung Galaxy Note 2 and LG G4 which uses Telus@Canada. I have proper electrical power supply which I used as charger (5V 3A) with custom made micro USB cable. I used Fluke digital multimeter measure voltage, ampere, power regulation, and phone to measure the temperature.

From my observation, there are several factors involved:

1. Battery temperature - If using the phone, keeps the battery below 24C/75F for higher current, especially USB charge (not wall charger). It looks like it only uses ~500mA when actively using it, e.g. Google Map, but will draw more current if screen is off. Trick: Removes the back cover to lower the battery temperature

2. Screen brightness - Keeps it below 10% if using computer's USB port. It is able to charge if brightness is low, even using computer USB port. 40% brightness is going to exceed the 500mA charging current from computer and losing charge. The full brightness going to consume ~ 600mA

3. Reconnect the USB cable - both my Note2 and G4 don't detect it is connected to AC wall charger, and I have to reconnect for it it redetect. Checks "Battery & power saving" screen to ensure AC is detected

4. Uses quality charging cable such as original Samsung/LG G4 - If uses custom cable, then ensure it is 24/28AWG (24AWG for power cable and 28AWG for data). I will recommend to buy 20AWG, which can both be long and future proofed. Trick: use a short 1 ft cable if you don't have quality cable

5. Shutdown all programs (Square > Clear All) - if possible, shutdown Google Maps or all background programs. I found Google Maps uses some power in background, and terminate it will improves the charging time when using USB port, but no different when using wall charger

6. Uses direct 12V to charge - If the phone battery level is < 40% (yet to measure the historical charging voltage vs battery level with QuickCharger 2.0), then you can use 9V or 12V. Car has 12V ready to be used, but you will need to fabricate the charging cable yourself

7. If you are using custom ROM, beware that some of them has a bug where the hvdcp daemon is not running, and not negotiating the QC 2.0 protocol, which limiting itself to 1800mA (for G4, but will be different for other phones) instead of 5V/2500mA or higher

Apps to recommend to monitor charging. Installs both:

1. Battery Log - Allows export to csv file (prompt for name). Display volt, battery %, temp, USB/AC, graph

2. Battery Monitor Widget - Shows battery temperature and charge current, but can't export to different file (overwrite previous file). This is more fancy, but I dislike that it doesn't support export

If you have a Quick Charge 2.0, and wanted to know what is the D+ and D- voltage from phone which tells your Quick Charger what voltage to support, or for in depth troubleshooting/reserve engineering, following is the lookup table

Forget about "Ampere" app if you use above 2 apps. It doesn't has export, or history

Reference:

1. Power Integration CHY100 Charger Interface Physical Layer IC

2. Mouser online electronic ordering

3. Texas Instruments Class A reference design circuit literature tidu917.pdf

Note 1: QuickCharger 2.0 has Class A, which is 5V, 9V, 12V, and Class B, which is 5/9/12/20V (extra 20V). Currently all QuickCharger 2.0 are either partial Class A that is up to 9V, or full Class A (upto 12V). Class B charger is yet to release, but the IC has been released. It is the same IC, according to my research, but manufacturer are either using cheaper IC that doesn't support 12V, or considering that 12V output will demand a qualify cable ($$$$$)

Note 2: These comments are for testing purpose. I am not liable for any damage to your asset or personal injury for any statement I disclosed

Note 3: On weekend, where I don't use the phone much, the battery can last for 3 days before recharge, compare to Samsung Galaxy Note 2 which I still have to recharge daily in similar usage