A typical USB phone charger even with a lit LED will only draw about 14mA that would mean you would have to leave it plugged in and not run your bike for about 2 weeks for any noticeable drop in battery power. I leave mine in 24 / 7 and it has an LED on it that is on constantly ( looks like an alarm and night ), I have almost 13,000 miles on my bike and have never noticed or had a low battery. I ride my bike almost daily and have not let it sit without riding for more than 3 to 4 days.
Not sure where you get your discharge statistics for how long it takes to fully discharge a G310 battery. Many folks have gone on vacation for 2 weeks only to return to a dead battery when having a USB charger in the circuit. Unfortunately, it has usually been dead long enough to sulfate the plates and then the battery has to be replaced. My recommendation to everyone is to have a switch in the circuit or simply unplug the USB charger so that you do not have to risk having to replace a $70+ battery after an extended time of discharging at 14 or 15 mA.
The battery in the G310 is an 8 amp hr battery at a typical draw ( measured on a multi-meter ) of 14mA ( that is With a Lit LED ) it would require 100 hrs or 4 full days to draw down 1.4 Amp hrs of the reserve power until you get below 50% of your batteries reserve capacity or approx 12 days you would still be able to start your bike. I have done this with cars, trucks, etc and use a standard Lead Acid battery running an inverter that runs a buffing motor at motorcycle shows polishing jewelry etc for multiple weekend events without recharging the battery in between. I have gone over 2 months worth of shows on the same battery charge. My buffing equipment draws far more power than an LED light. I suggest you have some other draws on your system that you can discharge the G310's battery in a couple weeks and destroy the battery. But hey your entitled to your opinion. As stated above I have had my USB Charger in and left on for the entire time I have owned the G310GS ( over 6 months ) I never remove or shut it off.
Try a simple test: Do a nice long run on your bike ( to get a full charge ) - park it - using a volt meter measure your battery voltage - leave it over night and measure it again in the morning - see how much the voltage drops ( oh and use the highest resolution on your multimeter to get the most accurate measurement ). This will give you a baseline.
Now Plug in your USB Charger ( without any attachment ) - leave it in overnight and measure it the next day - the voltage drop will be next to nothing compared to the day before ( be sure that any drop in voltage on the base line is subtracted from your USB test to show only the amount of drop from the USB device and not the system as established in the baseline )
Extrapolate that out for a number of days to see how long it will take to draw the battery down below 12volts.
I believe you will find around 2 weeks is a safe amount and still being able to start the bike.
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