How to charge 765 GPS using computer and how do I know

Charging via a PC is not the quickest way to charge a GPS because many USB ports can not supply enough current, so the battery will take longer to charge. Using an AC adapter is a better way. Leaving the unit to charge over night should be fine. The way to tell if the GPS is fully charged is to look at the battery icon on the main screen of the GPS. It consists of 4 bars. 4 bars = fully charged. 3 bars = 75%, 2 bars = 50% and 1 bar = 25%.

Best to charge unit from Mains or Computer USB, then you know it's fully charged. Navman has charge in battery but you need to fully charge it yourself , then you know where you are.

Which would suggest the battery is not charged, or retaining its charge.
Will it start up if connected to a power supply (USB or DC charger) ?

If you are charging your unit with the USB cable STOP - use only a wall plug. The unit will not charge using the USB cable. If you don't have the wall unit - try charging it with the car plug charger for 15 minutes. Just let the car charge it running or not. You will see it work. Once it is charge over half way - the rest will charge up. It is a problem Gamin is working on.

Using cable data /charger port you can do charging both at system or in car under car charger option. those specifications placed in manual and normally lasts for 3 hours

Very simple: connect the cable that came with the GPS: USB end to computer, mini USB to the GPS. When the unit is charging, the battery icon will disappear.

Unfortunately, Garmin did not think you would want to know the percentage of charge completed (they confirmed it to me in email), so you have to guess that after 2-4 hours your GPS is fully charged.

Charge it for at least 12 hours, directly off the charger, then try to usb connect it and run updates. This should work unless you left it long enough to damage the battery.

Hope this helps!!
Jim

This is kind of a late response, but this post may help other people. I had the same issue and was ready to have Garmin exchange my battery until I called them today and spoke to Tech support. They helped me upgrade the firmware on the unit (didnt take long) and so far the problem has been resolved. I have had it on for two hours without fully charging it, which is better than before when it only stayed on for about 20 minutes. I recommend that people with this issue call them first since it appears to be a common issue with this model nuvi.

if the greens bars on the upper right indicates fully charged

Cig-lighter offers 1A current. The battrey will be fully charged for 4 hours if the unit is not used.