Nuvi Charging
   13 years
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Because it always uses battery power (even when "OFF") my Nuvi 855 often needs charging. I've noticed that it will only charge some of the time when it is receiving external power and the battery has a partial charge. Why doesn't it charge all of the time?
I've confirmed this with the battery diagnostics page which has a Charging YES/NO indication. Often it will show not charging even though it is receiving external power and the battery isn't full. It won't drain the battery, but won't charge it either. If I remove the external power (USB or Car 12 volt plug) for a moment and plug right back in, the charging indication shifts to YES and the unit charges.
When you power off the battery while still receiving automobile 12 volt power, there will be a momentary display "Charging Complete" or Unit Charging. Oddly it will say "Charging Complete" even if the battery is 50% full but it isn't charging.
Why doesn't the unit charge whenever it has a battery less than full and is receiving external power? Checking to see if it is actually charging and then momentarily disconnecting the cord seems silly....
Any ideas?
One theory
One theory I've read suggests that the charging will cease when the nuvi is warmer than a set temperature point which in this summer season, may well be reached on many dashboards, even with the AC running, given that our 855s are black solar heat absorbers.
If charging ever becomes a big issue, bring the nuvi inside and charge it via a computer USB cable, remembering to set the computer not to go to sleep or hibernation.
Charginge
It would make sense to stop charging when too hot. However, temperature (as measured by the unit) does not seem to be an issue. I get the same results, whether through charging via the 12 volt cord or the USB connected to a computer or charger. Either way it will say Charging: NO, and disconnecting the USB or 12 volt cord for a moment will cause a change to Charging: Yes 2 seconds after re-connected.