Is it dangerous to charge a battery after partial use?
I have protected 18650 cells and a smart charger.
not any more dangerous than charging batteries that are fully depleted..i've read that li-ion prefers being topped up rather than being depleted to 3v before charging..if the battery is in use and you are not storing it then topping up is fine..if you are not using the battery discharge to 3.7v and store it..
all the time
but if storing batteries for longer periods, leave them at around half charge - they will be less permanent capacity loss over time. So cells that you know will be sitting around for a month or more should be discharged half way.
Storing at full charge causes permanent capacity loss over time. This, plus heat is a major factor in the death of laptop batteries for laptops that are permanently connected to a charger.
What voltage is considered a fully discharged voltage?
I think you are mixing Ni-Mh with Li-ion batt.
Fully discharged is a big no no in li-ion world.
What exact battery cell (brand) you are talking about?
Most electric cars use Li-on batteries, and their owners usually plug them in EVERY night. Shouldn't be an issue.
Although if you Google "bricked Tesla" you will get some interesting reading.
I realize that, I just haven't discharged my li-ions to the point of shutting down via the PCB. I have only seen them get as low as 3.6 volts. I was just wondering how much of my effective voltage was used up.
I have NCR18650A's that came with my xtar 2 charger and a couple trustfire flames, both are 3,000 mah.
Excellent,
1-NCR18650A Panasonic is the only battery cell that has true 3100mAh rated capacity. It is highest capacity 18650 battery cell on the market, period.
2- there is no 3000mAh 18650 battery that is charged to 4.2V with commercially available chargers.It doesn't exist.
3- All xxxfires 3000, 4000mah are batteries with fake capacity rating.Your Trustfires (flames) can go max 2600mAh under ideal conditions.
Now about safe discharge level:
Theoretically regular li-ion cell should not be discharged below 2.75V.
Panasonic NCR18650A that you have are the best even in discharging department:) . They can be discharged to 2.5V w/o problem.
I would say that 3V is generally accepted level of safe discharge for bare cells. For li-ions with protection - properly working protection should terminate at 2.7V (2.8V).
But if you really want to take a good care, and get max from your cells charge them whenever you have time. No, li-ion cells do not need to be fully charged. No memory effects.
If you want to store them for a longer period of time. Do not store them at full charge; 3.6V -3.7V is an ideal charge storage level.
Keep them in cold place, they do not like hot storage temp.
Previous poster mentioned Tesla . Your NCR18650A are cells that Tesla Motors company is using for their Roadster > 8000 of them ;)
Thanks for the info
I was going to say my NCR's charge to 4.2 volts, but I just realized I have no idea what their rated capacity is. I still have a few NCR's coming in the mail from Intl-outdoors.
http://www.intl-outdoor.com/panasonic-ncr18650a-3100mah-protected-batteryone-p-333.html
I think these are going to enjoy their new home in my UF980L. 
You should know that my Xtar WPII charger charged the Intl-Outdoor NCR18650 3100 mah batteries to 4.2 volts.
I think he ment other true 3000 mAh batteries charge to 4.35 or so to be classed as fully charged, the NCR18650A is classed as fully charged at 4.2 volt
Remember if you pull a battery out of a torch and it reads 3.6 volt under a 3 amp load you would find the cell voltage would be lower around 3 volt + or -
With voltage sag most batteries drop from 4.2 to 3.7 volt as soon as you load them up say around 3 amps depending on how good the cell is, some more some less.
And i top mine up all the time and i oftern leave mine fully charged, yes im a bad boy.