Hi.
I´ve got a Trustfire 18650 3.7V, “2400mAh” that I´ve never used so far. It was double pack and the other one is fine, but this one shows 0,01-0,02V on my multimeter.
What should I do with it?
Protected or not? If protected, than PCB might be tripped, so put it in a charger and observe. If it heats up, THRASH IT, if it charges normally still keep an eye on it.
If unprotected, follow the post above mine.
Yep, it is protected.
I wonder why they (not sure if that came from DX, but I think so) sell it this way.
make sure your meter works.
It works with my other batteries, so I think it is ok.
Put it in a charger for a few minutes, take it out and re-measure it. If the protection cct tripped, charging it up a bit will usually recover it. If it recovers keep a close eye on it. If it doesn’t recover after 10 minutes or so, it’s probably gone.
A lot of chargers will not attempt to charge a cell that measures two low of a voltage… they won’t reset the protection board. There are some threads here on how to reset the protection using a good cell.
Will that work? Maybe through a resistor from a 12 v car battery?
Not a good idea… unless you have written me into your will.
You’d be better off using a fully charged 18650 cell and a 10 Ohm resistor. Connect it briefly first to see if it comes back right away. Measure the voltage across the resistor to see if any current is flowing (voltage/10 is current in amps). I’d expect something in the one Volt area (4.2V - 3.0V) if the cell is still in a recoverable state.
If no current flows, it’s probably not going to come back. In that case, probably best to follow this suggestion.
Can any of these cells be trickle charged at C/100 or less for 100 hours or more without being harmed?
Yes, Li-Ion can be charged at a low current as long as you ensure that it stops at the end voltage (4.2V). Even charging at C/100, given enough time the cell would overcharge.
This is different than how NiCad or NiMH respond to trickle charging (where the term more appropriately applies). You can provide them with a very small current and fully charged cells will pass it without overcharging. This is a common way to balance NiMH battery packs.
To repeat the important point, you must not charge a Li-Ion cell past the fully charged voltage (usually 4.2V). This includes ‘trickle’ charging.
Thanks.
I guess a trickle charge (resistor) with a precision or programmable 4.2v Zener diode or Zener IC across the Li cell would make a workable charger.
$2 plus shipping . . .
NO! It would not shut off charging when the cell got full.
You can float charge LiFePO4 cells (at least the A123 ones) at 3.40-3.45V
No more $2 
Update:
I´ve put it into the charger for <5 mins and now it shows 3,14V.
How come?
Tripped PCB 
The battery must have been shorted.
So it´s ok?
You know, it´s all new stuff to me and I am glad you guys do always help me out
When I last bought them they were 5 pieces for $8…
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180921085924
Also search Ebay for TP4056