I've purchased batteries from this eBay seller in the past with excellent results.
http://stores.ebay.com/RL-batterydepot?_trksid=p2047675.l2563
http://stores.ebay.com/RL-batterydepot?_trksid=p2047675.l2563
Talked to a guy at one of the biggest European importers for batteries. He said (he wasn't selling me anything) they are using the Sony cells and if you look up the test data for 30a Efest and compare it to the Sony ones the test data is the same. They are priced the same too, so if you don't trust what I'm saying go with the Sony ones.
He also told me to stay away from the 35a Efest batts until some solid stress testing has been done on them.
Best bang for your buck is the Sony US18650VTC4 30amp or the Efest 18650 30amp.
i did notice the comments on temperature, for which i think it's likely he was referring to the vtc5s which have been noted to heat up very quickly at higher currents.
He didn't mention the VTC5 by name though.
akkutest said:Careful: in a continuous discharge, this cell will get extremely hot - up to 80 ° C at 20A, up to 125 ° C at 40A
THAT'S HOT STUFF!
Very true and very interesting beckdg ! It outperforms the VTC4 at 20A, but heats up significantly. That would normally make me think the amp rating was overstated, but in batteries where I've seen that done they also don't last long at the elevated discharge rate.
Food for thought for sure.
i'd say the amp rating is definitely over inflated. you can't do a continuous discharge to cutoff voltage at 66% of it's amp rating without it overheating. i doubt that cell would survive anywhere near half the 500 cycle "rating" at 20 amps full discharge let alone 30 amps. highly doubt it. though, i'm sure 40 amp bursts would hardly effect the cell for quite some time looking at how the voltage stays in those graphs. so still a very good cell for our purposes as long as it's not in the wrong hands. a hard short could be catastrophic quick with temps like that.
I don't think the VTC5 will turn out to be dangerous when it vents. It would still be great to see a hard short test done on one though.
It's battery temperature that causes a cell to vent. Straight ICR cells become unstable at around 150*C, straight IMR at around 250*C. My bet is these cells are at least as stable as IMR or Sony wouldn't be putting them out there. Then there are LiPos. My model airplane LiPos get quite warm on every flight and handle that very well for a good number of charge cycles.
There's also new technology being introduced all the time these days. In was amazed when I saw the video of a NCR18650A take a hard short for an hour and still be charged and used.