In reply to MGC:
Most of the times they will lie about the batteries capacity and Lm output.
LEDs used in this unit are:
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Dire...
Maximum output is 1040 lm @ 10 W per one LED. 3 of those will output 3120 lm and consume 30 W. It's can't be as advertised 4200 lm. But 3120 lm is very unlikely as well. For few reasons. First of all those LEDs get very hot operating at full power. Given where are 3 of them you could boil water with all the heat radiated by them.
Another problem is to source 3.1 V at 9 A. That's quite a lot of power. If they are not lying and power bank is 8.4v 6400mAh. That would mean that it would work for 2 hours before fully depleting battery. However stabilization in cheap flashlights is very bad and sometimes it will lose 25% on it.
Those sellers understand that numbers they are providing is very difficult to test and proof them wrong at home unless you are engineer in electronics or remember physics lessons from school.
As for the battery it's a bit easier to test. Many modern chargers will show how much power they have put in to the cells. But even without equipment it's obvious that 8.4V 6400mAh Li-ion pack is 4 cells 18650 with two pairs connected in parallel. That means cells used must be 3200 mAh 18650. One cell like that in China cost around £3.50 at the moment if buying in bulk. So either seller is loosing money selling this set or lying.
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Even said that it's an absolute bargain for money. I would just recommend to look after o-rings and make sure they don't leak and it will last.
