Most helpful positive review
64 of 68 people found the following review helpful
Very good, but doesn't deliver 12,000mAh
on 31 January 2012
This is, certainly, an amazing piece of kit. The amount of power it packs into a sleek case that will fit in the back pocket of your jeans is truly impressive, and for the price (under 35 pounds at time of writing) it's an absolute bargain.
However, I do have one niggle: It wont provide 12,000mAh.
Now, I have to say that I'm using this for a totally unsupported purpose; namely, to run a set of LED bike lights. Two sets, actually. The stock battery that came with each was a 6V / 2,400mAh unit of nearly the same size and weight as this TeckNet unit, so this will run twice as many lights for far longer without a weight penalty. Happily the 6V lights work perfectly on 5V or 5.5V, so a minor adaptation of a USB cable and hey presto, one of the best bike light batteries you could wish for.
This in turn led me to my findings about its usable capacity. The lights draw about 550mA according to my multimeter. A quick bit of maths should tell you then that a 12,000mAh battery should run the lights for about 22 hours between charges. In fact, the run time is about 14 hours (yes, I know, still longer than any night ride!), meaning about 7,700mAh of usable capacity.
I questioned this with the seller and very promptly they replied and reminded me about the inefficiencies of charging batteries; when you charge a battery it gets warm as, while most of the power charges the battery, 30-40% will be lost as heat. Which is fine, except, (a) I wasn't charging a battery, and (b) I was measuring the current flow from the battery to the load. What it did when it got to the load, I have no care (and neither does it), the point is that the battery put out 550mA for 14 hours before it became sufficiently discharged to turn itself off.
I suspect that the truth may have more to do with the clever electronics inside the unit. The instructions say that the output is shut off when the battery voltage drops below a certain value; my suspicion is that this value is reached significantly before the actual chemical cells are exhausted. In other words, the cells may well have a 12,000mAh capacity, but the electronics prevent you from getting much more than about 60% of that capacity out of them. On the plus side, those clever electronics keep the output voltage constant throughout the discharge, as is required for a USB supply; this means the lights stay on full brightness until they switch off.
I have no reason to believe this discrepancy is unique to this battery; it wouldn't surprise me one bit to learn that all similar batteries did the same, and, had I only ever used this to keep my phone charged when away from the mains for long periods, I'd probably never have discovered any of the above, and I'd have nothing but praise.
Speaking of which, I am buying a second one, for its advertised use this time, because despite the above niggle, the capacity/size/weight/price combination of this thing is still amazing - just be aware that the quoted capacity does not reflect the usable power while you're adding up what you want to charge or run from it.
As some others have pointed out (below), the mAh rating applies to the chemical cells, which, being LiPo, output 3.7V. This gives a "true" output of 44.4Wh which is marked (in tiny writing) on the battery itself.
Convert the voltage to 5 or 5.5V for USB, and this power rating gives around 8,800 or 8,000mAh at the actual output voltage (minus whatever inefficiencies the voltage conversion introduce), in line with my findings above. It's a niggle / annoyance that this battery (and probably all similar LiPo USB-voltage packs) is sold quoting a mAh capacity that cannot be achieved at the supplied voltage. It's like saying my car can do 100mph and 100mpg - it can, but, not at the same time!
Still a great piece of kit and still highly recommended, just a shame batteries like this aren't sold quoting the Watt hours which would give a much more accurate customer expectation.
Sellers *please* take note!