Customer Review

64 of 68 people found the following review helpful
4.0 out of 5 stars Very good, but doesn't deliver 12,000mAh, 31 Jan. 2012
Verified Purchase(What is this?)
This review is from: TeckNet® iEP392 12000mAH Heavy Duty 2.1A/1A Dual USB Ports External Battery Pack for the new iPad 3, new iPad mini, iPad2, iPhone 5, iPhone 4S/4/3Gs/3G, iPod Touch all versions; Samsung Galaxy Note/Nexus/S3/S2/S; HTC Titan, Sensation, ONE S/V/X, EVO Thunderbolt, Desire; LG Optimus series; Blackberry Bold, Curve, Torch; Motorola Razr HD/MAXX & Bionic, Atrix/2; Nokia Lumia 700/800/900 and GoPro (iEP392 Heavy Duty 5V/2.1A/1A high speed charger) (Electronics)
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.

Later edit:

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!

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Tracked by 2 customers

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Showing 1-10 of 10 posts in this discussion
Initial post: 19 Feb 2012 19:48:39 GMT
Sebastian says:

I am thinking of ordering this article but from some reviews it seemed like the battery bank itself could be charged with a standard USB charger as well. Can you confirm that?
Thank you very much for your help.

Posted on 27 Mar 2012 12:28:47 BDT
Hi A. Hawkins

I was interested in how/what you use to as LED bike lights, how do you convert the USB output to an input that can be used by LED lights.

Any way i thank you for a very through, review, it was very useful.

Posted on 11 Apr 2012 13:40:29 BDT
Oox says:
Most of these batteries will say for example 5000mAh @ 3.7V even though USB is 5V

If yours does this and you are pulling a 6Volt load instead of under 4V you have forgot to factor this into your calcs. 12Ah x 3.7V = 44.4Wh. 12Ah x 6V = 72Wh big difference

6V x 550mA = 3.3W
44.4Wh / 3.3Wh = 13.45 hours

Sounds like it is more than 12Ah if you got 14 hours out of it. If they would use Wh instead of Ah people wouldn't feel like they've been ripped off when in fact they've got a better deal than was advertised.

In reply to an earlier post on 3 May 2012 16:33:47 BDT
Last edited by the author on 3 May 2012 17:14:57 BDT
Grumpian says:
I was just about to comment along the same lines. I read in another portable battery's description that the Ah rating applies to the actual lithium battery inside (3.7V nominal output voltage), not to the device output (5V). So, here is the same explanation in a different way: to provide 550mA at the 5V device output, you need to draw higher current from the 3.7V battery. Simplistically, 5/3.7 times higher if you assume 100% efficiency of the electronic inverter. But typical inverter efficiency is more like 85-90% (and also varies with the current). Say it is 87%. So you need current from the battery that is 5/3.7/0.87, i.e. 1.55 times higher. Divide the lithium battery's 12,000 mAh capacity by 1.55, you get capacity of 7725mAh at the 5V output, matching the observed capacity.

I absolutely agree that it would be far better and unambiguous to quote the capacity in Wh instead of Ah.

Now I need to update my own review of the iEP392...

Posted on 10 Jun 2012 11:20:46 BDT
Thanks for this useful review. I live on a boat, so have to have a fair bit of knowledge of battery life! What is coming into play here, I think, is Peukert's Function: 'Peukert's law describes an exponential relationship between the discharge current (normalized to some base rated current) and delivered capacity (nomalized to the rated capacity), over some specified range of discharge currents.' (Wikipedia). So because your lights are demanding more than the current at which the battery was tested to 12kAh, the delivered capacity is going to be a lot less.

Posted on 10 Jul 2012 12:43:47 BDT
"so this will run twice as many lights for far longer without a weight penalty"
When it comes to chemical batteries there are no miracles I know of. The more it delivers the more it weighs. You cannot get half the size and weight and double the capacity just out of magic.

In reply to an earlier post on 11 Mar 2013 17:22:58 GMT
A. Hawkins says:
You are correct; what I failed to mention was the battery supplied with the lights was of a different chemical type - NiMH - and in a heavier case.

So this battery runs twice as many lights for far longer *than the NiMH battery* without weight penalty.


In reply to an earlier post on 11 Mar 2013 17:31:20 GMT
A. Hawkins says:
Note to the various people who pointed out that the mAh rating applies to the chemical cell (at 3.7V):

You are all correct, yes! This dawned on me only recently and in fairness, the tiny label on the battery does say 44.4Wh/12,000mAh (do the math and this gives 3.7V, the output of a LiPo chemical cell).

This equates - if the voltage conversion was 100% efficient, which it obviously won't be - to about 8,000mAh or 8,800mAh at the 5.5V or 5V outputs. This relates nicely to observed run times.

It is a bit annoying / misleading to sell a battery that outputs at 5V or 5.5V but to quote a mAh capacity relating to a 3.7V output though. I can get 100mph and 100mpg out of my car, but there's laws against claiming that because it wont do both at the same time...

However, I stand by the original rating and this is, still, an awesome piece of kit for the size, weight and money.

Recently I have found it will run a GoPro Hero3 video camera for 9+ hours (it was still supplying charge when I turned the camera off!) so still highly recommended.

In reply to an earlier post on 11 Mar 2013 17:31:56 GMT
A. Hawkins says:
Hi Sebastian,

No, the USB sockets are output only. Charging is via a dedicated PSU.

In reply to an earlier post on 11 Mar 2013 17:33:58 GMT
A. Hawkins says:
bzxd5z, they happen to be LED lights built to run at 6V (rather than the higher voltages that many serious bike light systems use). Both USB outputs will run the lights although the "tablets" socket gives (very slightly) longer run times.
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