measuring USB power bank capacity

I think you are confusing the capacity of the powerbank bank with the rate at which it discharges. If the powerbank can discharge at 500mA for 4 hours you have a capacity of 2,000 mAh (500 x 4). The units you are reading on your meter are mA not mAh.

I agree, that is confusing

for someone not an electrical engineer : )

for example, I measured 0.13 amps for 4 hours = 0.5 Ah = 500 mAh

If you're not familiar, The Wirecutter does great product testing/reviews, and it looks like even the most accurate claims are pretty far off the mark for those of us who measure product weight in grams. It seems like most of the manufacturers estimate using ideal cell capacity and assume 0% loss in transmission, and then round up. To the nearest Ah.

http://thewirecutter.com/reviews/the-best-portable-battery-pack-for-daily-use/

"No batteries are ever as fast or efficient as advertised, but in our lab testing, the L60X came reasonably close to Limefuel’s projections. We measured the real-world output speed at 1.38 A, versus the advertised 1.8 A. And it’s about 63 percent efficient with a phone, which means it’s good for about 3,780 mAh of energy compared to the advertised 6,000mAh."

From the footnotes of the review:
"Almost all of the USB packs we’ve tested are 55 to 65 percent efficient, and the L60X falls nicely into that range. Also, your real-world results might be better than ours, since your charge circuit has less resistance than our testing setup—we’ll assume that you don’t usually put an ammeter between your power source and your phone."

Thanks for that, I checked it out

I wouldn't call it 55 to 65 efficient, I'de call it 55 to 65 percent of meeting spec : )

Haven't they looked at Consumer Reports? Or a BPL review? They should have a table with specs like size, weight, spec'd capacity, actual capacity, etc. for each model tested.

Here you are, kindly answering my question and giving me a source, and all I can do is complain about it, sorry…

Funny that all USB power sources exaggerate their specs by almost a factor of two

"Funny that all USB power sources exaggerate their specs by almost a factor of two"

That is because they assume that 99% of their potential customers are unable to actually measure the results. That may be from stupidity or from lack of proper instruments.

–B.G.–

It's your fault that I bought the cute little tester, otherwise I wouldn't know

I got the tester because I had a cigarette lighter USB charger that I found put out 12 volts which killed a device.

Seems everything about USB chargers is a bit screwy

That 63% with a phone is interesting.

How inefficient is the phone side of the equation?
(Ie how many WAh do I have to put down the cable to fully charge my battery of some WAh rating).

– use a powerbank which has good base batteries … either build your own or look it up … some like the powerpond that MEC sells use Panasonic inside

– good powerbank will have 80%+ efficiency …. you can find ones that have been tested online … heres the old ENB with 80%+ …

– dont cheap out on USB chargers, especially the wall chargers … theres been multiple cases where cheap knockoffs have failed and caused fires, or damaged the protection circuit on devices leading to a li-on bat on yr phone which could then be overcharged … KABOOM

li-on bats and chargers is one place you DONT screw around and try to save a buck

;)

eric, does someone have a review, that has a table of, for each USB bank, speced mAh, actual mAh, weight,…

"How inefficient is the phone side of the equation?"

Of course that isn't a property of the USB bank. If they spec'd amount of mAh to charge a device, it should include any inefficiency.

For the USB bank, it doesn't really matter whether you have a bigger battery and less efficiency, or smaller/better efficiency. What matters is mAh out of the USB bank. That's what they should be specing.

Or watt hours but that's pretty close to the same because the voltage is about the same. If the voltage is a little higher, I wonder if the charging device would require a little less mAh to fully charge…

you can read some reviews of powerbanks here … but many are those you build yourself (supply yr own bats)

http://www.lygte-info.dk/info/indexUSB%20UK.html

also note that with chinese manufacturers that just because an older model has been tested as good, doesnt mean the new one is of the same quality …

for example the old miller charger was considered very good, but the new v7 version has been tested with an overcharging issue and is no longer recommended

but really what you are look for is good quality base batteries ….

heres a test with poor power banks where the tester replaced the bats with panasonics … the performance was greatly enhanced

http://goughlui.com/2013/08/30/follow-up-usb-power-bank-tested-and-repacked-5000mah-30000mah/

so make sure you get panasonic, sony, lg or samsung bats in that powerbank beforehand

;)

mAh or Ah are bad energy storage method measures. We really need wH.

These manufacturers use a discharge rate of 1/10th of the rated capacity. So a bank rated at 2,000 mAh uses a discharge rate of 200 ma to calculate capacity. Hook up your dead iPad pulling around 2 amps and it is not going to get any where near a full charge out of 2,000 mAh.

The deep cycle batteries I use for my solar system are measured as the total amps which can be delivered in 20 hours. Here is a chart for my two 6 volt batteries wired in series and "rated" as 260 Ah.

5-Hr Rate 215 amps

10-Hr Rate 239 amps

20-Hr Rate 260 amps

100-Hr Rate 287 amps

The mAh rating, which is standard, allows us to shop brands. It is not a predictor of how it will charge your devices. Charge rate, condition of you battery, etc. are all variables.

this is what you need …. again if you know the base batteries or build your own power bank, you KNOW what you are going to get …

to put it simply if you build yr powerbank from 3400 mAh quality panasonic 18650 bats …. you WILL get ~3000 mAh (at 3.7v, convert that to 5v if you want) before losses at a 2A draw … choose a 80%+ tested efficient bank and you WILL get ~2400+ mAh total output when the bat is new … after around a 30% degradation after 300-500 FULL charging cycles, this will be ~1700 mAh out at the cable (again at 3.7v, multiply it to get the Wh and thr divide to get the mAh at 5v, but thats pointless since the iphone is a nominal 3.7v anyways youll need to convert it back)

the link i posted above shows the HUGE difference putting in quality bats into a even a cheap powerbank makes …

;)

Since the USB voltage is a constant 5 volts (+- 0.25) it doesn't matter whether you talk about amp-hour or watt-hour, although the +- 0.25 makes a small difference. I wonder if devices being charged utilize that +- 0.25

I use deep cycle batteries and yeah, slow discharge rate yields more amp-hours

I was playing around with my USB power bank with lithium battery and seemed to make no difference. Lithium batteries must be different.

exactly WHICH usb power pack are you using … is it a li-on one or a AA/AAA one?

;)

cheap lithium from amazon.com – no name – shipped from China

Im sorry to say that its likely pretty useless then

My recommendation is that you get one of the well reviewed power banks and 2 high quality 18650 bats to power it

It should cost around 30 dollahs and you will get around 15-20 Wh out of it

;)

"It should cost around 30 dollahs"

Are those maple dollahs or Yankee dollahs?

I pay about $10 for each Panasonic 18650. I'm only a few dozen charge cycles in with them, but they perform perfectly.

–B.G.–

my charger cost $4, 2.5 ounces, sufficent for charging mp3 player

like ive said li-on chargers and bats is the one place NOT to cheap out … and im a fan of quite a few cheap things

;)

"Since the USB voltage is a constant 5 volts (+- 0.25) it doesn't matter whether you talk about amp-hour or watt-hour, although the +- 0.25 makes a small difference. I wonder if devices being charged utilize that +- 0.25"

Does it? I don't know. Most devices have 3.7 volt batteries. What we don't know is whether or not your device has a voltage regulator. What is the charge strategy of your device? At some point before full charge does it go into a float mode? Point being is different devices act differently. My Sony RX100i will charge off the USB port of my Dell laptop with the power supply not plugged in – my iPhone won't, the computer must be plugged into 120 volts. The iPhone will charge off my MacBook Pro when the computer isn't plugged into the wall. My iPad won't charge off some battery banks… Enough voltage, but not enough amps. The iPhone and iPad will give you a message when the charge parameters are right, which means not enough voltage and/or amperage. My RX100 won't charge if the USB wire gauge isn't thick enough, same with the Apple devices — even though some other devices will charge. I guess the bottom line is there are many variables here.

Conclusion (to agree with Eric), I buy quality stuff. I only use Apple cables and chargers with my Apple devices, although I have 3rd party battery banks.

But, for the most part, these battery banks will deliver 10% of their mAh rating for 10 hours. The construction of some of these batteries may not do as well at higher charge rates as the top quality brands.

I guess an analogy is your RV. Trojan deep cycle batteries do better than similar capacity batteries, because Trojan uses thicker plates, etc.

It is an interesting conversation, but since I leave most electronics at home when backpacking, it's not critical for me ;)

USB power banks have a DC-DC convertor to produce the 5 +- -0.25 required for USB

First one when I do the google says "xxxx 10,000 mAh Portable Dual USB Power Bank Charger" and then down below in the details says "10,000 mAh lithium polymer battery"

Ahhh… I see my problem, I thought they meant that the USB power bank as a whole was 10,000 mAh, not the battery. From Eric's plot, battery has about 3.7 volts, which the DC-DC converter turns into 5 volts, but it takes more current. Plus the inefficiency of the convertor. That's why the USB power bank as a whole is little more than half that spec.

Still, seems a little misleading…

They could use a 1.2 volt NiMh battery that has 10,000 mAh, DC-DC convert it to 5 volts, and the power bank as a whole would have only 2,000 mAh but they could call it a 10,000 mAh because it has a 10,000 mAh battery…

Ha!

Marketing 101

Confuse 'em with facts.

Sorta like my SUV. It can tow 9,200 lbs… But with only a 150lb driver and NOTHING else inside the SUV!

And yeah, usually best to get quality stuff

cheap good to see if you can even use it. If it breaks, and you use it a lot, then go ahead and get a better one.

or if all you need is something to use for a while then cheap can be good

But watches have moved on, no hands any more, digital…

I suppose you have a manual transmission vehicle???