Are powebank capacities half of the advertised?

My Anker 26800 is nonsurprisingly advertised as having 26800mAh capacity, yet I plug a 5v USB load to one of its putts and only measure 17500mAh coming out.

I have lots of electronic gadget but it's my first time buying a powerbank so I'm surprised that it's only 65% of advertised.

After scratching my head a bit it came to me that maybe these things are like computer hard drives, where they're advertised as 10Gb as raw but after putting a file system you only get 8Gb or so.

So I took the damn thing apart and manually measured each 18650 cell amperage throughout the spectrum of voltage ranging from fully charged to minimum voltage and got over 3000mAh per cell, but when I put it back in the case fully charged it only delivered 65% of that.

Garghhhhh!!!!!!

It could be the stage upping the voltage to 5V, but I thought transformers were like the most efficient machines on earth with upwards of 98% efficiency.

Help me /g. What gives here?

Other urls found in this thread:

batteryuniversity.com/learn/article/what_is_the_c_rate
batteryuniversity.com/learn/article/discharge_methods
twitter.com/SFWRedditGifs

Do the math, one of your cells is faulty.

What the first post says is correct.
And no. The most efficient “machines” are heating resistances, specially made with high grade materials.... such as CPU’s, they are extremely efficient at producing heat from electricity

If the advertised capacity is measured at 3.7v, real life capacity at 5v would be around 30% less, plus the 2-5% loss at the transformer, and some 5% at other ICs and cable.

check how warm it is during a discharge.

not all powerbanks are efficient at conversion.

no.... just... no
how the hell are you measuring mAh?
no, it's not like like a hard drive, you just have inefficiencies.
there are no transformers in the thing. the cells are probably ran in series and regulated down, allowing for higher voltage output on any of the "smart" usb ports

this.

Yes, my initial thoughts exactly...

however, I then used a battery tester and went through each cell individually to confirm they hold around 3000-3300mAh each.

If they used a shitty regulator, mabey the combined cell voltage is falling below the dropout voltage of the regulator before they get to their actual minimum working voltage.

this is exactly why everything should be measured with Watt hours

batteryuniversity.com/learn/article/what_is_the_c_rate
>When discharging a battery with a battery analyzer capable of applying different C rates, a higher C rate will produce a lower capacity reading and vice versa. By discharging the 1Ah battery at the faster 2C-rate, or 2A, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same since the identical amount of energy is dispensed over a shorter time. In reality, internal losses turn some of the energy into heat and lower the resulting capacity to about 95 percent or less.
Maybe should look at this too.
batteryuniversity.com/learn/article/discharge_methods

>how the hell are you measuring mAh?
Current multiplied by time.

An 18650 that outputs 1.5 amperes in 2 hours is 3 amperes per hour (3000mAh).

>cells are probably ran in series and regulated down
Actually they're ran in parallel and regulated up (based on the 4 powerbanks I bought last week).

Yes indeed... but such a small difference is not enough to justify a whopping 35% discrepancy.

I actually tried with one device connected to the USB port of the powerbank consuming 1.67A and a single fully charged 18650 inside the powerbank supplied 1800mAh measured at the output, while to a device consuming only 100mA it was able to supply a bit more at around 1900mAh.

Look up Buck boost converters.

If the advertised capacity is measured at 3.7v
Nooo way... is that it?

That's deceptive advertising user... every manufacturer selling USB powerbanks knows they're for use with 5V devices, not 3.7.

Geeezzz.

I have a 5000 mAh pack, it dies pretty fast when charging a cell that's only 3000 mAh, and the power bank will not even power up after it finally dies.

But the moment I plug it into a charger to recharge it suddenly it says it's half-full which is utter BS.

Maybe it's defective, maybe not, guess I'll never know but I won't buy products ever again that's for sure.

read the fine print. caveat emptor

And after aforementioned losses a phone well also step down the 5 volts to store at 3.7V in its internal battery losing an additional 15-30% in the conversion (which is why they get so hot when charging).

You want to charge a 3.7V battery with 3.7V supply? Why do you think they step it up to 5V to begin with?

Li-on is 3.7V
USB is 5V

3.7V * 3Ah * 3 = 33.3 Wh = 5V * 6.66Ah
So you should expect 74% the combined cell capacity. 65% means there is 88% efficiency in the boost conversion.

26800 is how much it stores.
How much you get out is ruined by the efficiency loss. They measure how much energy they put in to the cells, not how much they get out.

...

Why did it take so long for power banks to become a thing? You would think they would have started coming out with the first USB charged devices.

Didn't know that. Thanks user

Battery technology isn't that great, its very expensive.

It's not really deceptive, because if you charge a Li-ion device from the power bank then that 5v gets converted back into the roughly 3.7v the device hold.
There are losses in the dual conversion, but if you have a 26800mAh power bank you should be able to fully charge a 2680mAh phone about nine times, maybe a bit more, depending how bad the losses are.