8 cell power bank

Perhaps a noob question, perhaps not. I got 8 cells for one of those ebay usb power banks. I got 2800MaH cells

and charged all of them in my Nitecore D4. Then I ran a load test and came out with 13.78 Ah. Now I understand that there are losses here. But here is how I figured things
each cell downrated to 2500MaH x 8 = 20Ah
Take off 15% for loss and I should be at at least 17Ah
Or am I wrong?
I tested the bank at about 1.6 amp draw.
TIA for any insight

As far as I know the best Chinese cells made in China are 2600 mah. Claiming 2800 mah could mean anything. Your measurements showed that you may have been ripped off. Rechargeable Lithium Ion Battery 3.7V 18650 Battery Cell - China 18650 and Lithium Ion 18650 Cell price Read the product description “Normal temperature capacity:1800mah or customized”

Are you talking about you’re only getting 13.78 AH through the Power Bank using a USB device —- That is about normal conversion up to 5v —- 60% / 70%

Exactlyload test off the USB outlet.
I did not figure the losses would be that high. I figured derate the cells to 2500 and then 15% but you are saying the loss is about 30% + that would definitely make sense of the numbers then.

8 x 2.5 AH = 20 AH x 3.7v = 74 watt hrs

74 watt hrs / 5.1 v = 14.5 AH and with some loss 13.78 AH

The best I’ve ever gotten — I tested the cells and when I put them in the Power Bank — they converted to 70% of the cell capacity— that’s why people always get mad and say the capacity is low —most PB capacity rating is from the cells not the actual output

I’ve got a cheap 8-cell bank. The cells are tested in a pretty accurate discharge system. The capacity output is a lot less than I had hoped for.
OTOH, it IS a use for those cells that otherwise would get no use at all. So, there I am.

Some time ago I found a site that tested a bunch of powerbanks and measured the conversion efficiency, and most of the better ones were above 80. I thought I had noted the url but I can’t find it in my files. I might look for it again with web search. How did you do your load test? One issue is that most li ion chargers don’t use buck converters to go from 5v to 3.6v or whatever. They just dissipate the 1.4v difference as heat. So there is already around 25 loss right there.

The whole concept of powerbanks seems like a clunky artifact of almost all mobile phone batteries being sealed inside the phone these days. Otherwise you’d just swap charged cells into the phone. Sigh.

Yep — Years ago the phone was as big as the Power Banks with interchangeable batteries

Now the phone is the size of the old battery and the power bank is the size of the old phones

I showed my granddaughter one of my old phones a couple years ago — she just laughed at how big it was — I laughed and said this was a smaller one of the time

Just get a decent battery bank like the Xtar PB2S and a quad of 21700s and keep them charged for emergencies.

I use 50Es in mine.


I’ve found Molicel, DLG and BAK all test like any modern 3000 to 3500mAh cell at 1A, within ~5-8% of advertised capacity under my conditions anyway. Can’t speak to their longevity though.

But that’s the 64 dollar question…how long do those Chinese cells last, before high I.R.s crap the bed?

Don’t skimp on lithium-ion ‘anything.’


Agreed. I just bought a few of the cheapies when they were… cheap. For testing LVP, flashlight internal charging, things like that. GAs are still my go-to for actual use in non-monster flashlights.

I’ve got the same one, but about 4-5 years ago. I think that mine states 6800mAh and that it uses LG/Samsung cells, but they are hardwired in.

I leave it at my gal’s place, so she can charge up her iPhone during a hurricane outage, which will work for her.

Good stuff, but I prefer to buy power banks with replaceable bateries.

Latest is the Xtar PBS 2 with LG 50Es and that will work for me in emergencies.


This is what I do, I like it so much I’ll probably buy a second PB2S in the near future. I probably need to see if I can come by a few more 50Es first, finding those in stock is the hard part right now.

If you have good cells, they’ll probably last longer than the PBS2.

Always have a spare, if my way of thinking.

Two is one and one is none.


I’m going to concede to DLG and BAK making good higher capacity cells now. Molicel however gets much of it’s R&D from Canada.

I used to think the same way but it seems to me that preassembled powerbanks are generally cheaper and more compact than the raw cells would cost by themselves, at least from where most of us get our raw cells. I have an Aduro 30,000 mah powerbank that is pretty nice, supporting micro-usb/usb-A, usb-c, and Apple Lightning for both input and output. I don’t know if it does QC or PD. But it only cost $30 or so which would be $3 or $4 per cell not even counting the rest of the stuff, and I know it’s not going to pop open and dump cells all over the place

I’ve had terrible luck with single cell powerbanks but have done pretty well with the bigger ones.

FYI- Illumn.com sells the XTAR PB2S for $16.99.

Yeah PB2S is a 2 cell (2*21700 but still) rather than an 8 cell, Review here:


Yeah, but the operative concept is that their replaceable.

Fixed cells need to be topped up when the PB is depleted, so that was my point.

If one has a dozen 21700s at their disposal and a way to charge them up via solar, in a SHTF scenario, they’re golden.