Channel Wisdumb Investigates: Power Banks
Do you have a portable power bank that doesn't seem to perform as advertised? Do you suspect you are not getting the Watts you paid for? You're not alone. Inspired by your local news team, Channel Wisdumb went undercover into the seedy underworld of power bank specsmanship to uncover the truth. What we learned will shock you.
By publicly admitting this, I am risking my career. I may be asked to turn in my electrical engineering badge and soldering gun. I admit this here as a public service so others don't make the same mistakes I did: I was dumb. I wasn't thinking. I know better. I expected a 30000mAh rated power bank to be able to completely recharge my 5000mAh phone from empty about 6 times. When I could only get about 1.5 recharges of my phone out of that battery, I knew something was as fishy as a tuna cat food tin floating in a vat of sardines. I decided to dig deeper.
(Ok, that's it for the film noir theme. I don't know how to work this upcoming math and electricity lesson into that framework.) My first, and biggest mistake was not recognizing that a 30000mAh rating (or 30Ah rating for convenience) tells you very little. From an engineering perspective, that is the current capacity of the battery. What is more important is the energy stored in the power bank. Note that energy is different from power. Power is an instantaneous measure. Energy is power over time. When you talk about a 100W lightbulb, that 100W is the power. If you run that lightbulb for 2 hours, you would use 200Wh (Watt-hours) of energy (100W times 2 hours). (Don't run that 100W bulb for 2 hours- get an LED bulb!) Now for a little more engineering math, power = current * voltage. Therefore, energy = current * voltage * time. 4 variables to consider. We can tell from the units, that 30000mAh gives us two of the variables already multiplied together, current and time. But we still need the voltage to calculate the energy of the pack. I had just assumed that the power banks were specifying the current capacity at the 5V level that USB typically runs at. So naive. So dumb. It turns out power banks are specifying the mAh rating for the internal battery itself which does not run at 5V. most Li-Ion batteries in these packs run in a range of 3.6V to 3.85V, with 3.7V being a common choice. Some power banks specify their battery voltage, but most don't.
How's your head doing with the electricity lesson? This math stuff isn't too scary. I hope.
If I assume 3.7V for my 30Ah pack, that gives the pack 111Wh of Energy capacity. Now we are getting closer to some answers. To use that 3.7V battery bus, the voltage must be converted up to 5V for output over the USB port. Then my phone needs to take that 5V and turn it into 3.8V to charger its battery. My phone battery energy capacity is 3.8V * 5000mAh = 19Wh. So, already we are now looking at a theoretical best-case number of recharges as 111Wh/19Wh = 5.8 times. This difference is entirely due to the specification difference between my phone's battery voltage and the power bank's battery voltage. This is why rating power banks on the mAh capacity is dumb. Hear ye, hear ye, Ryan Wisdumb hereby decrees that power banks should advertise their Wh capacity from now on.
But still, there is a big gap between 5.8 theoretical recharges, vs. 1.5 times actual recharges. There is another piece to the puzzle: efficiency. Every voltage conversion comes at a cost. The best DC to DC converters can achieve between low and mid 90% efficiency numbers. You probably don't want to pay for that kind of efficiency in consumer electronics, though, so let's assume the converters in your phone and the power bank run at a still good 88% efficiency. For the first conversion in the power bank, the 111Wh is reduced to 97.7Wh (111 * .88) to bump the voltage up to 5V. The phone then has an additional loss through its DC to DC converter dropping the 5V down to 3.8V, so that only 86Wh of energy can reach it's battery (97.7*.88). That brings us down to 4.5 recharges.
But wait, there's more. Do you ever notice your battery pack gets hot when recharging a device? The batteries themselves are not 100% efficient and lose power to heat on both charging and discharging. I don't have a lot of detail on how much they lose, but I have seen specs published by some manufacturers that overall efficiency for a good power bank is 80% and a mediocre power bank is 60%. These numbers include the losses through the DC converter. So, if we start with the 111Wh pack and assume 80% efficiency, we are down to 88.8Wh. If we assume 80% efficiency on the phone battery side, we are now at 71Wh, or 3.7 full recharges. If we assume 60% efficiency on the battery pack and 80% on the phone, that drops to 2.8 times. If both phone and power bank are 60% efficient, that is as low as 2.1 times
I view that 2.1 times as kind of a worst-case scenario. I'm fairly certain my phone is more efficient than 60%. I don't know about the power bank, though. Either way, that is still more than I was actually getting out of the power bank, so despite my lack of thought about specs and efficiency of the devices, there is still something wrong.
Before going through all this analysis, there was another datapoint that was bothering me. Mrs. Wisdumb gave me a heated vest that can be powered by a USB power bank. I went out and bought this 40000mAh battery pack for about $20 that I mistakenly figured would power it all day.
On my first use of it, I only got about 6 hours of use before the battery was drained. I figured that the vest could not draw more than 2.1A and therefore a 40Ah battery should last about 19 hours. You should know by now, that this thinking is flawed. First of all, the vest runs at 5V, so I should be thinking about it in Wh for comparison. 2.1A@5V= 10.5W which would equate to 14.5 hours runtime on the 40Ah*3.84V battery in that bank, assuming 100% efficiency. If the battery pack were 60% efficient, that should still provide at least 8 hours of heat. But it didn't. The plot thickens...
Now the undercover investigation begins. I imported a specialized spy device to help me get to the bottom of this. Another way to put it is I ordered this USB power monitor from AliExpress. I paid less than $5 for it, which is really unbelievable. It works very well, too. The first order of business was to insert this monitor between the vest and the power bank and measure how much power it was delivering. The vest is nice for measuring, because it is converting 100% of the power to heat, so it is effectively 0% efficient or 100% depending on your perspective. On high, the vest actually draws about 1.8A, or 9W of power at 5V. I plugged the 40000mAh battery pack in and let it run until it was empty and shut down.
The power meter recorded 71Wh of energy went into the vest. As a reminder, the pack supposedly has 152Wh of energy in it. This means the overall efficiency of the battery pack is only 46.7%!!! I was pretty PO'd at this finding, though it confirmed my suspicions. I immediately boxed it back up in anger and hit the return button on Amazon as I only had a couple days left to return it. Had I been a little less dumb, I would've taken the time to recharge it with the meter in place and see how much energy went into the pack to recharge it. As you will see from the testing on my next pack, this can be illuminating.
Because I still wanted a large power bank to run my vest all day, I ordered this 40000mAh pack that was also about $20 at Amazon.
I liked it better than the previous one as it was smaller and had a digital display rather than 4 LEDs. That display saved me some time as I didn't have to fully charge it to run my test. The pack arrived and said it was 68% full, so I put it straight into the test with the vest and ran it to empty. Once it was empty and shut down, the power meter recorded about 47Wh of energy went into the vest. Since that represents 2/3 of the capacity of the bank since we started with it 2/3 full, we can extrapolate to a full capacity of 47Wh * 3/2 = 70.5Wh. This is almost identical to the total power I got out of the previous 40Ah rated battery bank, and also means it is only 46% efficient. Or the specs are lying about the capacity.
This time I fully recharged the bank with the power meter attached. The data this test reveals is telling: 84Wh went into the pack to fully recharge it.
This is a dead give away that the problem isn't inefficiency, but rather the pack is not up to spec. If this were just poor efficiency of the DC converters and the battery waste heat, the power going into the pack would have been larger than the 152Wh and the difference in power would tell us loss due to inefficiency. Because the power was lower than the rated capacity, much lower in fact, we can conclude that this pack is nowhere near the 40000mAh @ 3.85V that they spec. Possible reasons are 1. they are just lying. 2. they are using extremely inferior battery cells that can't handle deep charge and discharge and limit the discharge to something like 25% capacity and the charge to 75% capacity resulting in a usable capacity of half of the total battery capacity.
That is 2 packs tested and 2 cases of suspected false advertising. A part of me wondered if I could trust my data or if I was doing something wrong. There's a first time for everything, after all 😉. I decided I would next test the more expensive 20000mAh, 65W battery pack that I got a few months ago specifically to have one that could power my new Canon R7 and recharge its battery in the field.
This battery pack, while also from an unknown Chinese brand was also about $20, but for half the rated capacity of the 40Ah packs I tested previously, so it represents a step up in dollars per W/h. This pack actually specifies its Wh capacity on its Amazon page: 74Wh. I took that as a good sign that they aren't trying to play games with spec. This means they are running their Li-Ion cells at 3.7V
This pack performed as I think a power bank should. It delivered 54Wh of energy to the vest. This gives it an overall discharge efficiency of 73% (54/74). Not great, but not bad! The next step was to charge it and see how much power it took to restore it to full.
I'll now get back to my original "30000mAh" pack that only managed about 1.5 phone recharges. It should theoretically have 111Wh of energy. It delivered 37Wh and recharged with 54Wh. So, it is more realistically a 11000mAh pack, not a 30000mAh. Junk. And just to see if the math works out, if my phone is 80% efficient, the 19Wh battery would require 22.8Wh out of the power bank to fully recharge. And what do you know- 37Wh / 22Wh is 1.68 recharges from that bank. Just about what I actually observed.
In conclusion, the low-priced end of the battery bank market is fraught with peril. Expect that brands you haven't heard of before are lying about the capacity of their banks. Products that list their Wh capacity or some other measure of total energy may be safer, based on my one data point. Reviews are unreliable unless they are documenting their testing with hard facts like I did in this blog. As I have never bought an expensive, brand name battery pack, I can't say if they play the same games with their specs or not. I hope not. Maybe someday I will get to test one and find out. In the meantime, look for a new post in the coming weeks with my measurements for every power bank I can get my hands on if you are looking for input on what to buy.
As always, I have received no compensation or promotional consideration for any of the products and services mentioned in this blog.
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