I have been given or purchased a number of various USB power banks. These power banks are designed to replace the AC outlet with a USB power supply so that when you're in the field and do not have access to an AC outlet, you can still charge or power your USB powered device. The ubiquitous cell phone is our main USB powered/charged device, but there are other things, such as Bluetooth speakers, which also require a 5 Volt power source to recharge.

But, what does it mean to have a 10 amp-hour (Ah) USB power bank?

When testing batteries for amp-hour ratings, the battery is expected to be able to run down to ~ 80% of its nominal voltage and still be able to be properly recharged. So, 80% of a 12V is ~ 10.5 Volts. Actually, 80% of 12V is about 9.6V, but since a 12V battery is really a 12.6V battery and the cut off is usually considered to be around 10.5V, this comes out to ~ 83%.

The Ah rating is normally found by running the battery down to ~ 80% of its nominal voltage with a current 1/20th of its designated Ah rating.

So our tests are run with a current of 1/20th the printed rating on the battery. We want to see if the battery will stay above 80% of its voltage after 20 hrs, which means it passes and is at least that Ah rating. This is not the way we normally use a battery. This is only the way the manufacturers have agreed to test the batteries they make.

Now, here's the problem. The USB power standard is that it will supply the rated current at 4.75 to 5.25 Volts. If the voltage falls below 4.75V, the USB power source is considered out-of-specification and the USB powered device may not function correctly. Yet, 4.75V is 95% of 5V and ~ 90% of 5.25V, so right off we have a conflict of definitions.

The question is then how have the various USB power bank manufacturers decided to define their Ah ratings, especially when they say the device is a 10 Ah USB power bank. I would expect that the device would then supply one amp for 10 hours or two amps for about 5 hours. This is not how the Ah rating of a battery is defined, so how is it calculated? My guess is that they are not using an amp-hour rating based on the USB five Volt specification but simply on the internal Li battery.

I tested two such devices I have, one being a UNIFUN 10400 mAh (10.4 Ah) power bank the other being a 2 Ah power bank. Since the 10 Ah bank came with two ports, one supplying 1 amp and the other able to supply 2 amps, I tested both ports at their specified max current rating. I will only be reporting here on the UNIFUN power bank.

If this were simply a battery, I would expect the voltage to gradually decrease until it went below the USB cutoff voltage of 4.75V. But, since I know of no battery that can start at 5 - 5.25 Volts and maintain a voltage above or at 4.75V, I was curious to see what was going on.

What I found with the 10.4 Ah device is that the manufacturer has a circuit inside the device that keeps the voltage above 4.75Volts until the internal battery falls below some minimum safe voltage. Once that happens, the USB power is shut off by a safety circuit.

In the case of a 3.7V Li-Ion battery, this means the voltage is probably allowed to drop to 3.0V because this what every reputable Li-ion battery manufacturer states as the lowest safe cut-off voltage.

The output voltage of the 10.4V power bank fluctuated in a saw tooth pattern from about 4.95V down to 4.75V. At that time, the circuit cut in to boost the voltage back up to ~5V. The test graphs show the voltages to be ~ 0.11 lower than what I have stated because of the test setup.







In this setup we see the yellow power bank with a USB V/A (USB Voltage/Current measuring device) plugged into it, then going to a resistor switch bank, which is presently bypassed to feed directly into the computerized battery analyzer on the right side of the photo. The voltage and current, as displayed on the USB V/A meter, are 4.75V and 0.99A, while on the display of the computer it says 4.64V and 1.00A. I suspect that the added wire and connections added a bit of resistance and thus a voltage drop going to the CBA IV. Nevertheless, the device is putting out a voltage within the USB standard at the specified current. The CBA will tell us how long the power bank was able to supply 1A from the 1A USB port before the protection circuit shut off the power to the 1A USB port.

NOTES:
After ~ four hours the lowest voltage got down to 4.72V on the USB V/A meter.

The results of the 1A test is the green line in the graph below. Result = 6.26 Ah







As can be seen on the above graph, the unit tested to be about a 6.26 Ah power bank when the 1A port was used supplying 1A. Do not worry about the “FAIL” rating since we’re simply trying to determine exactly what it means when a device such as a USB power bank is rated at some amp-hour rating.

10.4 Ah Power Bank 2A Test (The turquoise curve above):
I recharged the 10.4 Ah power bank over the evening and when fully charged, disconnected it from the charger. The next test done was the smaller 2.8 Ah power bank, then the 2A test was done for the 10.4 Ah power bank.

With this test, the voltage drop was greater. It was around 0.24V less on the CBA IV test device compared to the output of the USB power bank and the voltage. Even measured right at the power bank, it was lower than 4.75V most of the time. Being a two-amp test, if batteries were linear devices, one would expect the power bank to last less than half the 1A drain rate, or less than ~3.13 Ah. However, since the Ah rating of a battery does not follow a linear decrease, we’re never sure what it will test for higher, or lower, currents. The results are shown in the graph above. The rating was measured to be 5.79 Ah.

The next test done with this power bank was run at 500 mA (the purple curve above) and it should be close to the claimed 10.4 Ah but was measured at only 6.37 Ah. This means that it was only running for about 12 hrs 45 min., not the expected 20 hours.

One last test was done running the unit at 350 mA. If it lasts for 20 hours, that would mean the rating of this unit would be ~ 7 Ah. In the graph above, this test is shown by the red curve. This test was a little lower than the 500 mA test coming in at ~6.31 Ah.

The conclusion is that this device is not a 10.4 Ah USB power source. It is close to a 6.3 Ah device, since running it at 350mA, 500mA, & 1 A shows that it is consistently in the 6.2 – 6.3 Ah range which is quite acceptable but certainly not 10.4 Ah. The 2A test came in at about 5.8 Ah and that’s more than acceptable for a 6.3 Ah power bank.

I’m not concerned about the voltage drop for any USB Power Bank since I figured, as a charging device, it is supplying the required voltage at its output and it is the responsibility of the device needing to be charged to supply a USB cable capable of supplying the required Voltage and current needed. Unless, the voltage at the ouput port of the power bank is below the USB specification.

My guess is that no matter what the manufacturer says about their USB power bank, you can figure that you will get only around 50 – 60% the charging time claimed. So, for a 10.4 Ah device figure it will be able to supply 1 Amp at 5V for about 5.2 – 6.2 hours. This means that if your phone is a 2 Ah device, you will probably be able to charge it twice with this 10.4 power bank but probably not three times. This is all probably due to using a 3.7 V Li battery and boosting the voltage to ~ 5 Volts, plus having safety circuits monitoring the internal battery so it doesn’t over heat or get too low in voltage. All of this uses some battery energy/power which is then unavailable to charge your device.

I contacted the manufacturer and told them of my test results and their response was as follows:

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Amy’s technical information is in line with my findings. If the battery inside is a Lithium battery of some kind, then its nominal operating voltage is 3.7 V. Thus there needs to be a boosting circuit to get it to the USB standard of 4.75 to 5.25 Volts. The best switching power supplies are about 90% efficient so we lose at least 10% in this circuit. Next, there needs to be a voltage monitoring circuit and a protection, low-voltage shut-off circuit, I believe Amy called this the “Security” circuit. All of this, she says, reduces the amp-hour rating to about 62%. The big “loss” is from the conservation of energy she mentioned. This is “simple” physics, meaning we have the same power going from 3.7 Volts to 5 Volts but the Ah rating is reduced: P=IV or Power (Watts) = Current (Amps) times V (Volts). Amy shows us that the power of the 10.4 Ah unit is: P = IV = 10.4 A * 3.7 V = 38.5 Wh. Now we can’t gain or lose power, so when the 3.7 V is converted to 5 Volts the Amps have to decrease so we have (I = P/V) or, following the units, we see: Ah = Wh / V. Using the numbers, we have 38.5 Wh / 5 V = 7.7 Ah. Amy then tells us that the additional circuitry (the voltage inverter and security circuit) reduces the deliverable energy to only 84%. So of the possible Ah of 7.7 AH we only get 84% of that at the required 5V, or about 6.5 Ah.

Looking at my experiments the best I got was at around 500 mA at 6.4 Ah. (The best was 6.37 Ah) which for experimental to match theoretical is extremely good.

Also note, when charging a 3.7V Li battery, there is another set of circuits inside your phone or other device to change the 5V into the needed voltage and another circuit to protect the battery, such as a high voltage cut-off and temperature-monitoring safety circuit, so the power is probably reduced by another 84%.

So here’s my rough rule-of-thumb: Take whatever the Ah rating is stated to be on the USB power device then figure that the best deliverable power/energy will be about 50% of that.

Thus, the 10.4 Ah will be able to charge a 2 Ah cell battery 10.4 * .5 = 5.2 Ah and then 5.2 Ah / 2 Ah = 2.6 times. Round that down and it might just be able to charge it from full depletion to full charge two times. I always round down when figuring things like this to be on the “safe” side. In an emergency, it’s always better to have more power than you figured on rather than less.

For a 5 Ah USB power bank, we can see that this comes close to ~ 1.5 times, probably only once for a 2 Ah phone battery and if that phone battery is 2.7 – 3 Ah then this USB power bank will only manage to charge the cell battery once and then only if it is not fully depleted.

If we want to be ready for that power outage that will last for days or weeks, we need as much power as possible and a way to recharge our batteries as well. A 16 Ah USB power bank will give us ~ 8 Ah, for a 3 Ah Cell battery we get ~ 2.6 times, and for a 2 Ah cell battery we might get 4 full recharges (but probably only three).

I hope this helps explain what we can expect from a USB power bank when all we can see is the Ah rating of the Li battery inside and not what it really means for actual recharging and/or usage with a device that requires 5 Volts.



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