On an earlier post about power bank endurance and modification, Joy Test made a recommendation in the comments for me to test a Xiaomi, otherwise known as Mi, branded power bank. This is a Chinese manufacturer, which is most widely known for their high value phones.
I don’t normally chase products to review, as I often have no need to purchase so many products (especially of one type), but I was swayed by the rather positive things I have been hearing about them. Apparently, they set a high standard for other power banks to follow – lets see if that’s the case.
The most common unit is the NDY-02-AD Silver 10400mAh unit. The unit itself is listed for RMB69 on their website, which should be around AU$13, however, it’s almost impossible to find one at that price. Instead, I opted to pay a bit more for an Aussie seller to supply me the unit, at a cost of about AU$30. This might have been a wise choice, as it seems that counterfeit units are already hitting the markets.
My unit came in a nice matte cardboard box with a simple, understated, Apple-like grey print of their branding. The front had the logo, and the back had the mi.com URL.
On the side of the box, there were barcoded serial numbers and the model number.
On the opposing side, there is the product specifications and a scratch off verification code panel which allows you to check the genuine status of the power bank (more about this in the next section).
Inside the box, there is a short flat USB micro-B charging cable that also has the data pairs connected, the power bank itself and a Chinese manual. Luckily, for such simple devices, there isn’t really a need for a detailed manual (and you can start using it right away). The ports on the power bank are covered up with a white piece of tape in transit.
The power bank is made of an aluminium shell, with a similar finish to the “Apple” stuff. It’s quite amazing that there is such a quality feel to it, but it does also lend the power bank some level of protection against physical crushing. The ends, are made of plastic. Similar to the box, the front has the logo, the rear has the URL.
The front end houses a power button you can use to check the power status of the power bank. Four small holes allow for a white LED indication to be shown. A single microUSB B connector is used for charging, and a single USB-AF connector is provided for connecting the device to be charged. It’s a little disappointing to have just a single port, but for the price, it’s quite acceptable. If you need more ports, buy a second power bank! It’s also likely to offer the optimal charging condition as there is less risk of overload with just a single device connected at a time.
The other end features the models and specifications for the unit with the capacity in large letters, so you can’t miss it. The capacity itself is rated at 3.6v, but all of my testing is based at 3.7v, therefore the efficiency figures derived will be slightly lower than expected.
For the purposes of helping users detect a fraud (on the suggestion by a friendly reader from Germany), the weight of this unit was measured on a (cheap) set of scales at 252 grams.
Xiaomi have made a lot of quality claims on their website, including the use of LG/Samsung/Panasonic/Lishen cells, up to 93% conversion rate, minimum 6200mAh actual output capacity with many rigorous ESD and 50kg physical/mechanical stress tests.
Specifically, they also claim the availability of over temperature protection, over-current protection, over voltage protection, automatic load detection, over current protection and PTC protection in the battery cells.
I hope that we can verify some of these claims in action.
Genuine or Not?
No doubt some purchasers may have issues with their power bank that lead them to question if their power bank is genuine. One way is to perform a visual inspection of the device, following information provided by online websites, such as this.
Another method is to verify the unique code on the scratch-off panel. To do this, you need to first scratch off the silver coating on the label with your fingernail or a coin, to reveal a 20 digit code.
As many people might not be familiar with Chinese, and neither am I honestly, I will guide you through the process pictorially.
Visit http://chaxun.xiaomi.com and you should be greeted with this screen. Click on the purple power bank icon.
Then enter your 20 digit code, in four sets of five digits, starting from the top into the boxes from left to right. Then answer the math problem in the box with Chinese characters and click on the orange submit form button.
You should see a result screen like this if it is genuine. The screen provides a confirmation it is genuine, and the number of times the code has been entered. If the code has been entered many times, it is likely to be a fake. In this case, it has been entered one time (numbers from 1 to 10 in Chinese for reference: 一,二,三,四,五,六,七,八,九,十)
In case of it showing no tick, check that you have entered your number correctly. If you still have problems, then you may have been a victim of a fake!
Opening the Xiaomi power bank is pretty easy. The first thing to do is to carefully pry off the white plastic facia on the USB port side. This is secured to the chassis by clips and double sided tape. There is a white strip of label paper attached to the LED holes – these act as diffusers, making the indicator much less glary. The indicators themselves light solid/flash during charging, but only periodically flash the battery status during discharging to save power.
Then, you need to undo the four black screws that hold the black plastic shell in the aluminium chassis. Once it has been undone, you can give it a good shake and the innards should slide out.
Already, it is impressive to see the insides of the power bank as one of the claims is immediately obvious – the black lead that leads from the PCB to the battery pack is an NTC thermistor and is used to sense the temperature of the battery pack. It seems likely that the over-temperature protection is no lie.
Another thing that is very impressive is the use of orange mylar tape, which is normally used in proper battery pack construction (e.g. in quality OEM laptop batteries).
The PCB itself is a nice blue colour, and looks well made. The white component near the negative, marked with a 10, appears to be a fuse, thus corroborating their claim of having over current protection in the cells.
Flipping it over, we can take a close look at the cells.
There’s no lies here either, as it appears to be a genuine LG Chemical LGABB41865 cell. These cells are rated 2600mAh each, for a total pack capacity of 10400mAh exactly as stated. They didn’t even lie on rounding up… quite impressive. However, it seems that the cells do claim to be 3.7v, so by marking them as 3.6v, Xiaomi may be trying to gain a little bit more efficiency percentage in their literature.
The terminal tab spot welding looks pretty decent, save for some nicks in the heatshrink and in the tabbing on the left side of the right-most cell.
A closer look at the top side of the PCB reveals certain testpoints, and markings of PB02_MB_45_131230C. This suggests the PCB design may date from 30th December 2013. The PCB itself is dated Week 18, 2014. There is a fair amount of semiconductor transistors, diodes and MOSFETs on this side, as well as a 1 ohm SMD resistor, likely as a current shunt to implement over-current protection.
Looking at the other side of the PCB, there are more assorted components, but most interesting is the use of an ABOV MC97F1204S 8-bit Microcontroller (Week 14, 2014) to control the whole operation, and the Texas Instruments/BenchmarQ BQ24195 charger and synchronous boost controller. No doubt, careful design using a quality solution like this should see good efficiency results. There also seems to be several multi-layer ceramic capacitors to keep the power quality under control, although one spot seems to be unpopulated.
Curiously, they haven’t gone for a shielded inductor, instead opting for what appears to be a potted inductor. This is probably to ensure that there isn’t any coil vibration, thus limiting the audible noise generated. Also visible is the tops of the cells, and it seems that the heatshrink around the second cell from the left has been damaged during manufacture.
In all, it’s not a flawless construction, but it does imply a good quality design and attention to detail. It’s probably a good power bank based on the teardown alone.
Performance testing was run based on the new rig and methodology used in previous tests. This included the use of a Keithley Model 2110 capturing data on voltage and current in dual measure mode, with the power bank loaded by a resistive test rig that I built myself. Data was recorded at a rate >1 sample per second, and integrated in software to determine effective capacity delivered to load with some losses (at most, a few percent) due to voltage drop in short USB lead and banana cables neglected.
Charging the power bank took about 5.5 hours using a 2A rated charger.
The capacity results are as follows:
|Load (mA)||Run||Capacity (mAh)|
|Load (mA)||Run||Capacity (mAh)|
|Load (mA)||Run||Capacity (mAh)|
Keeping in mind that the calculations performed are assuming a nominal battery voltage of 3.7v rather than the 3.6v, at 500mA, the power bank put out a very commendable result of 9707mAh, for an efficiency of 93.3%. At 1A, this dropped slightly to 9461mAh, for an efficiency of 91.0%. At 2A, the resulting capacity was 8695mAh, for an efficiency of 83.6%.
Rebasing the calculations on 3.6v, as Xiaomi had published, brings the efficiency ratings up to 95.9%, 93.5% and 85.9% respectively.
It is very commendable that at no point did the power bank even near the minimum 6200mAh promised, and even at the 3.7v, at 500mA it was capable of reaching an efficiency slightly greater than the 93% claimed.
A look at the voltage profiles suggests that operation at 500mA and 1A were a breeze, with the voltages holding relatively stable, fairly close to the 5v mark. At 2A, the power bank’s regulation began to suffer a little more, but still delivering very solid results without ever getting close to the 4.75v limit of the USB specs. In short, no problems there.
The ripple and noise performance of the power bank was a little perplexing. It seems the power bank operates in two phases – one where there is a fair amount of ripple for a short time, and the other which is a much longer phase with a lot less ripple. As a result, the ripple figures reflect the peak-to-peak average of the tall phase. I did not bother measuring frequency because of this.
At 500mA, on the longer timebase, it is clear how there is a repetitive “noisy” period, and less noisy periods inbetween. The average peak to peak ripple is 91.73mV at the 5ms timebase, and 100.1mV at the 500us timebase (as it captures the spikes more cleanly). This is a good result, as it remains below the ~150mV p-p that most mains powered phone chargers put out.
At 1A, the ripple actually falls to an average of 18.89mV peak-to-peak at the 10ms timebase, and 23.78mV peak-to-peak at the 500us timebase. This suggests that the converter design may have been optimized for a 1A load range. This is surprisingly low, even below that of the ATX 50mV requirement.
At 2A, the ripple grows again, to an average of 98.19mV peak-to-peak at 5ms timebase, and 106.9mV peak-to-peak at 500us timebase. This is still a mighty good result, still below the 150mV peak-to-peak of wall chargers.
I think the results speak for themselves. If you’re looking for an inexpensive, simple power bank that performs well, this is the one! It looks to be well designed, and feels like a quality item. It’s built around quality cells, and a quality power conversion solution and has a very good level of efficiency and output voltage stability. The noise levels are very acceptable as well. The fact you can get it at AU$30 or even less just boggles the mind.
I think it shows just how much China can be capable of, when they take some care to design a quality product, rather than just putting out something passable for cheap.
This one definitely gets my recommendation.