As part of the last “gift voucher that needed spending”, I picked up a Powertraveller Powermonkey Discovery 3500mAh power bank. I didn’t really need one, but nothing else really interested me from the particular shop (which will go unnamed). The power bank cost AU$69.95, which is a little more than the Comsol which I just looked at. In fact, it makes the Comsol look like an absolute bargain when we consider that this is a single port 3500mAh unit.
It comes in a very pretty clear plastic shell. In fact, it’s arguably overpackaged, which already tells you that you’re paying too much. The package indicates the specifications – one concerning specification is the single USB output which is only rated at 500mA. For many modern smartphones, they demand charge currents from 800mA-1200mA, and a 500mA output just isn’t enough to get them charged quickly or in some cases, charged at all. The 500mA instead goes to “slowing down the rate of depletion” of the battery rather than charging it.
I suppose that already tells you that it’s a rather dated device, and it’s generally not going to be suitable for tablets (at least, if you want a good experience). Phones and music players are probably okay though.
This one also claims to be designed and developed in the UK, but engineered and made in China. I’m not sure that’s where I’d prefer my engineering …
The sides of the case have pretty pictures, and product blurbs. I’m not much a fan of the “now you can do anything” type of hyperbole … it’s really unnecessary.
The top doesn’t have much but the branding – the bottom illustrates the package inclusions. We’ll take a closer look at them in a sec.
The power bank itself positively looks like a sardine can. The bottom half is moderately solid aluminium, with the top covered by a plate. This seems to be adhered down – I couldn’t easily prise it up to get a good look inside – as a result, there won’t be any teardown. The top houses six LEDs – the leftmost one is bicolour (to indicate low battery) and the rest are green to indicate charge or discharge state. The lid itself depresses somewhat under pressure, so while the back is solid, the front is a little flimiser.
The “age” of the device can be seen in the connectors – just a single USB output, with a Nokia-style barrel connector for charging. No microUSB B cable here, so you better not lose the cable it came with. There is a single button to turn on and off the power bank.
In terms of provided cables and tips, there is a special “monkeytail” for the Apple phones and iPods, either because those devices need status on their D+ and D- lines on the USB connector to accept the charger, or there wasn’t the conversion tip available. Note that the USB to old-Nokia style lead doubles as the charge lead for the device, as well as the power bank to target device lead with the addition of a tip. Micro USB and Mini USB are included, which are most useful, as well as the “new Nokia” barrel tip. The others are for other particular phones and are less useful – but also note that other power banks reverse the adapter convention where the tips have a barrel connector on them and the USB cable have a barrel socket. This is not interchangeable with those tips!
Testing this unit was the same as testing the other units except for a little bit of an issue. The problem is that the output was rated for just 500mA, and my test loads are configured to be switchable for 1A/2A (well, really 5 ohm and 2.5 ohm).
As I didn’t bother building another load, I tested it at the 5 ohm load, resulting in a true load between about 800mA to 900mA. This would be what a fairly “hungry” modern smartphone would consume.
This is obviously above the design specification, but it didn’t kill the power bank or make it heat up significantly. But the result for efficiency and the regulation would likely take a strong hit.
Charge time for the power bank was about 7 hours connected to a Nexus 7 2A charger.
The results for capacity and efficiency for five runs is as follows:
PowerTraveller PowerMonkey Discovery 3500mAh 1A Run 1 2556.677227 mAh 73.04792077 % efficiency 1A Run 2 2601.556767 mAh 74.33019333 % efficiency 1A Run 3 2590.347717 mAh 74.00993476 % efficiency 1A Run 4 2585.301559 mAh 73.86575883 % efficiency 1A Run 5 2591.026392 mAh 74.02932548 % efficiency Mean 2584.981932 mAh 73.85662663 % efficiency StDev 16.89164569 mAh 0.482618448 % efficiency Range 44.87953977 mAh 1.282272565 % efficiency
The efficiency averaged just below 74% which isn’t a particularly good result, although it was overloaded. This was not helped by the fact that the power bank always blinks one LED to indicate the current charge state – this also consumes power. The actual usable capacity recorded was 2585mAh at a nominal 3.7v, which is in itself insufficient to charge the leading smartphones fully from a flat battery when you account for conversion efficiency losses in the phone’s internal charging circuit itself. Some phones are shipping with 3100mAh batteries, so being able to extract 2585mAh isn’t going to be enough to charge those.
Of course, loading it at 500mAh may give better results, but it might not be fast enough to charge a phone that’s in use. It’s a bit of a frustrating current limitation.
It’s clear from the graph that the power bank’s “regulation” really gave up not too long into the test. It may have hit its maximum duty cycle and maintained it merely 15 minutes into the test, and the voltage continued to fall as the lithium polymer battery continued to discharge. The voltage actually fell below useful levels at the last red-blinking stage, with the resistive load drawing lower currents as the voltage continued to fall. This still didn’t stop the power bank dipping below 4.5v measured voltage at the load (through 20cm of USB cable).
At the end of the discharge, the power bank was able to isolate the output, thus alleviating any possibility of overdischarge damage to the internal battery.
Update: Performance Test at 500mA
As I was feeling a little guilty that I couldn’t test the product within the design parameters, I specially built a test load for ~500mA consumption. A 10 ohm resistor was used, and the actual load was about 480mA. The results are slightly better for capacity, as less resistive losses in the cable would have been encountered and the switching converter would be operating at higher efficiency.
Powertraveller Powermonkey Discovery 500mA Run 1 2810.343085 mAh 80.29551672 % efficiency 500mA Run 2 2809.197654 mAh 80.26279013 % efficiency 500mA Run 3 2811.660121 mAh 80.33314632 % efficiency 500mA Run 4 2800.129705 mAh 80.00370585 % efficiency 500mA Run 5 2803.639776 mAh 80.1039936 % efficiency Mean 2806.994068 mAh 80.19983052 % efficiency StDev 4.904450003 mAh 0.140127143 % efficiency Range 11.53041654 mAh 0.329440473 % efficiency
The results were remarkably consistent – a range of just 11.5mAh! I didn’t expect that! Keep in mind also that Lithium batteries reduce in capacity as they are cycled, so it’s wise not to keep testing things over and over otherwise you will waste its life away in testing! That being said, no real signs of capacity degradation here (or at least, not certainly visible within a short five-run test).
The voltage versus time shows extremely good regulation under this regime. I suppose if your load only consumes 500mA, you will get very consistent voltages throughout, to the point that the meter reading error contributes to the test result.
I suppose it is a good power bank after all – but only for devices which aren’t drawing more than 500mA (i.e. older phones, MP3/MP4 players and smaller devices – definitely not the latest smartphones or tablets).
The Powermonkey Discovery seems to be a triumph of aesthetics over functionality. Its packaging was first rate, if not, a little overpackaged. Its price is not particularly attractive for the capacity that is offered, and despite the result implying the capacity is genuine, the efficiency of the power conversion is not particularly good although the test was conducted overloaded and it is likely that it would perform slightly better if this was not the case.
The output of just 500mA is not really sufficient for the more demanding tablets and smartphones of today, with just a single output, it’s not really that convenient for the traveller who may have multiple devices to charge. The use of the old Nokia charge connector is an “old hat” decision, and is now considered inconvenient especially if you lose the provided cable as they are not as easy to come by as a USB to microUSB cable.
In fact, the result from this power bank makes the Comsol power bank seem like an absolute bargain – I’m sure there are other better alternatives with specifications that meet modern expectations at the same price point. Not recommended.