By Barry Kauler
This is a topic that needs to be explained before reading this page. The storage capacity, or energy, of a battery is usually specified in mAh, or so many milliamps of current over one hour. However, this is an awful way to specify energy. A 12V battery with a rating of 1000mAh, actually stores over three times as much energy as a 3.7V battery rated at 1000mAh.
We run into this difficulty with the batteries tested on this page, as the USB voltage is 5V (or thereabouts), whereas the internal battery is typically 3.7V.
Energy should really be specified in Wh, or Joules. "Wh" is volts*amps for one hour. I will try to do meaningful energy tests based on Wh readings.
|miniisw SW-5W80 Foldable 5W 8000mAh
Solar Powered Panel
|KP-8000 Foldable Solar Powered 5W 5V 8000mAh Dual Port Li-polymer Power Bank||5
|10.5W USB Output Foldable Portable
Solar Panel Charger
|Miniisw SUNWALK-070 Portable Foldable
7W Solar Panel Charger
|WN-02 10W 2A Fold-up Solar Panel
Power Battery Charger
|Miniisw 6W Folding 1100mA 4-Solar Panel Charging Power Bank||6
|Miniisw SW070B Portable 7W 1050mAh Solar Panel Battery Pack for Cellphone||7
|Miniisw SW-050 Portable Foldable 5W
Solar Panel Charger
|W-01 6W Portable Monocrystalline
Silicon Solar Power Battery / Cell Charger for
Cellphone + More
|7W Folding Solar Panel Charger for
Mobile Phone + Camera + More
|Miniisw SP6W Portable Outdoor Folding 6W Solar Panel||6
|ZNOODA SW-050 4W 4-Section Folding
Solar Panel Power Battery Charger
|WN-07 4W 800mA Fold-up Solar Panel
Power Battery Charger
|WN-09 5W 1A Fold-up Solar Panel Power
|CS-450 Portable Folding Solar Power
Charger w/ USB Cable for Cellphones / Camera + More
Weight of the Solarol is 400g total, including light globe, USB cable and carry case.
The panel is rated at 4W, which is reasonable.
Opened, the battery hangs at the bottom, and measures 70cm x 19cm. A bit longer than I would prefer, when hanging off my backpack.
Here is a youtube video:
|Extremely well made padded nylon
carry bag. Inside is the Solaroll itself, a
USB-to-microUSB adapter cable, LED with 3m cable,
user manual and plastic hanger for the LED lamp.
The all-up weight is 405g. But, compare the 175g Solaroll with all the folding panels at the top of this page -- the Solaroll is by far the winner. Most of those folding panels are just that, panels, no battery, which makes the Epak even more impressive.
The case is just too lovely, I have to carry the whole package in my backpack!
This is a 2600mAh lithium-ion battery, with plug-on 1W LED lamp. It cost US$10.56, postage-free:
I bought this for three reasons. First, one of my solar panels (without builtin battery) can be plugged into this. Second, at night I can use it at night for illumination in my tent or room, or thirdly, carry it around as a regular torch. Weight, including lamp: 144g.
The lamp has on/off switch, battery has charging LED.
This is just what the doctor ordered: the Charger Doctor. Rated up to 7.0V and 3.0A, plug this between panel and battery to see exactly what your panel is putting out!
This lovely little device cost just US$3.98, including postage:
|When these batteries are new, it is
recommended to give them a good charge. Here I have
it plugged into a USB port on my laptop, and using
the Charger Doctor to see what is going on.
The display flips automatically between showing volts and amps, and it is showing the charging current is about 0.25A.
Well, it has been dropping, now at 0.21A.
The battery wasn't flat when new, but if it was then I would be looking at 11-12 hours to fully charge (from laptop USB port).
||I have used the
nominal voltage of 3.7V in energy calculations on
this page, however in actuality the battery voltage
is "all over the place".
For "3.7V" Lithium-polymer batteries, the voltage can be between 3.4V and 4.1V, as this tutorial explains:
I am hoping that using 3.7V is a reasonable average value for the purpose of energy analysis on this page.
|I need a second opinion, so I bought
another charge monitor. This one has a LCD display,
so will likely have less power drain than the
Charger Doctor (which has LED display), plus it
records the accumulated energy, in mAh.
This unit is the KCX-017, cost US$8.55 from here:
Nup, still get 0.06A with the "1W" lamp, about the same as above.
|The accumulated mAh
is very useful. Right now I am charging the
"2600mAh" battery, will find out exactly energy
needed to fully charge it:
|The bad news:
||Using the KCX-017
charge monitor, the Epak lamp and an AC power
charger, I have put the "2600mAh" battery through a
few charge-discharge cycles.
it takes about 1060mAh to charge the battery, which is 5.1*1.06 = 5.4Wh
I only get about 630mAh out, which is 5*0.63 = 3.15Wh.
That is really awful. The actual capacity is about (3.15/9.6)*100 = 33% of that claimed.
|The energy loss is
also bad news: (3.15/5.4)*100 = 58%
Losses would be incurred at three places: switching converter voltage step-down at input, storage loss in battery, switching converter step-up to 5V at output.
I hope other USB external batteries are not this inefficient!
I chose this for testing as its power rating is comparable to the Solaroll.
Right off, the small size and solid construction creates a good impression. Folded, it is only 18 x 17cm, opened it is 18 x 40cm.
The small size does immediately make me suspect that the output will not be anywhere near 5 watts, unless they are spectacularly efficient solar cells -- which is highly unlikely.
I eagerly weighed it, and it is 230g, same as officially claimed. Heavier than the Solaroll, but let's see if its other features tip the balance in its favour...
It is solid plastic, and doesn't seem to have any "smart electronics", just straight from panel to smartphone.
it "quasi-folds", but with a big hole in the middle, awkward to pack in my backpack -- though perhaps I could wrap it around the towel.
|By attaching various resistor loads,
I obtained enough points to plot a
Notice the linear voltage drop as more current is drawn, reaching 4.62V and 0.825A, the maximum power output of 3.81W.
After that, it falls off fast, and see how it curves back -- that would be due to electronics in the panel. I haven't cut open the junction-box on the panel, but there is likely some limiter device in there.
|The bad news:
||Tuesday 26 August,
3pm, full-sun. It was cloudy earlier, bright sun
now, but of course a bit down in the sky. For each
of these loads, this is what I got out of the panel:
In other words, I am only getting about half, or less, of what the GP Solar panel is capable of.
The battery and smartphone will only charge at a certain rate, and will not take full advantage of what the panel offers.
|The good news:
pass-through charging is very good news indeed. It
means that while walking with solar panel over the
backpack, charging its battery, it can
simultaneously be charging my smartphone. What is
particularly good about this is that if the sun goes
behind a cloud or tree and solar charging stops, my
smartphone will continue to charge from the battery
in the Solaroll.
This is important! it means that the Solaroll's battery is acting like a buffer, so that charging of the smartphone will not be chopping on and off as the sun disappears and reappears -- such chopping may stress the smartphone circuitry.
Pass-through charging also means that the Solaroll battery does not have to be greater capacity than my smartphone.
|This is demonstrating pass-through
The AC adapter is supplying 0.73A, the lamp is drawing 0.21A. Thus, 0.52A difference is partly losses and part going into the battery.
if I disconnect the AC adapter, the lamp continues to work.
I replaced the lamp with my Xperia SP smartphone, and it charged at 0.32A.
The AC adapter continued to supply about 0.70A.
Thus, about 0.4A is losses and going into the battery.
Curiously, the Solaroll user manual does not actually state that it supports pass-through charging!
It should not be taken as "assumed", as most external batteries do not support it.
|More good news:
Even if the efficiency of the cells in the Solaroll is lower, it seems pretty definite that this panel is going to deliver the claimed 4 watts. It means that Companion, the manufacturers of the Epak Solaroll, have been scrupulously honest.
|Testing efficiency of
||I have shown above that the Epak
Solaroll supports pass-through charging, however
there are a number of caveats. One caveat: it is
possible that the pass-through mechanism may not
work in an efficient manner.
Consider what happens inside the external battery: if being charged from a 5V panel or AC adapter, the voltage has to be stepped down to the battery voltage (3.7-4.2V). For the 5V USB output, a separate step-up is required. This is normally achieved with switching converters.
If you want to see what these voltage-converters look like, Tim's Blog has pulled apart a cheap external battery:
So, going through step-down-converter, store-energy-in-battery, then out through step-up-converter, each step has a loss, and I have got only 58% efficiency in the external batteries that I have tested.
The big question is, if I have a load connected at the same time the external battery is being charged, is the energy being drawn by the load going through those three stages. Or, is the circuitry able to divert current direct from input to output, without the 3-stage step-down-battery-step-up?
To illustrate what I mean. Say that the battery is fully charged. The AC adapter is plugged into the micro-USB input of the battery. Let's say the Epak lamp is plugged into the USB output of the external battery:
...I started with a fully-charged battery, and ran the above setup for a couple of hours to see any variation. It settled down at 5.15V * 0.36A = 1.85W going in, and 4.42V * 0.24A = 1.06W coming out of the battery.
In summary, the Solaroll, when charging from the solar panel, with my smartphone simultaneously attached, does have the feature of acting as a buffer, continuing to charge my phone even when the sun fades away -- that part of it is good.
However, it does so in an inefficient way -- regardless of whether the external battery needs charging or not, current flows through the converters and you will loose 43% of the solar panel's energy -- compared with plugging the solar panel directly into the smartphone, you lose 0%.
|Further investigation of
||What I am really
looking for is a pass-through external battery that
behaves like a UPS (Uninterruptable Power Supply).
This is how the battery in a laptop works -- when
the mains is plugged in, and battery fully-charged,
current comes in from the AC adapter, and goes
through directly to power the laptop -- the battery
is bypassed. Current only goes into the battery when
it needs charging. The laptop runs from the battery
when the AC adapter is unplugged.
These guys are describing the same problem:
This is also a change from the foldable and rollable panels that I have considered so far.
I bought it for US$25.38 from:
I like the look of it, seems like it should be easy enough to store in a backpack. Weight is 222g.
The active area of the panel is 19.5x15.5cm, an area of 302cm2. That means I shall expect a power output considerably less than the advertised 5W!
I bought this from DX, for AU$12.45:
It weighs only 45g, incredibly small and light. But, as I have come to expect, that "1500mAh" specification is highly suspect.
I tested pass-through charging, but found anomalies, so need to re-do it.
Will update this page with pass-through results for credit-card battery soon.
I purchased this in Australia, for $AU119 plus AU$9.95 postage (receipted delivery):
The solar panel weighs 241g (8.5oz) and the V15 battery weighs 126g (4.4oz).
Here is the manufacturer's promotion video for the kit:
There is also a video for the V15 battery:
Thanks to the support for pass-through charging, this kind of setup is possible (LED lamp permanently connected to battery):
|First impression: this is one strong
I saw a video where they hit it with a mallet and it survived -- I can believe it. The panel seems to have two layers of aluminium, between what looks like epoxy. The cells are coated with a UV-treated protective layer.
Weight (on my kitchen scales): 249g (8.79oz)
|The peak power point is about 5.1V at
0.80A, which is 4.08W.
OK, it is not the claimed 6W, but still very impressive for the size of the panel, certainly much more than I was expecting, given that the panel is only 320cm2.
|The good news:
For its size, this panel is impressive, giving over 4 watts. Compare the area of the Voltaic "6W" panel, 320cm2, with the GP Solar and Epak Solaroll panels -- about half the area!
The explanation, basically, is that the thin-film rollable panels have much less efficient cells. Even so, I am still surprised at how much power I got out of this panel.
I did some reading-up on the efficiencies of solar cells, and found an explanation for the relatively high power output of the Voltaic panel.
The Voltaic "6W" panel is made with monocrystalline cells rated at 19% efficiency. The Epak Solaroll is made with a-Si amorphous silicon cells, that may have efficiency somewhere between 6% to 12.5%, as explained here:
On the otherhand, monocrystalline cells have efficiencies from 15% to 21.5%, and also last the longest (25 years):
In fact, I can roughly calculate the efficiency of the Epak cells, taking power output of the Epak and Voltaic panels to be about the same: (320/860)*19 = 7.1%
Wow, that really is something to consider when choosing a solar panel to fit on the limited real estate of your backpack!
|Now for the V15 battery bank. I gave
it an initial charge, and it took 760mAh to fully
charge (from an AC adapter).
I noticed the sophisticated charging algorithm, as the charging current started at 0.73A, dropping to 0.22A, then 0.13A, then 0.07A, then 0.05A -- this tapering off of the charging current is to top off the charge very nicely to the maximum that the battery can take.
Note, that 0.73A is the limit of the AC adapter, possibly an external battery would draw more if it could.
After charging, I discharged with the Epak lamp, as shown.
|Testing efficiency of
||The Epak external battery supports
pass-through, but with an efficiency of only 57%. In
theory, higher-quality voltage-converters could
raise this considerably, as could extra electronics
to bypass the battery under certain conditions. I am
eager to see how the Voltaic V15 performs...
The battery started off fully-charged, the AC adaptor plugged into its micro-USB charging socket, the Epak LED lamp plugged into the USB output port of the battery.
There are some errors here, due to the Charger Doctor and KCX-017 not being exactly accurate on their least-significant-digit, and some switching action on the charging input to the battery causing fluctuation in readings.
After one hour, I unplugged the lamp, and then I realised a potential gotcha, as the battery started charging (after unplugging the lamp), and 123mAh went in before charging stopped.
Question: was the battery draining in the above pass-through test? Or, did the charging algorithm decide that a bit more could be pumped into the battery?
To answer this question, I ran the pass-through test overnight. Starting with a fully-charged battery (as per above). Eight hours and twenty minutes, to be exact.
Now, the logic of my calculations starts to get a bit convoluted, as I then had to recharge the battery, and log how much had been drained from it in that 8 hours and 20 minutes, and include that in the calculation (for the record, a top-up of 182mAh was required after completion of the 8-hour 20-minute test). I will just post the end result here...
|Summary of Voltaic V15 power bank:
||I had high hopes
that this battery would have a capacity near that
claimed, and use high-efficiency voltage-converters.
However, neither is the case, this battery being a
bit better than others but not by much.
The battery stores 10.6Wh, about 71% of that claimed. I got out 2,000mAh at 5.30V, which I estimate would be enough to charge a 2300mAh battery in a smartphone -- but then, claimed storage of smartphone batteries may also be exaggerated also, so maybe a "so called" 3,000mAh battery could be charged -- but that is a wild surmise.
The storage efficiency, that is, how much energy you get out compared with how much goes in, is 64%
Then there is pass-through charging. There I was getting 62% efficiency.
||I found someone who
has posted a review of this kit: