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3.0 out of 5 starsRequires very careful consideration of your usage scenario - poorly designed AC invertor and USB port design
BySonarTechon August 19, 2016
UPDATED 12/12/2016: I edited my original review to correspond with the changes that ChargeTech made to better identify the capabilities (and limitations) of this product. That additional clarification greatly reduces consumer risk (as well as the risk for disappointment), so I updated my review based on these new, reset expectations. That said, the Portable Power Outlet does indeed do what it says it does, and depending on your particular usage scenario requirements, it could very well be an ideal solution for you. Above all, though, just remember that portable, battery-powered AC outlets will always have pretty significant limitations when compared to a household or generator-based wall outlet. Make sure you understand what your hardware will require versus what this power supply can actually provide and for how long. If you can do that much, you'll go a long way towards reducing any potential disappointment you may have when you actually put this battery pack to use.
This review will focus on the hardware design and performance of the ChargeTech - 27,000mAh Portable AC Battery Pack w/ AC Plug Outlet & USB Ports, model AC-27K.
From a typical consumer perspective, this battery pack seems intriguing, as it provides both AC and DC sources for your gadgets. One might immediately think that you could power "anything" with an AC cord, but even with realistic expectations of performance, you'll find that the scope of use to power AC equipment is fairly narrow and limited to 85W max. More on that later.
The ChargeTech iteration of this hardware appears to be a based on a Samya Technology AC-18K PCB with a larger internal battery capacity of 27,200mAh. The internal electronics, however, are identical to the AC-18K. Even the PCB is still silk-screened with Samyatech's company name and the original AC-18K model number information (see attached pictures of the mainboard).
INTERNAL DESIGN
The ChargeTech AC-27K Battery pack consists of two primary internal parts:
(1) The battery pack, which is built using 8 Panasonic NCR18650B Li-ion cells, each rated at 3400mAh. Each cell fully charges to 4.2V, and 4 cells are tied in series creating a total of 16.8VDC. The other 4 cells are also tied in series, then these two battery banks are tied in parallel. The total effective maximum capacity of this battery pack is 3400mAh x 8 cells = 27,200mAh. These are excellent quality Panasonic cells with a very high capacity for the 18650 form factor, but the battery cells (or banks in this particular design) are not individually managed or monitored. As a result, when one or more batteries in the chain eventually starts to deteriorate due to age, the entire bank will suffer. In designs like this, the battery pack is always only as good as the weakest cell - and this is the case anytime multiple cells are tied in series/parallel configurations with no cell balancing electronics. The best you can do as a hardware designer is to ensure that the battery banks never get into those situations where they get excessively abused outside of their ideal operational constraints (i.e., over charged, over discharged, shorted, too hot, too cold, etc...). The consumer's responsibility to ensure long battery cycle life and maximize return on investment is to take care of the battery pack, and keep it charged and stored properly -- no extreme temperature exposure (i.e., too hot and too cold), and don't store a completely depleted battery for any long period of time (i.e., > 1 day). If you can do this, then this battery design should be able to wear evenly and work well for 500+ charge cycles. Unfortunately for this device, the 2 individual 16.8V battery banks (each made up of 4 batteries in series) are tied directly together on the PCB, so no individual battery bank or individual battery cell management is possible. This means that the designers are hoping the batteries wear evenly over the charge/discharge cycles. Since they used high quality Panasonic cells, this probably isn't a significant worry, but it's still a considerable concern in my opinion, especially given the availability of decent (and cheap) battery charge and cell management electronics.
(2) The DC-DC convertor and the DC-to-AC inverter PCB. As I mentioned earlier, this PCB is made by Samya Technology. The DC side (16.8V --> 5V) of the board uses a fairly unremarkable DC-DC buck converter, and I measured an efficiency of around 82% which is typical for a cheaper DC-to-DC converter design. I was happy to see cleaner DC voltage at the USB jacks than I expected, but I did still see some high-frequency DC switching artifacts sneaking through to the output. The 5.0V USB output circuit is powered by a single 2.4A source, and that current is shared between both USB jacks – it’s not 2.4A per jack, it’s 2.4A shared between them both. That seemed a bit disappointing, given what it would take to actually provide 2.4A to each individual jack. I was actually able to pull 2.6A out of the left jack, and 2.8A out of the right jack before the unit shut down on me. Each jack is illuminated by 2 blue LED’s that transmit light through a white plastic bezel surrounding each USB port. One very bad design note about the two USB jacks is how they’re activated. Newer, smarter batteries electronically measure an attached load and provide charging current to an attached device when a load is detected (and the charging process is manually initiated by the user). Once the load stops pulling current, the charger automatically shuts down the port. Unfortunately, the AC-27K uses a physical switch imbedded in the USB jack itself to turn-on the DC 5V output. That means that anytime any kind of USB jack is connected to the AC-27K that it will be producing 5.0V, even if that cable isn’t actually connected to anything… So, now you have to remember to physically unplug the USB cable(s) from the AC-27K battery once your device is fully charged, or you risk running the Portable Power Outlet battery dry just through the process of DC-to-DC conversion (and powering those 4 pretty blue LED’s)… Again, Samyatech was just being exceptionally lazy here. Physical switches inside the USB jack?! I didn’t even know these existed, and I can't imagine why this particular design choice was made over a more robust load-detection and load-switch semiconductor solution.
The 120V AC inverter design is very disappointing. The AC-27K provides an extremely basic "modified sine wave" which has some very nasty surprises when it comes to powering modern power supplies like laptop adapters and other solid-state power supplies. The Samyatech AC-18K hardware designers went about as cheap as they could for this design, choosing to only provide a single positive and a single negative alternation for the simulated AC sinewave. As a result, you end up with something that looks far more like a square wave than a sine wave, and modern AC power supplies (i.e., laptop power adapters) are just not made to work efficiently with this kind of alternating square waveform. I’ve provided pictures of the actual waveform from the AC-27K for reference. Below, I’ll cover why this single-step AC “sinewave” design is so inefficient and provide some real-world examples of why single-step modified sine waves are essentially awful in every way (with the exception of design cost).
What’s the big deal? EFFICIENCY. Consider what’s happening here. You’re taking 16.8VDC, boosting it to 59V by pulling in a LOT of current from the batteries, then by using 2 FET’s, you cycle one FET to throw the voltage UP, and cycle the other FET to throw the voltage DOWN 60 times a second, effectively creating 118 volts of “fake” alternating current (AC). Then that “fake” AC travels to your laptop adapter where it immediately gets converted back down to DC voltage to power your laptop. This is a VERY inefficient process… You’re essentially paying a large toll just for the ability to use the AC plug, and the missing parts of the sine wave are murdering your power conversion efficiency.
The amount of power lost by using a modified sine wave instead of a “real” sine wave is obscene, and reduces the useful utility of this pack a massive amount. For example, an 85W AC load on the pack only generates 48.1W of DC through a 24VDC class 5 laptop adapter, and that equates to an efficiency of only 56%, which further equates to a power waste of 37.8W. That’s terrible. A few more mosfets and design changes to use a real sine wave and this thing would have been so much more performant. Unfortunately, Samyatech cheaped-out in the inverter design, and it really shows in terms of lost efficiency. Here’s some interesting math to go along with this:
My Laptop Power supply is a Phihong PSA120U-240 and is efficiency Level 5.
Real AC (from the wall) powering a Phihong PSA120U-240 Power Supply = 117.7VAC @ 0.0567A = 6.67W idle / no load (laptop not attached)
Fake AC from Chargetech Battery Pack powering a Phihong PSA120U-240 Power Supply = 118.2VAC @ 0.385A = 45.5W idle / no load (laptop not attached)
Difference using Real AC from the wall versus Fake AC from the AC-27K is 38.837W! Again, the ONLY difference there was AC from the wall or AC from the AC-18K. The voltage from both sources was the same.
You lose 39W of energy due to Samyatech’s cheap modified sine-wave AC inverter design…
When looking at Max loads the AC-27K can provide, here’s the losses:
Pin (Power In) = 95WAC = 118.2VAC @ 0.8037mA ---> (24.17VDC @ 2.20A = Pout (Power Out) = 53.174WDC) ---> 41.83W Diff ---> η (Efficiency) = 0.55 = 55% ---> Pwaste (Power Waste) = 43.51W
Pin (Power In) = 85WAC = 118.2VAC @ 0.7191mA ---> (24.17VDC @ 1.99A = Pout (Power Out) = 48.098WDC) ---> 36.90W Diff ---> η (Efficiency) = 0.56 = 56% ---> Pwaste (Power Waste) = 37.79W
η = Pout / Pin
Pwaste = (Pout/η) – Pout or Pwaste = Pin – Pout or Pwaste = Pout (1/η - 1)
Pin = Pout + Pwaste or Pin = Pout/η
That is HUGLY expensive in terms of efficiency. Don’t believe me? Do the measurements and math yourself using your own ammeter: Power your device with real AC, take a current measurement, then repeat the test with the AC-27K. Obviously, your mileage may vary depending on what you’ve plugged in – for example, a 60W curling iron which is just raw resistance isn’t going to lose much in terms of efficiency since there’s no AC-to-DC transformation occurring, but a laptop power supply will suffer dramatically when using a modified sinewave.
BUILD QUALITY & WHAT’S IN THE BOX
As a Hardware Design Engineer, I have an eye for electronics build quality. In my unit, I found 5 large solder balls around the FETs, a dirty PCB (flux residue), 1 misaligned LED and a set of untrimmed leads on an output capacitor that were only about 1.2mm away from shorting out to each other. The battery pack was pretty clean in terms of build, and there’s a thermistor tucked inside the battery pack to trigger a shutdown when the temperature gets too high for safe operation. The battery cells were Panasonic, but there is no battery pack or cell charge-leveling technology in the device whatsoever…so as soon as a single cell starts to deteriorate, it’ll drag down the entire battery pack with it. Once that day comes in the future when one of the cells die, the rest of the batteries will wear considerably faster, ultimately reducing the entire Portable Power Outlet to non-user serviceable trash. As with ALL battery-powered electronics, the clock starts ticking the day you buy and charge the battery. From that point forward, the onus is on you to properly maintain and use the battery to maximize its life. If you neglect your battery maintenance responsibilities, you'll just end up trashing your investment far sooner than you might hope. Take good care of this battery pack, and these high-quality Panasonic cells should serve your needs for quite a long time - just not forever.
The 2-part plastic case design is decent enough, with a single Philips-head screw hiding under the bottom label and several press-fits around the outer edge holding it together. Very easy to get inside, if needed (but not recommended, of course).
The miniature slide-switch used to enable the AC inverter feature is mechanical and may need periodic lubrication.
The included Samyatech-made, super-compact DC charging adapter is 1” x 3” x 1” in size and provides 40W (20V/2A tip positive) DC. It is auto-switching, international-voltage capable (110-240VAC~47-63Hz 1A Max). I didn’t take it apart for inspection, but it says it is UL (E225703) and CE certified. It comes with a nice, super-short 6” AC inlet cable, often referred to as a “Mickey-Mouse” connector (often used by Dell).
The box also includes an international AC plug adapter, a carrying bag, and a mediocre-quality owner’s pamphlet.
PRACTICAL APPLICATIONS
Wow, this one’s going to be tough to answer. What can you “really” use this for? Since it only provides a maximum of 85W, and its hugely inefficient modified sine wave inverter design could cost you up to 45W of lost power, there aren’t near as many opportunities to use the AC plug on this as you might hope. Low-wattage curing iron? Very small heating appliances? Low current wall-warts? As I said earlier, make sure you know what you need versus what you're going to get out of this battery pack to avoid possible disappointment.
I took two real-world measurements, starting with a completely recharged AC-27K (graphs attached):
30W AC Portable Fan: Operated for 4 hours 55 minutes before AC-27K battery shutdown automatically
85W Laptop Power Supply: Operated for 1 hour 23 minutes before AC-27K battery shutdown automatically
95W Laptop Power Supply: Operated for 1 hour 5 minutes before AC-27K battery shutdown automatically
Also, think about the stress the modified sine wave is putting on your hardware. This square wave is brutal compared to a sine wave, and that does stress the electronics in your power supplies. Most AC motors and heaters won’t care so much, but modern power supplies aren’t designed to function well with a modified sine wave. Keep that in mind when trying to understand if this is a good solution for you.
OWNER’S BOOKLET
There’s a fair amount of misleading and/or confusing information in the owner’s manual. For example, page 5 says “The Portable Power Bank is capable of powering most 120V AC products that use 95W or less”, but the specifications on page 8 state that the maximum AC power output is only 85W. There’s also lots of editing errors scattered throughout the manual, but it is still quite useable and gets the job done.
SUMMARY
The hardware design isn't ideal. The battery cells/banks aren't individually managed, the AC inverter is terribly inefficient, the 2 USB jacks share a single 2.4A source, and the use of mechanical switches inside the USB jacks to trigger USB voltage is just plain silly. The battery cells are about as good as you can get, but many improvements could have been made to this design to make it more robust and efficient. It seems they opted to cost over quality, and I consider that fairly disappointing.