An interesting power bank based on the SW6208 chip for 8 18650 cells

A decent power bank for your 18650 batteries, although in my opinion it has a few drawbacks.

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Not too long ago, I reviewed a power bank kit for 21700 cells. Today's test subject has similar specs, but comes in a different case and is designed for 18650 batteries.

I bought a few at a time, twice, because the first order sold out almost immediately. I think they came to about $7.5 each with shipping. Or rather, the power bank itself costs $6.87 and shipping is $1.42, but the shipping cost doesn't depend on the quantity.


It looks like the familiar little brick, of which there are thousands.


The plastic has a carbon-fiber look. There are four tiiiiny holes for the LEDs at the top; if you look closely, you can spot them. Correspondingly, there are four little tubes on the underside of the cover to make sure the light only comes from its own LED.


The model and specs are printed on the bottom, but the seller clarifies that they simply used a case that fit, and the actual specs are different.


The end panel has:
1. Two USB-A outputs
2. One universal USB-C port
3. One microUSB input
4. A pair of LEDs.

There's a little button on the side. On one of the power banks, it was hard to press—maybe the plunger had shifted or something else was going on, I don't remember. It was a quick fix.
The button has several functions, described in the manual:
1. Short press - force power-on
2. Double press - force power-off.
3. Long press - activate low-current charging mode. This mode turns off automatically after 2 hours, or can be disabled with another long press. While charging at low current, the LEDs light up one after another periodically.
4. Long press - turn on the indicator LEDs, but I never managed to get them to light up. By the way, the previous power bank doesn't have LEDs, but it does have an output for connecting them, and I couldn't get those working after installing them either.


This power bank is sold in four versions, for 5, 6, 8, or 10 cells. The seller also offers a $1 discount on orders over $25—not much, but still.


A 7-cell power bank would have suited me better, since I was thinking of using 8 Ah pouch cells; two would fit vertically. But there wasn't a 7-cell version, so I bought the 8-cell one.


The design is very simple: metal strips along the sides, springs on one side and raised positive contacts on the other. You can use batteries with flat positive terminals and no protection, though cells with button tops might fit too—there's some room left.


There's a thermal sensor for overheat protection, but its wires are long enough that it ends up under the third battery. When installing the cells, you need to bend it so it sits between the second and third ones.


All four power bank sizes use the same board, rated for up to 22.5 W.


The product page describes settings you can configure:
1. Enable low-current mode
2. Enable flashlight mode
3. Set the capacity of the installed batteries, which is needed to display the charge percentage correctly.

In fact, the chip also lets you change the charge termination voltage to 4.2/4.35/4.4/4.5 V, and is generally pretty smart and capable.


The connectors, LEDs, and button are on the back of the board.


The board has:
1. SW6208 controller
2. XB8689 battery protection controller with built-in MOSFETs.
3. Four 8205L MOSFETs in the USB port power circuit, with a 5 mΩ resistor nearby for current sensing.
4. An unidentified resistor; I’ll assume it’s 1 mΩ, although the marking is odd. It’s awkward to reach in and measure it.


The controller supports a ton of modes and effectively includes everything needed to build a power bank, including the power stage.
The stated maximum power is 22.5 W, though there’s a catch: it’s only available in one mode. In the others, it can deliver up to 18 W continuously.


The datasheet includes a circuit diagram. I assume the power bank under review isn’t much different, but this one does show resistor values for batteries of different capacities. Someone once asked me how battery voltage is tied to the charge percentage. This is a good example: you set the battery capacity, and the power bank then calculates the percentage based on the capacity delivered.
The resistor values for selecting the charge termination voltage are also listed here, but there’s a catch: the protection chip must match the required voltages, or it will simply disconnect the battery. Too bad there’s no LiFePO4 mode.


It supports proper charging, with a trickle-charge phase at the beginning.

The chip can also work with a wireless charger and provides “smart” temperature protection. That is, it doesn’t just stop charging or discharging if things get too hot; it also does so if they get too cold, apparently in several stages.


As we saw at the very beginning, five LEDs are used for indication, but you can also connect a display that shows the charge percentage.


One thing I didn’t like right away was the steel contact strips.
1. The spring resistance is tolerable, at around 4 mΩ. Since there are eight of them, the current is shared between them.
2. But 45 mΩ for the busbars is way too much. So if you put a battery in the last slot, the parasitic resistance will be 45+45+4=94 mΩ!


There are five indicator LEDs: four show the charge percentage, and the fifth indicates fast charging.
In this case, during discharge:
all LEDs on — 75–100%
3 LEDs on — 50–75%
2 LEDs on — 25–50%
1 LED on — 5–25%
1 LED flashing — 1–5%

While charging:
all LEDs on — 100%
3 LEDs on — 75–99%
2 LEDs on — 50–75%
1 LED on — 25–50%
1 LED flashing — 0–25%

The chip can also control three or five LEDs to indicate the charge percentage.

On the left, normal mode; on the right, fast-charging mode.


For the tests, I used eight batteries:
2× LG HG2 — review
2× Vapcell N36 18650, 3650 mAh — review
4× Vapcell 18650 F35, 3500 mAh — review

According to the tests in the reviews, the total capacity comes to:
Ah — 2971×2+3610×2+3437×4=26.91
Wh — 10.83×2+12.99×2+12.61×4=98.08

I placed the temperature sensor between the second and third batteries. Also, it’s best to put the higher-current cells closer together: they’ll handle most of the load, and they’re more resistant to high temperatures.

The power bank weighs almost half a kilogram.



Protocol support is also excellent here. Naturally, it’s limited by the power rating, but even Power Delivery is supported, via USB-C, of course.


It’s a little odd that PD can deliver 9 V at 2.2 A, but only 2 A at 5 V.
Otherwise, everything is familiar: AFC, FCP, SCP. There are some quirks with the last one, though; more on that later.


As usual, I tested it with my phone. It’s pretty picky and doesn’t always request 12 V.
This is a little odd: the power bank supplies the requested 12 V without any trouble, but when I power it through another tester, it sometimes falls back to Huawei SCP mode and only 5 V. In principle, the difference is small: about 10–12 W at 12 V and about 10 W at 5 V, but it still looks suspicious.


The manufacturer lists the following protocols


Charging performance is also fairly good. For some reason, an unknown charger that also supports 12 V only gave me 5 V at 2 A. But when I powered it from a charger supporting QC/PD, it negotiated 9 V at the same 2 A. It behaved similarly when charging via USB-C.
It charges to 4.22 V. That’s not a critical issue; the important thing is that it then stopped charging correctly.


A few tests.
First, 5 V: I got up to 3.1 A at the output. It compensates for voltage drop in the wires: you can see the voltage drop at first, then automatically rise. The rated current is 3 A.


At 9 V, I got 2.4 A, also without any issues, since the rated current is up to 2 A in normal mode and up to 2.2 A in PD mode.


At 12 V, the maximum current was 1.8 A; the rating is up to 1.5 A.


The PD profile listed an 11 V, 2 A mode, so I tried that too. Because it was awkward to connect the load, though, I had to test it in QC mode. I did get 2 A, but not without some trouble.


I had to increase the load current gradually, but eventually got 2 A, albeit with noticeable fluctuations in the output voltage. I assume it would work fine in PD mode, but I can’t say for sure. I also measured the temperature here and initially thought overheating was the problem, but it turned out not to be.


The Huawei SCP mode caused a problem—not with the mode itself, but with sending the power bank the right command. In this mode, the power bank should deliver up to 4.5 A, but two testers absolutely refused to set it. The FNB48 changed the voltage in its settings, but the power bank didn’t change its output voltage. The small USB-C tester sent the command and the voltage even changed, but it went up instead, then reset to its original value.
Only when I connected my old YZXstudio tester did the power bank respond. It automatically set 4.5 V, with no way to change it. I easily got 4.5, 5, and even 5.5 A, though with a noticeable voltage drop—the power bank apparently started limiting the output voltage.


There’s no compensation for voltage drop in the wires here, most likely because of how the protocol works.


Earlier I wrote that for some reason I couldn’t turn on the flashlight. I thought the LEDs might not be connected to anything at all, but it turned out they were connected—and they even light up. Or rather, they blink, on their own, while discharging at a relatively high current.
What’s going on?


Out of curiosity, I also checked how much I could “download” from this power bank. I previously calculated the capacity of the installed battery at about 98 Wh. The claimed efficiency is 95%, but there are plenty of caveats, for example:
1. Only in some specific mode
2. Probably without accounting for voltage drop across the battery protection components
3. Without accounting for voltage drop at the output connector

I discharged it in what I consider the least efficient mode: 12 V output at 1.5 A.
I got almost 78 Wh. Near the end of the discharge, you can see a slight drop in voltage—that’s when the overheat protection started kicking in.


After that, the power bank shut down and wouldn’t turn back on, even though I knew there must still be charge left in the batteries. To get it running again, I had to connect it to a charger for about 30–40 seconds. This of course introduced some measurement error, but I don’t think it was significant.
I finished discharging at 5 V and got another 2.87 Wh. Taking into account the energy used to charge the power bank, we can round the total up to, say, 80 Wh, which means the actual overall efficiency was about 82%. Unfortunately, I don’t know the exact capacity of the LG HG2 batteries, since I’ve had them for a long time. Also, because of the current events, I let them overdischarge :(


The entire discharge cycle at 12 V took a little over four hours. At the end, I measured the board temperature. The first two photos were taken while the power bank was delivering a stable voltage; I took the third almost at the very end of the discharge, when the overheat protection started kicking in.
Overall, I was impressed with how well the power bank handled heat. It delivered almost its full capacity under suboptimal conditions and at maximum power, without any severe power reduction or output voltage fluctuations. I think it would work even better with a small heatsink attached to the chip.


Since the load was connected through a USB tester during the test, there were losses in the tester too. Its counter recorded 81.77 Wh; I think the actual figure was closer to 81 Wh.
A caveat about efficiency: I calculated the battery capacity from my test results, but those tests discharged it down to 2.5 V, while the power bank shuts off when the voltage drops below 2.96 V. So my estimate of the battery capacity at 98 Wh isn't entirely accurate.


The subsequent recharge ran at about 19 W and took 6 hours 30 minutes. Almost 110 Wh went in.


Conclusions.
Personally, I liked the power bank, although, as I mentioned, it has its drawbacks. The two main ones are:
1. The steel busbars connecting the batteries have fairly high resistance.
2. The power bank's controller could use better cooling. That would improve voltage stability toward the end of discharge, when efficiency is at its lowest.

The flashlight is also questionable. I don't really need one, especially one with indicator LEDs, but it's there and could come in handy sometimes (for example, when the power goes out across the whole city), so I'd like it to work.

Otherwise, it's pretty good: support for a bunch of protocols, up to 22 W output and up to 18 W input, though more accurately, it's really 20/19 W. It's also nice that there are versions for 5, 6, and 10 batteries, for those who don't find 8 convenient. It's good that they added battery thermal protection; that's genuinely useful. A percentage charge indicator would be nice too, but I guess I'm asking a lot for this price.

In short, I definitely recommend it—and if you can modify it, I recommend it even more.

That's all from me. I hope this was useful, and that you'll need devices like this as rarely as possible. I look forward to your comments. Maybe there's something else I should test.