Cuktech S15 Ultra power bank review: seriously powerful with some cool features

The Cuktech S15 Ultra is a higher-tier power bank similar to the one I reviewed in 2024. The S15 SE left a good impression, so it was interesting to see what a more powerful and feature-rich variant could offer at its rather steep price of $119.99.

Note: Cuktech provided the review unit without any editorial input or review pre-approval.

What is in the box:

• The power bank itself
• A 6A USB-C to USB-C cable with colored connectors
• A carrying case
• User manual

It is nice to see a carrying case in the box (it has a separate pocket for the cable—good attention to detail), but I would say it is something to expect from a $100+ power bank.

The bundled cable is also of good quality, and it can charge the S15 Ultra at its maximum rated power of 140W. I wish it were braided for a more premium feel (again, the power bank is pretty expensive), but hey, it is the charging speed that matters most (colored connectors are a nice touch).

Here is a more detailed spec table:

Design

The S15 Ultra is made of gray plastic with one textured size (still plastic) that states its rated and typical capacity: 70.56Wh and 72Wh (slightly smaller than in the S15 SE). It also says that the power bank is "airline safe," which means you can take this battery with you on a plane without breaking the FAA's 99Wh limit.

The front of the S15 Ultra has a glossy plastic strip with a button and discreetly hidden display (more on that later). It surely has a good-looking, stylish design, but because it is plastic, I expect it to collect quite a lot of scratches. During a few weeks with the S15 Ultra, I put a few small scratches on it.

As for ports, the S15 Ultra offers three: two USB Type-C and one USB Type-A. Unlike the S15 SE, the S15 Ultra can charge with either USB-C port, which is evidenced by two colored Type-C connectors.

Cuktech's signature cyan stripe around the ports is here, but again, it is just a small touch of color, not an LED or something. Still, good design, I like it.

The S15 Ultra is slightly bigger than the S15 SE despite having a smaller battery, which, I assume, is due to more powerful hardware and a display. It measures 152 x 44.5 x 57.1 mm or 5.9 x 1.75 x 2.24 inches. As for weight, it is 593 g or 1.3 lbs, a bit heavier than the S15 SE, but nothing extraordinary.

Charging the S15 Ultra

When it comes to charging, the S15 Ultra has a few cool tricks up its plastic sleeve. You can top this thing up at up to 140W using a single Type-C cable if you have a powerful enough charger. Alternatively, if you are in a hurry, you can cram 165W of power in the S15 Ultra using Beast Mode. To do so, you will need two cables and two chargers (or a single charger with two ports).

When in Beast Mode, the S15 Ultra changes its display to cool Star Wars-like graphics showing the charging speed and the current charge with an accuracy of up to two decimals—you can really see how fast it sips juice from the wall.

Of course, it cannot sustain all 165W from dead to 100%. It starts at its maximum speed and climbs down to 60-90W as you get closer to higher charge levels. Keeping the power bank cool during charging will help it sustain peak charging speeds for a bit longer.

When testing Beast Mode, I noticed that the power bank gets quite hot. It was 75F (24C) in my room, and the S15 gave up charging after roughly 10 minutes, showing a warning on the screen that it needed to cool down. It was a bit disappointing, considering that the user manual promises a maximum speed of up to 70% of charge at a room temperature of 77F (25C ± 2C).

When placed next to an open window for some winter breeze, the power bank charged in Beast Mode from dead to 100% without issues. Perhaps it would not have overheated had it been a bit cooler in my room, so I suggest keeping an eye on it when charging at full speed in warm conditions. Fortunately, all the protections and safety measures were in place, and there was no need to call 911 and firefighters.

As for lower-speed charging modes, I did not notice anything alarming. It remained reasonably warm even at relatively high single-port speeds (105F or 41C). Although you can use both ports for single-cable charging, the second USB-C is slower: 65W max versus 140W.

The fact that you can charge the S15 Ultra with one cable at up to 140W begs the question of whether there is even a point in straining the power bank with dual-cable charging only to get an extra 15W. Probably not if you have a very fast charger that can deliver 140W over one port. However, it makes sense if you have, say, two less powerful chargers. My main charger is 65W max, but I also have a charging station with two 100W Type-C ports, and an extra 65W makes charging faster. So yes, Beast Mode is pretty useful when you are in a hurry.

However, keep in mind that Beast Mode is only possible if the USB-C1 port is connected to a 100W or higher and the USB-C2 port is connected to a 65W charger or higher.

From a single port, I could get 130W max charging, and the S15 Ultra charged in roughly 50 minutes. Cuktech says a quick 20-minute charging in Best Mode gets the power bank to 70%, and I can confirm this statement. Full charging with two cables is not that much faster than the max single-port charging—40 minutes from 0% to 100%. As usual, charging speeds drop significantly as you get past 80%.

Here is an overview of charging speeds:

• 165W (two cables): 40 minutes
• 130W: 50 minutes
• 100W: 58 minutes
• 65W: 81 minutes

Charging with the S15 Ultra

The S15 Ultra is rated for 20,000 mAh. Here is a regular reminder that we measure the amount of energy in Wh, not mAh. Two 20,000 mAh power banks can have different battery capacities because they are measured by volts times amps. The S15 Ultra is rated for 70.56Wh.

Like in my S15 SE review, I tested the S15 Ultra's capacity by charging my iPhone 15 Pro from 10% to 90%

Overall, when charging 80% of the 12.7 Wh battery in my iPhone five times, we get 50.8 Wh of available energy or 72%. The rest is lost due to heat and charging inefficiencies. I also tested the power bank with my 13-inch M1 iPad Pro, and it charged the tablet from dead to 100% in 1 hour 42 minutes, with 33% capacity left.

I also had a Redmi Note 14 Pro+ 5G on my hands, so I managed to test its high-speed charging capabilities. The S15 Ultra supports miPPS (Xiaomi's fast charging), and it had zero issues charging the phone at 120W, resulting in the battery going to 100% in 27 minutes, with 71% left. If you want to learn more about that phone, check out Robbie's recent review.

Cuktech claims the S15 Ultra has 83.33% conversion efficiency, but I could not verify it myself because my USB tester died right when I started my tests; sorry for that.

Unlike the S15 SE, the S15 Ultra does not drop charging when you connect additional devices. Charging could slow down for a split second, but it does not halt altogether, so thumbs up for a better controller.

Cuktech says the S15 Ultra can deliver up to 140W output from a single port, but this only applies to the first USB-C port. The second USB-C tops at 90W, while the USB-A port can only deliver 33W max. I tried to stress-test the power bank by plugging in as many powerful devices as possible, but I could not overheat it even when having my laptop and another power bank connected to it.

I also could not verify the promise of 210W max on all three ports (I do not have that many powerful devices to connect all at once) but I managed to crank it all the way up to 170W, which is close enough. The temperature was around 111F (44C). Quite warm, but not hot. Also worth noting is that the USB-C1 port can only sustain its max 140W power rating if nothing else is connected. As soon as I connected another device, charging would drop to 90-95W.

You can charge the power bank while charging other devices. When charging from a 65W adapter, the S15 had no issues charging my iPad at 30-35W. Output power can also exceed input, but in such scenarios, charging halts after roughly six or seven minutes. All connected devices stop charging altogether while the power bank itself slows down to a mere 35W (without overheating). The same happened when charging the power bank at 90W.

I asked my contact at Cuktech about it (I could not find anything about simultaneous charging in the user manual), and I will update the review after I have received word from them.

Display and additional features

The Cuktech S15 Ultra has a pretty large (for a power bank) TFT display, which is full of useful information:

• Current charge level with accuracy up to two decimals.
• Estimated time to charge or discharge (goes to 99h 99m when charging itself and other devices)
• Voltage mode
• Total input and total output
• Total power, voltage, and amps for each port

The display itself is of decent quality and has perfectly fine brightness when indoors. Outdoors, though, you might have to cover the display with your hand to read it because it turns completely unreadable under sunlight.

Below the display, you can find a single multifunctional button. Single press wakes the power bank, while a double press toggles low-voltage mode for small-battery devices. When the power bank is turned on, a single press lets you customize the primary port—you can set it to input (can only charge itself), output only (can only charge other devices), and input/output.

Unfortunately, this option is only available for the first port. Cuktech told me that implementing this feature for both USB-C ports would have been too complicated.

The S15 Ultra also lets you keep the display always on when in use or time out after a few seconds. My testing showed that keeping the display in always-on mode does not affect charging much, which is nice.

Finally, it is worth noting that the power bank is quite intelligent and can detect handshake protocols. For example, upon connecting my iPhone, the S15 Ultra detects that it is an Apple device and displays that on its display. Neat! Supported protocols include PPS, QC, PD3.1, FCP, SCP, and MiPPS.

Conclusion

Yes, the S15 Ultra is on the pricey side of the compact power bank spectrum, but it offers quite a lot in return. Super-fast charging is great (assuming you have the right adapters), the built-in display packs a lot of useful information, and the output has enough juice to charge even seriously powerful laptops alongside your phone, tablet, etc. It is a good recommend for those who do not mind a $119 power bank; just keep an eye on that Beast Mode charging in warmer environments.

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