This is the English edition of a review from my Japanese YouTube channel, iestaLife. I review gadgets from my home studio in Japan, so product links go to Amazon Japan.
Quick take
The CIO SMARTCOBY ULTRA SLIM 3K is a 3,000 mAh magnetic battery for iPhone that measured almost exactly 5.0 mm thick and 84.1 g on my scale. It looks and feels premium and in my 30-minute test it added more charge than Xiaomi’s thin battery (from a different starting level). But it only adds a small top-up, the stainless steel back marks easily, and at around ¥9,000 it is expensive for the capacity. It suits people who put thinness and weight above everything else.
What I tested
- CIO SMARTCOBY ULTRA SLIM 3K (3,000 mAh), bought when it went on general sale in Japan at the end of January
- Xiaomi UltraThin Magnetic Power Bank (5,000 mAh)
- CIO SMARTCOBY SLIM 5K (5,000 mAh), which I already owned
- A 30-minute magnetic charging test of the CIO and the Xiaomi on the same caseless iPhone, then about a month of daily carry
What I found
- Thickness: CIO advertises 4.98 mm; my calipers read about 5.0 mm. The Xiaomi measured 6 mm and the CIO SLIM 5K 8.6 mm.
- Weight: 84.1 g (CIO 3K), 99.3 g (Xiaomi), 121.3 g (CIO 5K).
- Charging: from 29% the CIO added 17% in 30 minutes; from 46% the Xiaomi added about 9% and the iPhone showed “slow charging”. The CIO got to around 40 °C (the stainless surface was hard to read with my thermometer); the Xiaomi stayed around 34 °C.
- Magnets: the CIO holds reasonably well and is light enough not to fall off. The Xiaomi felt weak through a case but held firmly on a bare iPhone.
- Capacity: roughly 0.3–0.4 of a full charge for an iPhone 17. On my iPhone 17 Pro Max it is strictly a top-up for the end of the day.
- Durability: small marks appeared on the stainless back without any drops, and rubbing one with a finger changed the finish.
- It supports Apple’s Find My, so you can locate it if you leave it somewhere.
- Sticking a spare metal MagSafe ring on the back made the phone-plus-battery much easier to hold, and it still weighed only 86.8 g.
| CIO ULTRA SLIM 3K | Xiaomi UltraThin | CIO SLIM 5K | |
|---|---|---|---|
| Capacity | 3,000 mAh | 5,000 mAh | 5,000 mAh |
| Thickness (measured) | About 5.0 mm | About 6 mm | About 8.6 mm |
| Weight (measured) | 84.1 g | 99.3 g | 121.3 g |
| Price (Amazon Japan, at recording) | About ¥9,000 | About ¥7,000–8,000 | About ¥6,000 |
| Charge added in 30 min | +17% (from 29%) | +9% (from 46%) | Not tested |
Who it’s for
iPhone Air owners, people with older or smaller-battery iPhones, and anyone who wants the lightest possible emergency top-up and cares about looks. If value per yen matters more, I would pick the CIO SLIM 5K instead. On iPhone, the Xiaomi’s slow charging made it feel like a compromise to me.
Watch the review
The videos are in Japanese. In the first one, the thickness and weight comparison starts around 4:00. The charging and heat test, and my follow-up after a month with the ring mod, are here:
Limitations
The charging test was a single 30-minute run per battery, started at different charge levels (29% and 46%), with a basic thermometer, so treat the numbers as a rough guide. I did not test the Xiaomi with an Android phone, where it may charge faster. Prices were Amazon Japan’s at the time of recording.
Products
CIO SMARTCOBY ULTRA SLIM 3K: on Amazon Japan
Xiaomi UltraThin Magnetic Power Bank: on Amazon Japan
CIO SMARTCOBY SLIM 5K: on Amazon Japan
The Amazon Japan links above are affiliate links. As an Amazon Associate, I earn from qualifying purchases. Check the listing for the exact model, current price and shipping availability in your region.
More reviews in English
All Chargers & Cables (29) →Browse all English reviews →
This review is also available in Japanese.