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A perfectly sound form of insurance for your phone battery during long days. It won’t deliver anything close to 5000mAh output, the USB connection is dated and speeds are nothing special. But it’s cheap enough these aren’t necessarily a deal-breaker.


  • OK build
  • Convenient design
  • Battery level estimate button


  • Poor efficiency
  • Slow charging, in line with its stats
  • microUSB, in this age?

Key Features

  • 5000mAh batteryThis battery pack has 5000mAh capacity, and can supply around 3081mAh according to our tests.
  • LED indicator A 4-LED indicator up top offers a rough estimate of the charge level. They light when the side button is pressed.
  • Cylindrical shapeIt’s likely this battery pack is shaped to match the shape of the power cell used inside, but it’s compact nonetheless.


The EnergyCell Slim 2 Power Bank is as close as I’ll get to reviewing a totally generic battery. The label says it’s designed in California, made in China, but the design here is so generic any claims of US heritage are pretty meaningless. 

However, it costs £9.99 and is popular on Amazon. If the EnergyCell Slim 2 Power Bank can supply the goods it promises, there’s no major reason to complain. Does it?

Mostly. This is a solid no-nonsense battery pack you can sling in a bag or coat pocket for emergencies. However, thanks to its low efficiency it can’t supply anything near 5000mAh, and this suggests it doesn’t use the best-quality internals. 


  • Cylindrical shape
  • Portable size
  • Rather dated microUSB for charging

The EnergyCell Slim 2 Power Bank is a cylinder, like an oversized stick of lipstick. This sort of shape is practical for these small power banks because they typically use cylindrical battery cells. 

EnergyQC Power Bank front view
Image Credit (Trusted Reviews)

Its outer is plastic, including the silver band towards its upper section. There’s no metal to the outer shell, which would likely just attract more heat than the rest of the outside anyway. 

The EnergyCell Slim 2 Power Bank is almost as basic as these battery banks come, and it’s dated in one annoying way. It uses microUSB to charge, rather than the USB-C that has been ubiquitous for years. 

EnergyQC Power Bank in hand
Image Credit (Trusted Reviews)

Amazon’s listing suggests this model has been on sale since 2022. Almost all gadgets had shifted to USB-C long before then. However, it does include a microUSB cable so you won’t need to dig one out from your abandoned cable pile. 

There’s no USB-C at all here. The socket you use for charging is a USB-A, which may be an issue if you have a fairly new phone that has a USB-C to USB-C charge cable. 

A line of four LEDs sits by these two connectors up on one end. They stay lit while charging the battery, and you can get a quick glance at the charge level by pressing the button on the side. 

EnergyCell Slim 2 USB ports
Image Credit (Trusted Reviews)

The obvious point of failure is the LED Button. But, hey, some of these cheap battery banks last an age. I have a decade-old TP-Link one that’s trucking along just fine. 

No part of the EnergyCell Slim 2 Power Bank feels expensive, but it’s not worryingly made either. 

The EnergyCell Power Bank is perhaps a little large to slot comfortably into a pocket, but it is among the smaller units with 5000mAh capacity. A jacket pocket or bag is the ideal resting place for this little guy. 

It’s roughly 27mm in diameter, 105mm long, making it shorter than the average pen but way longer than the AA batteries it may remind you of from a glance at its official images.  

There are no ruggedisation claims here. There’s no water resistance rating, and no military-grade testing. And I’d be deeply sceptical of any power bank this cheap that makes those claims anyway. This is a no-nonsense, practical accessory for people who want to make sure their phone can stay charged a full day or two. 

EnergyCell Power Bank side
Image Credit (Trusted Reviews)


  • Claimed 5000mAh capacity
  • 62.7% charge efficiency
  • Slow charge speeds

There are no big numbers involved in the EnergyCell Slim 2 Power Bank’s performance either. 

As you can see from the markings on one end, it takes in power at up to 5V, 2A (10W) and spits it out at up to 5V, 2.1A. And it claims to have capacity of 5000mAh, or 18.5Wh.

I found that, from a fully depleted state where it would no longer charge anything, the EnergyCell Slim 2 Power Bank could take in 18.17Wh. That’s 4910mAh based on the standard nominal battery voltage of 3.7V. 

A full charge took 133 minutes, and the charging rate was slightly higher than claimed. My test device recorded 11.6W, rather than 10W, the excess likely lost through inefficiencies in the charge path. 

As you’d hope for such a low-power battery bank, the EnergyCell Slim 2 Power Bank doesn’t really get hot when charging or discharging, just a little warm. And its charging rate is OK until around the 80-odd percent mark (which takes around 80 minutes), where it slows down to around 5W. 

The EnergyCell Slim 2 Power Bank’s weakness is efficiency. Despite sucking up 18.17Wh of juice, it only put out 11.4Wh to the device being charged. 

EnergyQC Power Bank bottom
Image Credit (Trusted Reviews)

That’s equivalent to 3081mAh in the phone industry standard, giving the EnergyCell Slim 2 Power Bank just 62.7% efficiency. And that doesn’t account for additional power loss at the phone’s end. Have you noticed your phone getting warm when it charges? That’s more energy being lost. 

This is a cheap battery pack, and its performance is absolutely in line with that too. It wastes a good amount of electricity, and can only really fully recharge a 3000mAh-odd phone at best, not a 5000mAh one. 

There’s its charge rate to consider too. I recorded 11.7W output, similar to the advertised figures, We’re not close to real fast charging figures here. 

All of this matters much less if you want a power bank for emergency use, or to keep wearables or earphones charged up. Don’t buy one for laptops. There’s not enough juice, and many won’t even begin to charge thanks to the limited 5V output. 

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Should you buy it?

You want a cheap power bank

It’s cheap, it’s not too large and it has good claimed battery capacity. This isn’t a bad fit, particularly for lower-power devices. 

You want a good-quality power bank

Basic power banks like this are quite slow. But the part you can’t tell by the numbers is that this one is also quite inefficient, only delivering 60-odd percent of the power put in. MicroUSB also feels super-dated in 2024. 

Final Thoughts

Let’s drag out that old cliche: you get what you pay for. In most senses, the EnergyCell Slim 2 Power Bank is exactly what it appears to be. It’s a small, mid-size portable battery pack with limited output, suitable for slow charging phones or for less demanding accessories. 

Its power efficiency is really quite low, though, at 62.7%. Perhaps that doesn’t matter for occasional use, but it means you won’t get close to fully charging a 5000mAh phone, and it’s a bit wasteful power-wise. 

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How we test

We thoroughly test every power bank at Trusted Reviews, using a USB Voltimeter to not only test elements like maximum wattage but to measure its total output to gain insight into efficiency and more.

Tested input, output, maximum wattage and more using a USB voltimeter

Charged multiple phones during testing


Does the EnergyCell Slim 2 power bank have USB-C?

It only has USB-A and microUSB connectors.

Can the EnergyCell Slim 2 power bank charge a laptop?

Limited capacity makes it a poor fit for laptops, and some won’t charge at all thanks to the 5V output voltage.

Can the EnergyCell Slim 2 power bank charge a 5000mAh phone?

It can charge such phones, but will only fill them up by around 60% maximum according to our testing.

Trusted Reviews test data

Battery tested capacity

Full specs

Size (Dimensions)
Release Date
First Reviewed Date
Battery type
Battery technology
Battery size

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