Low on battery? Here we review the best iPhone 6s battery cases and iPhone 6s Plus battery cases, which also work with the older iPhone 6 and iPhone 6 Plus.

The Apple iPhone 6s has a battery life shorter than most owners would like it. Who hasn’t cursed as the “Low Battery” warning pops up? For all its fancy apps and super camera the more important feature of any smartphone is its battery, for when this expires so does everything else.

Apple went for super-thinness rather than significantly better battery life when it designed the iPhone 6 and 6s. So those of us who want our phone to last all day, and then some, have to lug round a battery power bank or forgo Apple’s slimness for a bulkier battery case that will recharge a fading iPhone battery back to full life without having to stop to find a power socket (see the Apple Smart Battery Case for iPhone 6s, £95.41, UK only)

The iPhone 6 and 6s battery cases aren't fully compatible with cases for the iPhone 7, iPhone 7 Plus, iPhone 8 and 8 Plus. See our separate dedicated round-ups for iPhone 7 battery cases and iPhone 8 battery cases.

If you fancy carrying a portable power pack (rather than a protective case) around with you, read our round up of the Best Portable Battery Power Banks. We've also rounded up the best iPhone 6 Plus and 6S Plus cases if you're after general protection for your device.

Battery capacity can be measured in Milliamp Hours (mAh) and Watt Hours (Wh). The higher, the more charge you should get from the extra battery enclosed within the case.

kilponen 6000mAh Battery Case

kilponen 6000mAh Battery Case

Capacity: 6,000mAh
Weight: 204g

The Kilponen battery case provides an additional 6,000mAh of juice with minimal bulk, while also protecting your phone. When plugged in, it charges both your iPhone and the battery pack at the same time, so you don't have to remove the case. Plus it's compatible with Apple Pay.

LED lights on the back also indicate how much power is left in the battery pack. You also get a 2-year warranty. Available on Amazon for £29.95/US$19.95.

Mophie Juice Pack Reserve for iPhone 6/6S

Mophie Juice Pack Reserve for iPhone 6/6S

Capacity: Up to 3,950mAh
Weight: From 75.1g

Pros: Market-leading battery case brand, quality construction made in same factories that Apple uses.

Cons: Bulkier than most and certainly not the cheapest; lacks some of the innovative features of others.

Mophie is the market leader for iPhone battery cases. What you get with Mophie is a long pedigree of quality, award-winning battery cases. From a protection point of view you can’t fault Mophie cases, which feature rubberised support pads to guard against drops and hard falls.

Mophie offers a range of battery capacities, and you can trust Mophie's claims more than the cheaper cases available online. Unlike most of the cases here the Mophie cases will sync and charge while connected to a computer. Each case comes with a Micro USB cable for charging and a headphone extension lead for those who use headphones with an angled or short jack.

The Juice Pack range has several models.

The Juice Pack Reserve (£43.39 on Amazon UK/$59.95 on Amazon US) is the most compact and affordable and will give your iPhone around a 60% boost, the Air (£39.99/US$69) should power a near-dead iPhone back to full charge, while the Plus and the Ultra will give you capacity to charge your iPhone more than once.

The 1,840mAh Juice Pack Reserve weighs 75.1g (2.65oz).

The higher-capacity and more robust 3,300mAh Juice Pack Plus measures a slightly larger 7.44cm x 15.57cm x 1.68cm (2.93in x 6.13in x 0.66in), and weighs 110g (3.9oz). The Mophie Juice Pack Plus is available on Amazon for £49.99, though as little as $20 in the US for the option in gold.

Mophie Juice Pack H2PRO

Mophie Juice Pack H2PRO

Capacity: 2,750mAh; 10.45Wh
Weight: 114g

Pros: Waterproof; market-leading battery case brand; quality construction made in same factories that Apple uses; decent battery capacity.

Cons: Bulkier than most and certainly not the cheapest.

Another case of a battery case that goes the extra mile in terms of features or clever design is Mophie's Juice Pack H2PRO, which offers healthy extra battery life at 2,750mAh capacity plus waterproof protection. Like the Juice Pack Air it should give you a full extra charge of a fading iPhone 6/6s battery.

Water is one of the iPhone's weak spots, and a drenched iPhone is mostly declared dead, although there are ways to save a wet iPhone. So protecting your valuable device from a watery death is highly advisable. It also feels like one of the more robust designs for drops and knocks.

It's easy to put together, and also to take apart – handy as this is a case you might save just for watery environments, and use another case for everyday dry use.

The Mophie H2PRO will especially appeal to underwater photographers, divers and general beach bums or swimming pool enthusiasts. Take your phone to the beach all day and you'll likely need a battery top up, so why not invest in a waterproof battery case at the same time. You'll thank yourself for it if it gets dropped in the pool or left in a bag with a wet towel.

Available on Amazon in the UK and on eBay in the US.

Emtec U500 Power Pouch

Emtec U500 Power Pouch

Capacity: 6,000mAh
Weight: 240g

Pros: Not really a case so doesn't bulk up iPhone when not being charged; interesting case alternative; large battery capacity; can charge other devices too.

Cons: Doesn't offer protection when not being charged; heavier than a normal case.

So many iPhone battery cases are similar that we rejoice when we get to test something a little different, such as the Tylt Energi Sliding Power Case, and iWalk cases.

The Emtec Power Pouch is more of a purse, in that you can slip your iPhone (there is also an Android/Windows Phone model) into a pocket and connect with the included Lighning connector only when you need to charge. For the rest of the time you can leave your iPhone naked or in a more subtle, slimline case than most of the battery cases reviewed here.

You can just leave the Power Pouch in your bag, rucksack or handbag for when you need the reserve power bank. And the 6,000mAh battery capacity is pretty huge, which could charge a fading iPhone over two times over.

There's also a standard USB port so (with the right cable) you can charge other devices from the Power Pouch, too.

Certified by Apple, the Emtec Power Pouch is well made. It doesn't really offere extra protection as it's not a case as such – although in a handbag it could protect from everyday scratches. At 240g it is a lot heavier than most battery cases, but as the iPhone doesn't need to be encased all the time it doesn't add any bulk to the phone when not in use – unlike most iPhone battery cases.

Available on Amazon in the UK and at Best Buy in the US.

Savfy iPhone Battery Case

Savfy iPhone Battery Case

Capacity: 3,200mAh; 11.47Wh
Weight: 102g

Pros: Slim case and reasonably lightweight; high capacity battery; inexpensive

Cons: Case a little tricky to remove

The iPhone battery case market is dominated by Chinese imports, and some are better than others.

The Savfy Battery Case for iPhone 6 (£14.99/US$17.99 - without stand) and iPhone 6s is one of the better battery cases available online. It’s slimmer than a Mophie but with a large battery capacity – 3,200mAh, which is just under the top Mophie’s 3,300mAh but above the less expensive Mophie battery cases.

That level of extra power should easily power up a faded iPhone for at least one full charge.

It’s a two-part design, with a solid-looking bumper and hard-shell backplate, which features a handy LED battery-level indicator. You fit the iPhone into the backplate, then snap the bumper over. Taking the bumper off was a bit fiddly, but you may not need to do this too often.

At 102g it’s around the same weight as the other recommended iPhone battery cases, although the Tylt Slider’s clever two-part design is still our favourite for lightweight protection and a separate battery case only when you need it.

Although supposedly Apple certified we did get an initial “This accessory is not certified and may not work reliably with this iPhone”. However, another push on the button and charging started immediately.

Apart from its lightweight and apparently robust design, plus decent battery capacity, the main appeal of the Savfy Battery Case is it’s online price of under £20/US$20 on Amazon.

It comes with a charging cable and pass-through jack so that angled headphone jacks will fit.

UNU DX-6 Battery case

UNU DX-6 Battery case

Capacity: 3,000mAh; 11.1Wh
Weight: 95g

Pros: Nicely designed case; easy to separate sections to remove iPhone.

Cons: Battery life not as high as others; LED display basic.

The UNU DX-6 has one of the smaller battery capacities (3,000mAh, though now you can find the 3,100mAh version using the uNu link provided above) but will still offer over a full charge for the fading iPhone. It still boasts more charge than most iPhone 5 battery cases, mind, as well as the entry-level Mophie Juice Pack Air.

When charging the case and the iPhone simultaneously from 0%, the battery case will charge for a minute or two then the phone will begin to charge. This is to protect the internal battery of the DX-6, and, UNU claims, will “dramatically increase” the total amount of charging life-cycles of the battery case.

To prevent turning the case on or off while the battery case is in a pocket or purse, the power button must be held for two seconds and then released. This avoids accidental switching on the battery, which can occur with some of the other  cases.

The case is well designed, with rubbery tough plastic for solid protection.

The usual cut-outs are included, plus you get a headphone extension cable in case you use headphones with a short or angled jack.

Available directly from uNu.