Whether it's a fitness tracker or a smartwatch, Google Glass or a medical monitor, almost every wearable device has the same Achilles heel: the battery. Wearable tech is evolving quickly, but batteries aren't.
Apple's Watch demonstrates the problem beautifully. Despite having a slightly bigger display, the 42mm model gets up to 34% more battery life than the 38mm model, and that's purely because its slightly bigger case has slightly more room for a slightly bigger battery.
The smaller the battery the less power it can store, and that leaves manufacturers with a problem: if they want their wearables to be slim enough and small enough for us to want to wear them, they'll need to compromise.
Must read: How Cute Circuit brought the magic back to fashion
Apple does it with a daily charging routine and a screen that switches itself off with indecent haste. Pebble uses e-paper. And some firms go without a display altogether.
None of these things are ideal. Why can't we just have better batteries?
The power problem
A battery is essentially a sandwich with positive and negative electrodes separated by a liquid or solid known as an electrolyte. In most of the rechargeable batteries we currently use, one of the electrodes is made from a lithium compound, usually lithium cobalt oxide. Lithium-ion batteries can store a lot of power in a small space, and they don't suffer from the memory effect that used to ruin the nickel-cadmium batteries we used to find in laptops and other devices.
There are some key problems with today's lithium-ion batteries, however. Their electrodes are flammable and pressurised, and that means they have a tendency to explode when damaged, faulty or connected to a dodgy charger. And of course, small ones don't store enough energy to power an typical smartwatch for a week.
That doesn't mean they can't be improved, though. Researchers from Kansas State University have discovered a way to boost lithium batteries' storage capacity and extend their lifespan via the use of silcon carbonitride-wrapped "nanosheets"; while researchers at Stanford University are experimenting with nanomaterials and additional metals to produce lithium batteries that can cram five times more energy into the same space as today's batteries.
Dyson and Volkswagen are among many big name investors in solid state battery tech, but it's believed to be several years away from production
Meanwhile, at Oak Ridge National Laboratory, researchers are experimenting with solid state batteries that use a solid electrolyte rather than the traditional liquid or powder and which deliver more stability and even more battery density.
Dyson and Volkswagen are among many big name investors in solid state battery tech, but it's believed to be several years away from production reality - and despite the hype about lithium air batteries, which use oxygen in their reactions and which are therefore much lighter than normal batteries, that technology is even further in the future.
Where lithium ion batteries can be recharged hundreds or even thousands of times before they start to lose capacity, the record for lithium air currently stands at 60 charge cycles thanks to researchers at Yale and MIT. Don't expect to see it on your wrist any time soon.
Improving lithium isn't the only option. Researchers and battery firms are making big progress by embracing completely different materials and manufacturing processes.
In the US, Imprint Energy has targeted the medical device market with miniaturised zinc-carbon batteries that work similarly to lithium-ion batteries but use zinc instead of lithium. By printing multiple layers of ultra-thin film, Imprint can create highly flexible batteries that don't have the downsides of lithium ones. Imprint believes that its technology could power ultra-thin medical sensors and even smart bandages.
In April, Stanford University hit the headlines when it announced a bendable, foldable aluminium-ion smartphone battery that would charge in just one minute - and which would be good for a massive 7,500 charge cycles.
Professor Hongjie Dai said: "We have developed a rechargeable aluminium battery that may replace existing storage devices, such as alkaline batteries, which are bad for the environment, and lithium-ion batteries, which occasionally burst into flames. Our new battery won't catch fire, even if you drill through it. Lithium batteries can go off in an unpredictable manner - in the air, the car or in your pocket... I see this as a new battery in its early days. It's quite exciting."
For now, though, we'll need to curb our enthusiasm: the battery's 2v output is good compared to normal AA and AAA batteries, but it's roughly half the power you need for a smartphone or mobile device.
In tech, you'll usually find that any headline-hogging new breakthrough turns out to at least five years from commercial reality. That's usually the case with battery tech news too.
Fight the power
There are lots of ways to get power into a rechargeable battery, such as smart clothes that harvest kinetic energy or solar panels that harvest solar energy - and with the development of truly transparent solar panels, the prospect of a display that doubles as a solar panel isn't too far fetched.
Researchers at the Chinese Academy of Sciences have even developed a solar cell that can be woven into fabric, although with just 3% efficiency and four-day life the prospect of solar-harvesting pants is a long way off.
For now, the problem with all of these energy capturing technologies is that they don't produce enough juice for the current crop of power-hungry gadgets.
Essential reading: MotionX creating a better future for wearable tech
In the short term, the focus for manufacturers won't be on finding ways to get energy in, but on finding ways to reduce the energy you're taking out. We've seen that happen with laptops, where ever more efficient processors have eked more power from less energy, and we've seen it happen in smartphones, where more efficient display technologies and more aggressive battery management have seen devices get bigger and brighter without a catastrophic effect on battery life.
What we're likely to see in wearables is a mix of things: improvements to battery density and flexibility, more efficient displays, processors and radios, more aggressive battery management, clever packaging and new materials that enable batteries to go where no battery has gone before - such as into the strap of a smartwatch.
The charger's days may be numbered, but unlike a wearable after a workout, there's a lot of life left in it yet.
How we test