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Today we’re all surrounded by personal electronic devices, not just phones but increasingly novel wearables, watches and products like e-cigarettes, which have become an essential part of our lives. They all have one thing in common; they contain a rechargeable battery. As manufacturers strive to make these devices ever smaller and better they must use the most ‘energy dense’ and cost effective battery technology available. At present, this is lithium polymer technology, or ‘LiPo’ for short.

LiPos are a type of rechargeable battery that use the flow of lithium ions though a polymer between two electrodes to create an electric current. They were first introduced in the early 1990s and have become ubiquitous because they store a lot of energy, can be easily manufactured to shapes that neatly fit into products and do not need a heavy metal case - factors that add up to better gadgets!

However the battery technology does have one potential drawback that designers must overcome. Due to its high energy density (the energy stored per unit volume of battery) even a small battery can be potentially dangerous if the energy is released uncontrollably. In fact, the energy density in LiPos is so high it is around a tenth of that in the explosive TNT!

But that’s not the only challenge. The chemistry and construction of these batteries has an unfortunate characteristic. If the battery gets very hot when being charged (above 60°C) then a process can start that results in the battery getting hotter and hotter uncontrollably, giving off large volumes of gas in the process. This is known as ‘thermal runaway’ and can result in the battery setting fire to its surroundings or even exploding if the gas builds up in a constrained space.

It was this type of failure that grounded Boeing’s new Dreamliner airliner fleet back in 2013, when the battery system caught fire. The issue is so serious that there are special regulations regarding the airfreight of these batteries when a large number are packed closely together. The concern is that if one battery has a fault and heats up it could cause a chain reaction through a whole consignment. 

So how are millions of these batteries in use every day without problems? The answer is because designers carefully protect them from overheating using electronics. They limit the power delivered to the battery during charging to keep it operating well below its danger temperature and within the limits that allow it to work reliably time after time. However designers also want to charge the battery as fast as possible which means using as much power as is safe. When the battery is charging, this power is converted into chemical energy in the form of electron separation for release later, but inevitably heat is also created.

Battery protection design solutions are well established, but consumers want ever smaller and cheaper products and the convenience of plugging them in anywhere. And this is the point where challenges arise, as we’ve all seen in recent press reports of gadgets left on charge causing fires. So why has this happened?

At Cambridge Design Partnership we have identified the root causes and looked into how designs can be improved. We found a combination of three factors at fault:

  • In highly competitive markets there is inevitable pressure to reduce the cost of charging circuits to an absolute minimum, which results in safety margins being pared back.

  • Users seek to charge their devices anywhere that’s convenient, while USB power sources have developed to meet market demands and can vary from what designers expected.

  • Market pressure to make products as small as possible results in charger designs that split the circuit between a ‘charger’ unit and the product itself. This creates a new interconnection that can cause problems.   

For example, in rechargeable electronic cigarettes, some manufacturers have split the charger electronics between a ‘charger’ and the battery unit. To charge the product, the user first plugs the battery unit into the charger provided with a special miniature plug, and then plugs the charger into any convenient USB socket. Unfortunately as several manufacturers use the same miniature plug design, the unwary consumer who buys two different but similar products can easily plug the charger from one product into the battery from another. 

Without realising it, they have created a ‘Frankenstein’ system that no longer has the essential protection required. Everything may appear to work fine until they plug their device into a high power USB 3 adapter intended to fast charge the latest generation of energy hungry phones and tablets. Then the ingredients for a real problem have come together.

We have tested hypothetical scenarios in our lab and shown that when charging these combinations can exceed the threshold temperature so that thermal runaway occurs in spectacular fashion, as the photographs at the top of this page testify!

So what is the solution? If separate chargers are essential then either the interconnecting plug must stop consumers connecting to another manufacturer’s product, or the product itself must be designed to protect from over charging whatever the user connects. This is likely to be a slightly more expensive circuit to manufacture but with the integration of the electronics onto a small piece of silicon called an ASIC (application specific integrated circuit) it can be achieved in an extremely small space.

At Cambridge Design Partnership product safety is always our first priority and we have robust quality processes to ensure this is achieved.  Our electronics design team work closely with our product designers to implement a risk based design methodology to make sure products work both intuitively and safely while remaining cost effective to manufacture. This includes when the user operates the products as intended, but also when they make mistakes or manufacturing processes go awry.

For more information contact us on +44 (0)1223 264428 or email

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