More of everything needs careful thought!
Not So Mobile Devices!
It was only a couple of months ago that I was writing about another product recall of rechargeable battery powered equipment. Now, just a few weeks later, there is another very high profile issue with batteries in a mobile device overheating and creating a potentially hazardous situation. So much so, this ‘mobile’ device is now being restricted as to which forms of transport it can be used on. A reported temporary software fix apparently limits battery state of charge to 60% until new ‘safe’ devices can be shipped to users. But what leads to these types of situations developing in the first place?
Energy + Power = ?
The drive has always been more energy in a smaller and lighter format, and misguided criticism of battery manufacturers has targeted the way battery performance has failed to keep up with the developments in electronics. When you look at the raw figures, yes the exponential increases in processing power look stratospheric compared to the pedestrian increase in battery power and energy density. But this overlooks a basic fact, the battery contains the potential to deliver energy whilst the rest of the kit just consumes it. If the µP is attacked with a hammer it will just stop working, do that to the battery and you will get a very different result!
Safe Battery Operation
The battery has to behave properly over a range of conditions, and great efforts are made to ensure that this is the case. Even abusive treatment can be tolerated to some extent. But the more energy, and the ability to deliver it at higher power levels, are demanded by users (and the reverse case of shorter charge times), the consideration has to be made of what happens to all that energy when the battery is abused. If the protection built into the battery is bypassed, either by an internal fault to the battery or external influence, all that energy crammed into a small space has to escape somewhere, and if the battery is designed to provide high power as well, then inevitably the result will be that things will get hot, very hot!
Triggers
Battery overheating due to external physical events, such as puncturing, can be considered extreme – or just stupid. If you really want to know what happens when you put a knife into a battery there are plenty of videos on-line that show the result. It appears that many of the current crop of failures are a result of batteries getting hot resulting in an internal failure of insulation and leading to what is often referred to as a thermal runaway. The high energy contained within the battery coupled with the high power capabilities make this quite a spectacular event. So much so that they have been referred to as explosions, even though in reality they are venting / fire events. It is the downside of those long run times and every higher energy batteries consumers demand that have eroded the scope for safe operation.
Battery Solutions
Designers have to be aware that the equipment they are working on is safe and fit for purpose. This has to take into account the need for reasonable physical protection, but to protect against someone determined to puncture a battery is clearly not viable. However, the risk of an event leading to an internal failure can be reduced by taking simple measures. As already mentioned, the temporary fix for one problem has been to limit charging once the battery has reached a set percentage of its theoretical maximum charge. This will result in shorter operational run times, but ironically will give the kit much longer overall battery life in terms of the number of charge/discharge cycles that can be achieved.
Run Time vs. Battery Life
When looking at new mobile ‘phones, some of the most prominent data presented is in terms of talk & standby time. Consumers are being advised the longer this time the better the device is – trouble is that this is all down to how hard the battery is being worked. Perhaps the figure being promoted should be how many years the battery can be cycled for. As discussed in a previous blog, longer battery calendar life can be achieved by charging to a lower level. So perhaps the answer lies with the marketing teams!
Striking a Balance
Battery selection for a new product is much more than headline figures for voltage and capacity. Careful thought needs to be given to the interaction of the battery, the device it is powering, the environment it is operating in and the skill set of the user. The availability of ever more powerful and energetic batteries, and the rules and regulations regarding transport and disposal, mean that battery selection is not as straightforward as seems at first. The use of an independent expert to give guidance to your design team can save a huge amount of time and cost to new project development. If you are interested in finding out more, please contact DM Power Solutions using the contact form on this website.