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!

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.

Understanding Battery Data Sheets

In the perfect world it would be possible to get all the information you need, and compare different manufacturers product, by referring to their data sheets. But this is not a perfect world, and battery manufacturers seem to have gone out of their way to make it difficult to compare like with like. Even product from the same manufacturer often does not include a consistent presentation of performance across their range of products.

If you are looking for general performance guidelines cell and battery data sheets are fine. A lot of effort goes into compiling them and making sure data is accurate. Data is normally presented for stand alone tests, at cell level this means a discharge curve is at a specific current under controlled conditions such as temperature and orientation. The test subject will be a cell that is no more than a couple of months old, and correct test methodology, such as separate current and voltage monitoring connections, will have been implemented. It is also likely that connections to the test cell are through resistance welded tabs, especially for higher current tests.

Sometimes there may be some representation of pulse capability, where a higher current pulse is superimposed on a low base load. Again, this is useful general information on how a specific cell will perform, but quite minor changes from the load profile presented are likely to give different results.

The use of data presented on charts with logarithmic scales gives the opportunity to present lots of data in a relatively compact form. But as always, the devil is in the detail. Having a chart that shows performance at -20, +20, +40 and +60°C is not much good if you want to know what happens at +5°C. This is a particular problem if you have a high amplitude pulse infrequently applied, you may be able to make a reasonable estimate for what happens to the base load, but the pulse is little more than a guess!

And have you ever looked at a cell data sheet where a value for peak current is given in the text, but the charts stop a long way short of showing what actual performance you can expect? These things are often wrapped up in caveats in the small print, involving asking for the manufacturer to evaluate your application on an individual basis. You can be sure that all sorts of detail about how the product will be used will be required such as detailed load definition and environmental temperature profile before any sort of prediction is made. Even then the information the manufacturer provides will be hedged using terms such as ‘expected’ or ‘typical’ performance! And of course they will only tell you the good bits – try and get a cell manufacturer to guarantee the performance figures they give you.

This all indicates that the performance shown on data sheets is unlikely to be reproduced in a real world application. If you are trying to decide what cell type, from which manufacturer, you want to use in your new product then relying on data sheets alone is a risky option.

Even more risky is using cell data sheets to predict likely performance of a battery made up of multiple cells to give higher voltage, higher capacity batteries. Issues such as localised heating can affect performance considerably. In some cases additional features need to be included in multi cell batteries for safety reasons. When these should be implemented and how they affect to overall battery performance are critical issues. There are so many potential combinations it would not be practical to include these on data sheets, expert advice is a must. Once you have chosen your battery supplier they will of course make sure the product they provide is safe. But if you have done your initial development work based on what you think the voltage output would be, only to find that you need a couple of diodes in the battery, there is a nasty performance shortfall on its way.

Experience is key in understanding how data sheet information relates to likely real world performance. Using an expert early in your new product design process to select what cell type, and in what configuration will give the best performance based on a realistic assessment of manufacturers data will help reduce you project risks. If this article has got you thinking, please contact me using the enquiries form on the contacts page of this website. I am always happy to give a no obligation chat through the issues of battery selection and use