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