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Bench Talk for Design Engineers

Bench Talk

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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics


Power Storage
On February 4, 2016 in Power by Caroline Storm Westenhover
Caroline Storm Westenhover

 

I try and keep my blogs at a level that an avid hobbyist or engineer-in-another-discipline can relate to and understand. However this blog I am going to go more technical. There have been some really interesting innovations in electrochemical storage devices (e.g. batteries and supercapacitors) in my area of focus. These days, bigger products demanding more energy are moving towards battery power. The other day at my uncle’s house we talked about a battery-powered chainsaw. It had a 36V lithium ion battery with something like 80 Wh. I have noticed that many mid-power range loads (somewhere between cell phones and electric vehicles) tend to have a higher voltage and lower-than-expected amps, somewhere around 1C rate for typical lithium ion cell (2-3A). The reason for this, as I am learning, is that putting batteries in parallel is tricky. However, there are times when the job calls for higher instantaneous power, therefore high current as well as high voltage. To achieve the energy required for these systems, many cells or modules, are assembled in series/parallel. There are two battery pack types. The first type builds modules made from a set of parallel cells connected in series, then multiple series strings are connected in parallel. This type of arrangement is costly and complex, but the sharing of current and balancing of cells is more controlled.

 
The second option, both cheaper and easier, is made by first connecting the cells in parallel then assembling them in series. This reduces the part count by skipping the first step and just setting all the cells in parallel to achieve the desired current. This also bypasses some control requirements, because there are fewer modules to monitor. Ideally, each of the parallel cells in the second option will share the load current equally, however due to minor variation in cell equivalent series resistances (ESR) due to both manufacturing variability and uneven aging, significant imbalances in current sharing occur.
 
The potential problems that uneven sharing brings are significant; uneven load sharing can push a cell within the battery well outside of safe operational limits. Pairing cells with an inferior cell inevitably causes premature aging and can drastically reduce the lifetime of the other cells. However, little research has been done for high instantaneous power loads to develop a model for expected lifetime impact that exhibit such current discrepancies, given initial ESR and chemistry. Such research could be the foundation for a cost-benefit analysis in the practice of including inferior cells in parallel packs. This is just one step among many to making battery powered products, including electric vehicles, cost effective for everyone.      
 

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My name is Caroline Storm Westenhover. I am a Senior Electrical Engineering student at the University of Texas at Arlington. I am the third of seven children. I enjoy collecting ideas and theories and most enjoy when they come together to present a bigger picture as a whole. Perhaps that is why I like physics and engineering.  My biggest dream is to become an astronaut.


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