Beginners Guide to Processing 18650 Cells | Cell Database


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Relatively low tech active cell balancing scheme, schematic and working model
#1
I have short term memory issues that make me a lot better at answering questions than writing a long piece about something so please bear that in mind and if you have questions I hope you ask. Also I'm taking pictures with my decrepit old cell phone because it's convenient, please excuse the crappy image quality.

The idea is simple, you store energy from the high cell/pack in your series connected string in a parallel pair of cells deliberately kept near the low point of the charge curve so they have the best voltage differential with the  high cell. At a point that depends on several criteria the parallel connected pair is disconnected, rewired through relays to a serial connection and discharged through the low cell/pack of your series connected string.

The balance pair has nothing to do with the rest of the battery besides accepting and dispensing energy from and to the high and low cells/packs respectively, it does not enter into capacity calculations and can be a lot smaller than the rest of the packs in the battery in capacity and still function.  Monitoring and switching will be done by the Arduino (or whatever) that controls the rest of the BMS which I have not drawn or particularly thought about yet other than the scheme in the second schematic to connect and disconnect to individual cells.

Schematics and images of a conceptual working system  follow.





Parallel connected drawing current from the large pack





And series connected putting charge back into the big pack through the lamp.




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#2
In multi-cell batteries, because of the larger number of cells used, we can expect that they will be subject to a higher failure rate than single cell batteries. The more cells used, the greater the opportunities to fail and the worse the reliability. Unbalanced ageing is less of a problem with parallel chains which tend to be self balancing since the parallel connection holds all the cells at the same voltage and at the same time allows charge to move between cells whether or not an external voltage is applied. The high energy storage capacities needed for traction and other high power battery applications can be provided by using large high capacity cells or with large numbers of small cells connected in parallel to give the same capacity as the larger cells.
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