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I am looking into building my own battery management system, and I am beginning to understand the concerns of overcharging/discharging, overheating, and equal use of each of the cells. I am planning to build a battery management system that uses around 120 3.7V 18650s to power a 24V DC motor, so I am planning to put 6 in a battery and have 20 batteries. At this stage this is all I know.

My question is this: should I be concerned about uneven discharge or charging within a 6-cell battery? I know the battery management system takes care of these problems from one battery to the next, but I don't know if this would be an issue within a battery.

On a slightly more vague note, any other recommendations on (1) sites where I can learn more about building this system and (2) routes I could take for this project.

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    \$\begingroup\$ Use consistent terminology, especially where it matters. A cell is a single 18650, there is absolutely nothing you can do about unevenness of the chemistry inside this. A battery is any assembly of cells. A battery with 3 cells in series using 6 cells would have 2 parallel strings and be called as 3S2P battery, not a cell. Assemblies of batteries could be called a battery, or could be called a battery bank. Please edit your question so that we understand what you are asking, and you understand any advice you get. You do need a battery managment system to handle all series cells \$\endgroup\$ – Neil_UK Jul 11 '16 at 5:35
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lithium cells have a nice steep monotonic charge-to-voltage curve, so if you connect the 18650 cells in parallel they will self-regulate to have the same charge level.

you'll still need a management system to regulate the charge in each set of parallel 18650s

eg: a "10.5V" battery:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ are you saying not to use a battery management system at all? \$\endgroup\$ – louie mcconnell Jul 11 '16 at 2:25
  • \$\begingroup\$ Management is still needed to balance the sereies components, also to protect against undercharge and overcharge, see edit. \$\endgroup\$ – Jasen Jul 11 '16 at 2:49

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