Battery Modules and Designing the Pack

Here’s a question that’s confused me for a bit.  Say my target voltage is 72V, and I’m looking for a capacity of 40ah.  Does it make more sense, if I’m putting together two modules, to build a module that’s at full voltage and half the capacity, or better to build it at half voltage and full capacity?

That is, in the first scenario, do you series your cells to get up to the voltage you want first, and then parallel the two modules?  Or, in the second scenario, parallel them to get your capacity up, then series them?

Well, when we put it that way, the answer is a little more obvious.  You always want to parallel the cells first, then series them.  First, if you’re using a BMS, each set of paralleled cells only needs one BMS, right?  It also gives you a better balance since cells in parallel like to keep each other happy.  Parallel first, then series.  For more details, see the Li-Ion BMS white paper on paralleling cells vs strings in parallel.

Now, what has me a little stumped is the RC lipo equation.  I’m using lipo packs that are configured in 4s, to give me 5ah at 14.8V.  I’m thinking I need to parallel these to get me to my 40ah, treating each pack like it’s an individual cell…  then string them together to get to my target voltage.  Unfortunately the mathz doesn’t quite work for an even two packs.  …maybe time to reconsider my basic structure, hmmmm?

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