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Hello all,
I've got a bit of a question. I'm looking to upgrade my battery pack to a home-made 18650 pack, but I know in the future I'm going to be doubling my voltage and upgrading the motor, controller etc. I can't afford to do this now, but I'm wondering if I can build half my pack now to use with my current system. My plan is to then build a second identical pack in the next year or two and connect them in series? Is this possible to do with two battery packs of different ages? I know in theory they should be the same, but I know that with use the capacity of my original pack should end up decreasing slightly, hence my question...

For reference I'm doing this on my electric motorcycle build with a 48v system, and then I would be upgrading to a 96v system.

Thanks~
Alex
 

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To the extent you treat them very gently little problem, but expect longevity to be as if the whole pack was bought from day one.

Which means big hit if you hammered it hard.

Better of course for the whole set to match, sell off the old half buy all new together.
 

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Since you mention 18650 cells, I suppose you are going to connect several cells in parallel and then connect these larger "cells" in series to 48 volts.
What you could do is to make sure your parallel assembled cell has an even number of 18650 cells and is easily dividable in two smaller paralleled cells. So when you decide to upgrade the voltage with new cells split your pack in two parallel pieces and connect them in series. Then add parallel to that the new cells. Something like this (where 'i' is the old cells and 'I' is the new ones):

old:
ii
ii
ii
ii

new:
iI
iI
iI
iI
iI
iI
iI
iI

The old cells will automatically balance with the new ones. I suppose you will connect them in parallel when both cells have same voltage to avoid huge equalising currents. The problem that comes out of this is that if the old cells are close to their end of life, when a cell dies it will drag its parallel buddy with it. A risk worth considering. A BMS could probably warn you in time, but still.
 
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