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Discussion Starter #1
Hi,



I am new to the forum, and working on a plan to convert a Beetle. Mostly because it's a well-trodden path and I can get the bits I need easily (I hope).


I have decided to use 5 Tesla modules in series, but, can I 'mix and match' said modules from different donor battery packs? I have been told it's not a good idea, because some modules will have had more cycles than others, but how bad a problem is it?


If I had 4 from one pack, and one from another, how bad is it?

What can be done to 'balance' them (excuse the poor choice of technical word) such that modules from different packs can work well together?


Thx!
 

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I'll let others debate the importance of matching the cycle life and condition of the modules, but I will note that even with the assumption that we are talking only about Model S or X (not Roadster, Model 3, or Tesla-branded packs for the Smart ED or RAV4 EV), Tesla has made different sizes (number of cells in parallel) of modules... clearly they must be the same size. Even the cell specs probably varied with time, so from the same model year of the same model variant (e.g. all P85) would seem wise to me.

I suggest the term 'match', rather than 'balance', due to the very specific meaning of 'balance' in the context of EV battery management.
 

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Yes, same cell chemistry and pack **voltage** is critical, safety issues there.

Age / condition mismatching just means the whole bank's longevity is limited to that of the weakest link, but hey if cheap enough who cares?
 

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Yes, same cell chemistry and pack **voltage** is critical, safety issues there.

Age / condition mismatching just means the whole bank's longevity is limited to that of the weakest link, but hey if cheap enough who cares?
Nominal voltage match is easy, since all Model S and X modules are 6S configurations. Matching the state of charge before connecting them is just good sense. If the same voltage doesn't lead to the same state of charge (with the same nominal cell capacity), then this is a condition mismatch; so the question is whether this is an issue or not.

The approximate overall pack voltage is a simple result of the number of modules used (for these Model S/X modules) regardless of the module variant or condition.
 

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Discussion Starter #5
Thanks for all the replies.


So it sounds like, provided the actual modules are the 'same' (ie, same voltage, cell configuration, etc.) it's not critical that they all come from the same car. They could come from a number of different cars, but one is limited by the performance of the weakest module (I'd be running 5 Model S 5.3kWh modules in series).
 

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Yes, the BMS-type protection logic should key overall capacity off that of the the lowest one, and it will likely wear out the fastest.
 
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