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Discussion Starter #1
Hi! I'm working on a few projects involving EV batteries but there is one thing that I'm having problems wrapping my head around.

I have 4 BMW i3 12S modules which I want to use in a parallell configuration - a 12S4P setup. I've removed the stock bms and plan to use a so-called smart active balancer/equalizer. I've tested it on single modules and they seem to work well.

BUT.... I don't want to install a balancer on each module and that means all modules must be connected at the serial level. The original i3 module wire size is very thin - maybe not more than 17 or 18 gauge.
I'm obviously going to make sure the packs are well balanced before connecting them and HV ports are connected first.
In theory there should be very little current going "sideways" between module1, cell 5, module 2 cell 5, module 3 cell 5 and module 4 cell 5 and so on for each serial level.
Is there any scenario where that theory would be wrong and high currents would move across the thin balancing wires?

Thanks.
 

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To the extent the packs get unbalanced.

Also power flowing unevenly "through" those paralleled packs is very likely, causing such imbalances, and uneven wear.

Ideally the paralleling should be done at the lowest 1S voltage level, the bottommost corresponding groups within each pack connected to each other, not just at the power pair exiting the packs at high voltage.

Then one BMS can do the balancing for all as if one big pack.

In effect getting the same result as atomizing all tge packs and rebuilding so everything is just a single series string of paralleled groups of individual cells.

I have no idea if what I am suggesting is practical for your situation though
 

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Make a simple drawing of your cells and wiring diagram, then examine the current path for a scenario such as:

If a single cell were to fail internally in a high resistance or open-circuit condition. (e.g. broken current collector on an electrode)

If a single cell were to fail internally in a low resistance or short-circuit condition.
 

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Discussion Starter #4
To the extent the packs get unbalanced.

Also power flowing unevenly "through" those paralleled packs is very likely, causing such imbalances, and uneven wear.

Ideally the paralleling should be done at the lowest 1S voltage level, the bottommost corresponding groups within each pack connected to each other, not just at the power pair exiting the packs at high voltage.
Absolutely, this is what I plan for. Every 1S voltage level has it's own lead (original BMS sensors), and I plan to join these - with the active balancer. Parallell cells will balance themselves, and the active balancer will make sure serial balance is in place. However, my concern is that the original BMS sensor leads could be too small of a size.
The leads on the active balancer (which balance at 2A max) are the same size.

Then one BMS can do the balancing for all as if one big pack.

In effect getting the same result as atomizing all tge packs and rebuilding so everything is just a single series string of paralleled groups of individual cells.

I have no idea if what I am suggesting is practical for your situation though
The only thing I would do differently if I built the battery bank from scratch with 1S modules, was to use some slightly larger sized cables to connect all the 1S modules in parallell. I'm not sure if that would be necessary though, and that is what I'm trying to clarify.

Thanks for chipping in!

Ørjan
 
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