Electronics > Projects, Designs, and Technical Stuff

Battery Modeling - What am I Doing Wrong?

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SolidBanks:
Hello all,

I'm looking to model a system where there are 3 sources tied in parallel. Two batteries of the same open circuit voltage but with different capacities, and a current source. The point of modeling this is mainly for my edification but I've become quite frustrated with myself for not being able to figure out why my model outputs an eventual and unexpected decay in SOC and voltage of the two batteries.

I'm attaching a presentation I put together that describes my approach to the model and the result I'm talking about. Could someone smarter than me please take a look and tell me what I'm missing? I've been staring at this for far too long. lol

-Cheers

Daixiwen:
On page 4 when you are using the superposition model, I think you made a mistake with the current source. When you turn off a source, a voltage source becomes a short circuit (U=0) but a current source becomes an open circuit (I=0).
The way you did it the batteries will discharge in both RL and R2, and it could explain why the SOC of your batteries decreases so quickly.

SolidBanks:
Daixiwen,

After thinking through it, I'm inclined to agree with you that I might have made an error in my superposition analysis. I'll re-run the calculations and see what the model puts out. Thanks for responding!

SolidBanks:
So I doublechecked my calculations and I do treat the current source as an open when it's removed during superposition, my charts were wrong as it showed that I didn't. I've since updated the charts to correctly reflect how it's being calculated. Thanks for the feedback though, made my charts better.

Daixiwen:
What conditions are you using when you do your plot? If Is is off, the batteries will discharge in RL and their SOC and voltage will fall to 0. If Is is on, then both voltages will converge towards RL*Is and their current will fall to 0 at the end of the simulation.
By the way you don't need R2 in your model, it is just in series with an ideal current source and can be removed without changing anything.