Electronics > Projects, Designs, and Technical Stuff

Battery Modeling - What am I Doing Wrong?

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Daixiwen:
Oh I see. So you are using a purely linear model. It's not very accurate, but should be good enough to see how things work, and it shouldn't cause any instability.
I didn't notice at first how huge your value for C1 is. I wonder if this would cause some oscillations or weird values in the currents through the batteries. As I suggested before I think it would help to have on the same graph that you have on the last page the currents through the batteries. If you can also show the voltage across C1 maybe it will give some clues about what is going on

SolidBanks:
I'll work on it this weekend and reply again when I'm done.

amosborne:
I have done this sort of modeling before for the closed loop control and energy balance of spacecraft power electronics, solar arrays, and lithium ion batteries.

You should consider the energy balance of your model. You are injecting energy with your current source, and the load is consuming is energy. But you are also losing energy due to the batteries internal resistance. I bet that you are losing more energy than you are pushing in.

You said in a post above that the conditions are pulse source 60min every 36min and pulse load 8min every 86min. I don’t know exactly how you time that, but I guesstimate the beat frequency to be around 1/1000min, which explains the scallops.

It you are modeling the nonlinear open circuit voltage to state of charge relationship, that easily explains why the voltage craters at the end. Batteries don’t hold voltage well at low state of charge.

The fact that you have a “communication” model puts me on a spectrum somewhere between curious and concerned. I suppose it can work, but I guess you’re not taking advantage of ODE solvers to perform numeric integration.

By the way, the best way to determine the RC dynamic battery impedance is with a step response. And these parameters also end up being a function of state of charge ;)

SolidBanks:
Attached include the following:

* Both battery currents
* Current source profile
* C1 Voltage

SolidBanks:
My responses:

You should consider the energy balance of your model. You are injecting energy with your current source, and the load is consuming is energy. But you are also losing energy due to the batteries internal resistance. I bet that you are losing more energy than you are pushing in.
I provided the IS profile in a graphic in another response. That might be the case as I'm only source around 15A each time the IS is switched on.

You said in a post above that the conditions are pulse source 60min every 36min and pulse load 8min every 86min. I don’t know exactly how you time that, but I guesstimate the beat frequency to be around 1/1000min, which explains the scallops.
The graphic I attached in another reply might clear up what I was trying to communicate. It has the IS and load profile.

It you are modeling the nonlinear open circuit voltage to state of charge relationship, that easily explains why the voltage craters at the end. Batteries don’t hold voltage well at low state of charge.
So I understand completely that battery performance worsens as it reaches low state of charge but I've been more confused about the battery performance earlier on in the simulation when things just seem to suddenly take a turn for the worse. Happens around 1489 minutes.

The fact that you have a “communication” model puts me on a spectrum somewhere between curious and concerned. I suppose it can work, but I guess you’re not taking advantage of ODE solvers to perform numeric integration.
Yeah this could be where my not knowing will get me. I'm trying to learn so it sounds like this could be something I use to improve the model. I never liked the separation I have between the circuit and the battery, where the results of one have to feed the other in order to continue progressing but I've heard that it's not uncommon.

By the way, the best way to determine the RC dynamic battery impedance is with a step response. And these parameters also end up being a function of state of charge
Indeed it is. I used a ballpark method in an attempt to estimate the parameters for RINT and the RC branch. That ballpark method included starting the battery model at a relaxed state and then applying a load to draw a steady current for an amount of time that allowed the magical 5 time constants. Now I had no idea that they'd be a function of SOC. Are you referring to the calculation of terminal voltage? When the OCV, as a function of SOC, is reduced because of the voltage drop across RINT and then the voltage drop across the RC pair? Or are you referring to something I'm completely missing?