Battery Passive Cell Balancing | Simscape Battery Essentials, Part 4 - MATLAB & Simulink
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    Battery Passive Cell Balancing | Simscape Battery Essentials, Part 4

    From the series: Simscape Battery Essentials

    Simscape Battery™, a new product in R2022b, has been developed to provide a technology development framework that is assembled specifically to create a bridge between battery cell and battery system. The bridge directly supports upskilling as well as design exploration and design rigor, meaning you can navigate the battery system technology development cycle rapidly and with confidence. You will learn how to:

    1. Define the components and geometry of a battery pack that includes passive balancing circuits on each parallel assembly.
    2. Connect the battery pack with a constant-current–constant-voltage (CC-CV) charger and a passive cell balancing algorithm.
    3. Simulate the system and observe the effect of passive cell balancing on state of charge.

    Published: 20 Sep 2022

    Hello, everyone. My name is Graham Dudgeon. And welcome to part 4 in a series of videos where we'll provide insights and work examples on the use of Simscape battery, a new product in the Simscape portfolio.

    Simscape battery has been developed to provide a technology development framework. It's assembled specifically to create a bridge between cell and system. The bridge directly supports upskilling as well as design exploration and design rigor, meaning you can navigate the technology development cycle rapidly and with confidence. Today, I'll show you how to configure a battery pack with a passive cell balancing circuit and then connect and test a charging system to that battery pack, which includes passive cell balancing.

    We're going to be working from this example, build model of battery pack with cell balancing circuit. And in parts 1 and 2, I spoke in more detail about how to build up from a cell through parallel assembly, module, module assembly, and pack. So today, I've actually created the pack object, which contains all the components. But what I'm going to do is point out the specific settings we need to make to add passive cell balancing circuits to this pack.

    So let me just scroll down. We're going to this section here, define cell balancing strategy. So we do this at the parallel assembly level. So what you can see with a parallel assembly, we can connect the balancing circuit, which consists of an ideal switch and a balancing resistor. We then control that switch from a suitable control system, which we'll see in just a moment.

    So here's what we need to set. The object property balancing strategy, we set it to passive. So let's just take a look at battery pack. I'll show all properties. And you can see here we have balancing strategy set to passive. That means when we build this pack using the build battery function, it will include the balancing circuits on each parallel assembly.

    So let's just visualize the pack. We're using the battery chart function. Let's just take a look. So here's our pack. The pack consists of 64 cells in total with each parallel assembly having four cells.

    If we look at the simulation strategy, we can see because we have defined the simulation strategy as detailed, every cell in the pack is going to be modeled as its own individual cell model. So just scroll a little bit further down. We build this pack using the build battery function. So let me just restore the current folder. I'll just bring the pack model up so we can take a look.

    So here we have the pack, positive and negative terminals, various measurements that we're familiar with from previous sessions. But the difference is we have this balancing signal into. And I just double click on the pack.

    We've got two module assemblies in this case. Going to module assembly one, two modules. And I double click on the module. We're just using default parameter values here. And if I go to the description, we see each module consists of four parallel assemblies with four cells pair parallel assembly. And we're using detailed model resolution.

    Now what I'd like to do is show you the test model I put together. Here, I'll just expand this to the full screen. So I have my pack model. I'm showing the dimensions of the signals here for each of the signals. The way you do this is you go to debug, information overlay. And then you can select signal dimensions. There's other settings you can choose as well. But signal dimensions is what you select in order to see the dimensions.

    So what we need for the passive cell balancing, we need a battery constant voltage charger, which produces a current which we feed into an ideal current source. And what we're doing is we're monitoring the 16 parallel assembly voltages. And I double click on this. We're just taking the maximum value, which is into the constant voltage charging system.

    20 amps current when we're charging. And if we were discharging, which we're not in this case, it would be minus 20. We can set those separately as you can see. But today we're only charging.

    And here's the second component, the passive cell balancing control system. So we just double click on this, set threshold as 0.002. I'm updating sample time of TS. So TS is set to one second in this particular case. We're going to simulate over five hours.

    So you can see the input to the passive cell balancing algorithm is the parallel assembly voltages. It then gives out 16 commands, which go into the balancing port. And then those 16 commands are then appropriately distributed among the parallel assemblies.

    I did change a parameterization for this example. I changed the initial state of charge of each parallel assembly. Let me just show you where that was done. I'll just look at module 1 in this case.

    So I've selected cell model's state of charge. And I'm going from 0.7 0.702, 0.704, et cetera. So we can see at time 0 that each parallel assembly has a different state of charge. And we'll be able to visually see passive cell balancing in action.

    OK. Before we simulate, just show you set solver configuration to use local solver, sample time of one second. So we'll just run this. It's a five hour simulation. It will only take a few seconds to simulate.

    OK. Simulating now. You can see how quickly it's simulating. Let me just open up the simulation data inspector for that line. And what we'd actually like to take a look at is the state of charge of each of the parallel assemblies. We select socPA, convert to individual channels, and I'll click on all of these. And we'll make some observations once I've got through them all.

    OK. You can see how the parallel assemblies have all started from different states of charge. So we have an initial ramp up. Then the charging starts decreasing as we start approaching the voltage threshold. You can see that the state of charges are starting to equalize.

    And then at the end, we are within the tolerance that we set the passive cell balancing algorithm. I'll just zoom in on this. So we're within 0.002. And 0.002 is the threshold for balancing that we set.

    So to recap, what we've done is we built a battery pack with balancing circuits included on each parallel assembly. We then put the battery pack with balancing circuits into a test harness, which includes the battery constant current cost of voltage charging algorithm and also the passive cell balancing algorithm. And we saw that the passive cell balancing algorithm operated as expected. I hope you'll find this information useful. Thank you for listening.