MATLAB: I want this code to run only once. Battery Charge/Discharge

Question

Hi guys, newbie here on Simulink, this is my first project.

I'm trying to make a battery charge and discharge simulation, I'm using a project i saw in a youtube amateur tutorial, but the example itself is flawed and I'm trying to do my changes. I have this code here:

What it does is when the SOC is > 80% the system is feeding the Load, when it reaches below 40% it turns of the Load and starts charging the batteries.

function [LoadOn, ChargingOn] = fcn(SOC)
    LoadOn = 1;
    ChargingOn=0;    
         
    if (SOC>80)
        LoadOn=1;
        ChargingOn=0;
    end
    
    if (SOC<40)
        LoadOn=0;
        ChargingOn=1;        
    end

The problem that I'm having is that on the iteration that it reaches >40% it correctly changes the variables to Load=0 and Charge=1, but on the next iteration, due to the second and third line of code, it changes back to LoadOn=1 and Charge=0, when I wanted it only to happen at SOC>80%.

And If I remove the 2nd and 3rd line it gives me an error. Basically what I want to do is that the 2nd and 3rd line only run once.

Any suggestions?

Answer 1

One way of doing this would be using Persistent variables - but just as with global variables you will need to be careful - to get back to your starting condition you should use 'clear fcn'.

Additionally the persistent variable should not be the output of the function.

e.g.

function [LoadOn, ChargingOn] = fcn(SOC)
    persistent LoadOnPersistent;
    persistent ChargingOnPersistent;
    
    if isempty(LoadOnPersistent) % Initial condition
        LoadOnPersistent = 1;
        ChargingOnPersistent = 0;
    end
         
    if (SOC>80)
        LoadOnPersistent=1;
        ChargingOnPersistent=0;
    end
    
    if (SOC<40)
        LoadOnPersistent=0;
        ChargingOnPersistent=1;        
    end
    
    LoadOn = LoadOnPersistent;
    ChargingOn = ChargingOnPersistent;
    
end