batterysim.py

import matplotlib.pyplot as plt
import numpy as np
import time

max_ah = 18 # Maximum Amp Hours of the batteries
dt = 0.01 # Time delta for graph points
res = 0.5 / 1000 # Internal Resistance
tol = 0.01 # Tolerance/Delta between the 2 battery voltages before stopping

# Ask user for state of charge percentage
batt1_soc = int(input('Battery 1 Charge %: '))
batt2_soc = int(input('Battery 2 Charge %: '))

# Convert SOC to decimal
batt1_soc /= 100
batt2_soc /= 100

# Return the voltage from the state of charge using a linearized equation
def calcVoltageFromSOC(soc):
    return soc + 11.8

# Return the voltage from a capacity
def calcVoltageFromCapacity(cap):
    return calcVoltageFromSOC((cap / 3600) / max_ah)

# Return the current from 2 voltages using internal resistance
def calcCurrentFromVoltages(v1, v2):
    return (v1 - v2) / (2 * res)

# Calculate the initial capacity from SOC
def calcInitialCapacityFromSOC(soc):
    return (max_ah * soc) * 3600

# Return the last value in a list
def lastIndex(indexlist):
    try:
        return indexlist[-1]
    except IndexError:
        return 0

# Initialize initial values
batt1_voltage = calcVoltageFromSOC(batt1_soc)
batt2_voltage = calcVoltageFromSOC(batt2_soc)
batt1_capacity = calcInitialCapacityFromSOC(batt1_soc)
batt2_capacity = calcInitialCapacityFromSOC(batt2_soc)

currentList = []
batt1_voltageList = []
batt2_voltageList = []
timeList = []
done = False

# Main loop to calculate current and voltage
while (done == False):
    currentList.append(calcCurrentFromVoltages(batt1_voltage, batt2_voltage))
    batt1_voltageList.append(batt1_voltage)
    batt2_voltageList.append(batt2_voltage)
    timeList.append(lastIndex(timeList) + dt)

    if (batt1_voltage > batt2_voltage):
        batt1_capacity -= calcCurrentFromVoltages(batt1_voltage, batt2_voltage) * dt
        batt2_capacity += calcCurrentFromVoltages(batt1_voltage, batt2_voltage) * dt
    else:
        batt1_capacity += calcCurrentFromVoltages(batt1_voltage, batt2_voltage) * dt
        batt2_capacity -= calcCurrentFromVoltages(batt1_voltage, batt2_voltage) * dt
    
    batt1_voltage = calcVoltageFromCapacity(batt1_capacity)
    batt2_voltage = calcVoltageFromCapacity(batt2_capacity)

    if (abs(batt1_voltage - batt2_voltage) < tol):
        done = True

# Current Plot
plt.subplot(2, 1, 1)
plt.title("Battery Current/Voltage over Time")
plt.plot(timeList, currentList, label='Battery Current')
plt.ylabel('Current (a)')
plt.legend()

# Voltage Plot
plt.subplot(2, 1, 2)
plt.plot(timeList, batt1_voltageList, label='Battery 1 Voltage')
plt.plot(timeList, batt2_voltageList, label='Battery 2 Voltage')
plt.xlabel('Time (s)')
plt.ylabel('Voltage (v)')

plt.legend()

plt.show()