Getting a warning when trying to return batteryPack. How do I fix it to make this code run correctly?

Question

I'm keep getting a warning when trying to run the code below: "warning: reference to stack memory associated with local variable 'batteryPack' returned [-Wreturn-stack-address]". How can I fix this to make it run correctly?

#include <iostream>
using namespace std;

class Battery {
public:
    Battery() {} // default constructor
    Battery(double voltage, double maxStore, double curEnergy);
    Battery BatteryA(double voltage, double maxStore, double curEnergy);

    double powerDevice(double curEnergy, double voltage);
    double maxTime(double curEnergy, double time, double voltage);
    double reCharge(double maxStore, double curEnergy);
    double testBattery(Battery);

private:
    double voltage;
    double maxStore;
    double curEnergy;

    friend Battery& operator+ (Battery&, Battery&);
    friend istream& operator >> (istream&, Battery&);
    friend ostream& operator << (ostream&, const Battery&);
};

Battery::Battery(double battVoltage, double battMaxStore, double battCurEnergy) {
    voltage = battVoltage;
    maxStore = battMaxStore;
    curEnergy = battCurEnergy;
}

Battery batteryA(double voltage, double maxStore, double curEnergy) {
    Battery batteryA;
    voltage = 12.0;
    maxStore = 5.00e+006;
    curEnergy = 5.00e+006;

    return(batteryA);
}

istream & operator >> (istream &is, Battery &batteryB){
    is >> batteryB.voltage >> batteryB.maxStore >> batteryB.curEnergy;
    return is;
}

ostream& operator<<(ostream&os, const Battery&batteryA) {
    os << batteryA.voltage << fixed << "-V," << batteryA.maxStore << scientific << "J," << batteryA.curEnergy << scientific << " J" << endl;
    return os;
}

double Battery::maxTime(double curEnergy, double time, double voltage) {
    time = 52083 - (curEnergy/(voltage*8));

return (time);
}

double Battery::powerDevice(double curEnergy, double voltage) {
    double w;
    w = 900*voltage*4;
    if (w < curEnergy) {
        curEnergy = curEnergy - w;
    }
return (curEnergy);
}

double Battery::reCharge(double maxStore, double curEnergy) {
    curEnergy = maxStore;

return (maxStore);
}

double Battery::testBattery(Battery) {
    double time = 52083;
    Battery BatteryA;

    cout << "How long the battery's remaining energy can charge (in seconds): " << endl;
    cout << BatteryA.maxTime(curEnergy, time, voltage) << endl;

    cout << "How long the battery's remaining energy can charge after recharging (in seconds): " << endl;
    cout << time << endl;

return (0);
}

Battery& operator+ (Battery &A, Battery &B) {
    Battery batteryPack;
    batteryPack.voltage = A.voltage + B.voltage;
    batteryPack.maxStore = A.maxStore + B.maxStore;
    batteryPack.curEnergy = A.curEnergy + B.curEnergy;

return batteryPack;
}

The issue seems to revolve around the final lines of the above section. The remaining code is shown below.

int main() {
    Battery batteryA(12, 15, 13);
    Battery batteryB(13, 16, 14);
    Battery batteryPack;

    batteryPack = batteryA + batteryB;

    cout << "Initial battery as constructed: " << batteryA << endl << endl;
    cout << batteryA.testBattery(batteryA) << endl << endl << endl;
    cout << "Enter battery to place in the battery pack: (";
    cin >> batteryB;
    cout << ")" << endl << endl;

    cout << batteryPack.testBattery(batteryPack);

    system("pause");

return 0;
}