Implementing Classes Java

Implementing Classes Java

Question in book: Write a class Battery that models a rechargeable
battery. A battery has a constructor public Battery(double capacity) where
capacity is a value measured in milliampere hours. A typical AA battery
has a capacity of 2000 to 3000 mAh. The method public void drain(double
amount) drains the capacity of the battery by the given amount. The method
public void charge() charges the battery to its original capacity. The
method public double getRemainingCapacity() gets the remaining capacity of
the battery.
My Question: Are my instance variables right? How do you figure out what
needs to be in the private instance variables? (If that makes sense) Can
this code be written in a better way?
My Code:
public class Battery
{
private double fullCharge;
private double batteryCapacity;
public Battery(double capacity)
{
batteryCapacity = capacity;
fullCharge = capacity;
}
public void drain(double amount)
{
batteryCapacity = batteryCapacity - amount;
}
public void charge()
{
batteryCapacity = fullCharge;
}
public double getRemainingCapacity()
{
return batteryCapacity;
}
}