LeBron James invented a solar charging system for iphone 15 . The system equips with a solar panel with 1.0V,500mA output under direct sunlight; in order to fit the voltage of the USB connector on your smartphone, an IC chip that convert 0.5V to 5.0V is used. When charging the iphone 15,15% energy dissipates in the form of heat and therefore your battery temperature increases. Assuming the IC efficiency is 75%, how long does it take to fully charge a 4000mAh battery in your smartphone (you may assume we have 24 hours sunlight a day!!)