The 20-80 rule means keeping your battery's state of charge (SOC) between 20% and 80% instead of fully charging to 100% or draining to 0%. By avoiding extreme voltages, you reduce chemical stress inside the cells and significantly extend battery life.
Solar Technology Has Ancient Roots but Modern Applications: While humans have harnessed solar energy since the 7th century BCE using burning mirrors and passive solar architecture, the scientific foundation for modern photovoltaics wasn't established until 1839 with Becquerel's.