Mass energy

• How Much Does a Smartphone Battery’s Mass Increase When Charged?
  A charged smartphone battery is slightly heavier: estimate the change from its stored energy using Δm = ΔE/c², typically well below a nanogram.
• Does Matter Become Energy at the Speed of Light?
  Matter does not turn into energy at light speed: massive objects cannot reach that speed, and their rest mass remains part of their total energy.