Lithium ion battery is in charging stage has higher mass than in discharging stage. Why this is the case? If difference in mass-energy has gone into the lighting of bulb, then the emitted photon must have the same mass and corresponding the same energy. The emitted photon must have the same energy which is equivalent to the Gibbs free energy of the battery. What is the chance that, then emitted photon has non zero very small mass? No matter how small this mass number is, but then photon mass will NEVER be perfect zero as current established physics suggest?