What is Round trip efficiency?

The process of battery charging and discharging is always associated with some energy loss. Round trip efficiency is the combined loss of energy added to and withdrawn from a battery. It is given as the percentage of the energy put in. Round trip efficiency of lead-acid batteries is commonly about 75 percent. Li-ion batteries are claimed to boast a round trip efficiency of 96 percent or even higher. However, the actual value is normally lower. Thus, the round trip efficiency of Li-ion-powered Tesla Powerwall is 92.5 percent.

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