Battery efficiency is most easily described as the ratio of energy out to energy in. The more efficient a battery, the lower the losses will be during charging/discharging. Heat generation is a significant loss mechanism for most batteries, but lead-acid batteries also lose electrical energy to gassing. Gassing is the decomposition of water in the aqueous electrolyte into hydrogen and oxygen gas. This process, more commonly known as electrolysis, increases at the higher voltages associated with the absorption charging phase.
Heat generation and gassing are responsible for ~20% electrical loss in lead-acid battery operation, giving an overall round-trip efficiency of ~80%.
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