In an online UPS, the batteries are always connected to the inverter, so it isn’t necessary to have a power transfer switch. When a loss of power occurs, the rectifier just drops out of the circuit and the batteries keep the flow of power steady and unchanged. When the power is restored, the rectifier resumes carrying most of the load and also begins charging the batteries. However, the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
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