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This is regarding home UPS/inverters which are typically known to convert the DC from the battery to AC, so the AC can be used for household purposes.

What I don't understand is how the batteries are charged.

From my understanding the 12V or 24V batteries be it lead-acid or Li-ion are charged using DC current. The batteries don't understand AC current either for input or output. They take DC and output DC. But the typical household gets AC current.

Which device is converting the AC to DC for the batteries to get charged?

Is it done by the UPS/inverter device?

For example take this device (branded and sold as UPS not an inverter) - Home UPS 900VA Zelio+ 1100 and these devices (branded as inverters and sold as UPS.) - Exide UPS/Inverters

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  • \$\begingroup\$ Note that there are inverter-only devices, used when you want AC power in your car, boat or recreational vehicle. There are also inverter/charger units that use external batteries - these are used in boats and RVs. \$\endgroup\$ – Peter Bennett Oct 28 at 19:25
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They have a power supply that takes AC mains in and converts it to DC outputs to charge the batteries.

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  • \$\begingroup\$ And buried in that power supply are several rectifiers, probably in the form of diodes. Probably four "main" ones forming a bridge, and numerous diodes in the power supply circuit itself. \$\endgroup\$ – TimWescott Oct 28 at 18:35
  • \$\begingroup\$ gr8 - buried in that power supply are several rectifiers, probably in the form of diodes. - this answers @TimWescott. \$\endgroup\$ – samshers Oct 28 at 18:38
  • \$\begingroup\$ +1, so though the UPS's are mostly marketed as inverters they bundle the functionality of rectifier too which is often unmentioned. \$\endgroup\$ – samshers Oct 28 at 18:40
  • \$\begingroup\$ @samshers Well charging the battery one way or another is kind of expected feature from an UPS device, just like it is expected to make phone calls with a phone or a car to include a steering wheel (or rectifiers to charge battery from the AC alternator). \$\endgroup\$ – Justme Oct 28 at 20:25
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enter image description here

Figure 1. Image source: Electrical Notes.

The battery charge converts mains voltage down to a suitable voltage and rectifies it to create DC for battery charging. In the old days this would have been done with a heavy iron transformer and rectifier. Modern units will use a switched mode power supply which is built around a much lighter high-frequency transformer and rectifier.

The linked article may be of interest.

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  • \$\begingroup\$ Actually modern switching off-line converters rectify first, then convert the voltage down. But -- po-tay-to, po-tah-to, there's still rectification happening. \$\endgroup\$ – TimWescott Oct 28 at 18:58
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If it's a UPS it must have a battery and for that battery to be charged there has to be one or more forms of rectification to convert the incoming AC to a DC form that can be regulated to charge the battery.

so though the UPS's are mostly marketed as inverters they bundle the functionality of rectifier too which is often unmentioned. – samshers

Whether or not it is a UPS based device (that must use an energy storage device like a battery) there will be rectification and diodes. The presence of a rectifier is implied whether it be UPS based inverter or, an inverter without an internal battery. Diodes and rectification are needed in other areas of the circuit too such as the output stages that convert a low(ish) DC voltage to a high AC output voltage.

Which device is converting the AC to DC for the batteries to get charged?

Probably, in most scenarios of low to moderate power it will be a flyback converter (that will use diodes).

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