Lead Acid Battery charging Discussion

Hi all,

Does any one knows how to charge a battery using active rectifier?? I have seen in some UPS that they are using active rectifier for both charging in mains mode and for inveter in battery mode. Can anyone explai how this happen?? How they converting AC/Dc

In some UPS systems that re 3 phase I have seen SCR's used.
Pulsed at the correct time in the AC sine wave to get the correct voltage needed for the DC bus.
Not sure how to attenuate the current.