Question regarding AC to DC converter fundamentals

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convoluted

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Hi all. I'm hoping you kind folk can help clarify some fundamentals regarding AC to DC converters. I'm building a hybrid vehicle as a part of a team competition and we would like to utilize an AC to DC converter to charge a 12V battery from mains (AC). What I understand is that when connected to AC mains, AC is converted to DC and the battery begins charging. What I am unsure about is when disconnected from the AC mains, will there be a risk that the converter could get damaged as the load (DC side) is now a power source and the AC end is open circuited? Or is there some internal built-in protection (like a diode) that disallows any reverse operation/damage? Can the same be said about DC to DC converters? Thanks
 
I've never met a mains powered battery charger that cares about the battery load when it's unplugged. I believe they are all immune to the battery load because they use diodes to convert AC to DC and the battery load doesn't go backwards through the battery charger anyway.

As for DC to DC converters, some convert the DC to AC, then back to DC, and some just "regulate" the incoming DC to a lower voltage. These would be in danger if you applied a higher voltage to their output than they were designed to produce by themselves.

I think you don't have a problem, but I don't think I know all the answers, so I hope other people will throw in their two cents worth.
 

Diodes are your friend.

In a power supply circuit you have a rectifier arrangement to rectify the incoming AC into pulsed DC, then a series of filter networks filter that into steady state DC. Moreover, most charging circuits also incorporate an anti-back feed diode in them as well to prevent the battery from back feeding into the charging source. As far as the charging circuit is concerned, it doesn't see the powered circuit that the battery is powering due to it having a direct connection to the battery.
 
As far as I know, all converter-chargers are designed to be connected full time to the battery, whether AC is supplied or not.

For good battery life, you want a three-stage charger. Starts at a high voltage (14.4 for 12v), backs off when the battery is mostly charged, then drops even more when the battery is fully charged. Old style single voltage chargers take much longer to charge the battery and then "boil" the electrolite when the charge is complete.
 
Thanks everyone! Really appreciate the replies. Blars, that's a neat tip about the three stage charger, I'll have to look into that more. Cheers all.
 
Battery chemistry plays a large part in the design of a charger. Batteries in series generally do poorly charging. Li-ion has a very strict charging requirements and that's why a "gas guage" type system is incorporated. Battery temperature in indicitive of a problem, so PTC fuses are usually incorporated into the pack.

Your battery selection also depends on deep cycling. Lead acid batteries are availabe in deep cycling varieties. They have less short circuit current. To get high short circuit current, the plates have to be close together. If you deep discharge these batteries, the plates short.
 
Diode in the case is used to oppose the flow of reverse current, and is also used for charging to stop after a certain extend (like when the battery is completely charged). I would suggest you to use a wall-wart for 12V, in series of which you need a Diode. That would be a simple go.
 
r.vittalkiran said:
Diode in the case is used to oppose the flow of reverse current, and is also used for charging to stop after a certain extend (like when the battery is completely charged).
Click to expand...

To further elaborate on this and explain how the diode stops the charging once the battery has reached full charge, what happens is that when the difference between the charging source potential and the battery potential is equal to less than the forward turn on voltage of the diode (0.7 volts theoretical for silicon diodes), the diode turns off and charge current stops.
 
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