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backup detection

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Quank

New Member
Hello,
I'm looking for a simple circuit that does then next:
- when 12 V applied in the input connect the input to a battery charger and the same voltage to the output.
- when no voltage applied in the input, connect the output to the battery charged in the first stage.
Any ideas. Where to begin?
Thanks

Quank
 
Here's a circuit that should do what you want using a DPDT relay. This assumes that you can leave the battery connected to the charger output (which should not be a problem).

Relay Ckt.JPG
 
If you can afford to have the supply vary between 13.8V and 11.4V, for the primary and battery supplies, simply use two diodes, to 'mix' the two.
No relays, and no 'interruption' of the supply, when changing between the two (if that's important?)

Assuming you are using an SLA battery, which needs a constant voltage charge, you can provide a simple charger current limit with a polyfuse across the battery 'diode', and set the primary supply at 14.4V, to set the charging voltage.
 
ronv, crutschow: Thank you for your answers. I think it is a very usefull solution. Why to use a DPDT relay and not to use a DPST? For a 150 W Input Power Supply is a very big relay? I have a little limitation in space...

rogs: I don't understand very well the diode configuration you are explaining. Could you attach me a simple "crockis". My battery is a LiPo battery and is charged in diferent stages with a controller. Is this a problem?
Thank you

Quank
 
Why to use a DPDT relay and not to use a DPST? For a 150 W Input Power Supply is a very big relay? I have a little limitation in space...
If you look at my schematic you will see that a DPST relay will not work.

For a 150W @ 12V you will need a relay with contacts rated for 15A DC .
 
rogs: I don't understand very well the diode configuration you are explaining. Could you attach me a simple "crockis". My battery is a LiPo battery and is charged in diferent stages with a controller. Is this a problem?
Thank you

The simple 'diode mix' idea I was suggesting is only really suitable where you can set the primary supply to act as both a power source, and a constant voltage charge source for an SLA battery. The concept is not really suitable for LiPo batteries I'm afraid!
 
Okey, I'm gonna use a relay a DPDT relay. Is there any way to make a "no interruption" output supply when making the transition from "ALIMENTATION OFF" (backup is giving supply and is not fully discharged) to "ALIMENTATION ON" (charging battery)?
Maybe to put a diode network on the output?
Thanks
Quank
 
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