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Connecting a power supply and battery in paralell

lagman

Feb 25, 2012
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I have a home made backup sump pump solution consisting of an AC powered sump pump plugged into a 750W inverter which is powered by a 12v deep cycle marine battery.

I also have a 13.8V 25A DC power supply. If I connect the power supply to the battery in parallel, will the power supply power my sump pump as long as the power isn't out (then if the power goes out will the battery power the sump pump)?

Also, if I leave the power supply plugged in, will it keep the battery charged or will it overcharge?

I hooked everything up as explained above and measured the voltage to be between 13.77 and 13.82V which seems to be in a safe range.

Any advice would be appreciated.

Thanks,
Dan
 

davenn

Moderator
Sep 5, 2009
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hi lagman,
welcome to the forums :)

Yes the PSU is going to act as the main PSU to the inverter as well as a battery charger. Lead Acid batteries charge at a fixed voltage with a current drain that decreases as the battery charges. In your situation the battery will act as a huge smoothing capacitor across the 13V supply.
Ohhh if you read the label on the battery you will probably find that the charging voltage is more like 14 - 15V, meaning that at ~ 13.8V it may not be getting charged properly.

cheers
Dave
 

lagman

Feb 25, 2012
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Feb 25, 2012
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It looks like the charging voltage the battery wants is 14.4V. Am I doing damage to the battery if I charge it at 13.8V or is it just that I am not quite able to charge it to full capacity?

I do have a 1.2A battery maintainer.. If I also hooked that in parallel maybe that would bump it up closer to 14.4V?

Thanks,
Dan
 

jackorocko

Apr 4, 2010
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Yes you will damage the battery. Read up on sulfation, it happens when the battery is consistently undercharged.
 

lagman

Feb 25, 2012
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Feb 25, 2012
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Yes you will damage the battery. Read up on sulfation, it happens when the battery is consistently undercharged.

Thanks for the answers everyone!

So, obviously I do not want to damage the battery. If I add a 1.2A 12v battery maintainer/charger in parallel (along with the 12v power supply) I assume it will be at the proper voltage to fully charge the battery and will turn off once the battery is fully charged, correct? Or will the constant drain of the inverter (~0.35A) cause the maintainer to stay on, thus overcharging the battery?
 

duke37

Jan 9, 2011
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I was under the impresion that a 12V battery should be charged at 13.8 to 14.4V. Your big charger will do this and will not overcharge.

If you leave the battery charger connected when driving the invertor, the battery voltage will fall and the charger will try to maintain it. The invertor uses more current than the charger can give and may stress the charger.

Your diddy 1.2A charger will be of little benefit other than using less mains power once the battery is charged.
 

twister

Feb 12, 2012
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Feb 12, 2012
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Every car I have ever owned has the voltage set a 13.8. But what do the car manufacturers know?
 
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