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12v to 12v charging circuit.

markalishousfantastico

Oct 5, 2010
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hi all, this might sound like a silly question however i am only new to electronics and i am unsure. i need a circuit that charges a 12V 7.2AH battery that is to be installed as a secondary battery a vehicle. this battery needs to power a electronic device once the ignition is turned off without draining the starting battery of the vehicle. any help would be greatly appreciated.
 

poor mystic

Apr 8, 2011
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Hi there markalishousfantastico
(I surely did copy & paste that user name)
If your electronics isn't a heavy power user you might get away with a 50 cent diode. But let's enquire.
I take it you want to leave the battery permanently installed in your car, and have a small load permanently running from it? What is the load to be, please?
I also guess that your battery is a sealed lead-acid unit?
If you'd kindly answer those questions we'll be in a better position to help.
see ya!
:)
 

poor mystic

Apr 8, 2011
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Hi again
A surprisingly large load!
Perhaps you could simply connect it in parallel with the existing battery, but with a diode in the positive lead to prevent the new battery from feeding back into the vehicle's electrics. This idea has the virtues of economy and simplicity.
The diode will slightly undercharge the battery due to the voltage loss of half a volt or so, but your battery is large compared to your needs so that might not matter.
 

poor mystic

Apr 8, 2011
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Look I've been thinking i should have said to get a really high specification diode with good current characteristics, because it will last. Maybe a specialist auto-electrician or professional electronics supplier can get you the good diode you want.
Execution is the whole of reliability in this job too and it all needs to be solid and well screwed down.
 

markalishousfantastico

Oct 5, 2010
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Oct 5, 2010
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would this circuit work?
dhlschem.jpg

i hope that works.
cheers.
 

markalishousfantastico

Oct 5, 2010
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i cant get the image to upload but would a PX6007 rectifier be alright to use and should it be placed on the positive lead of the secondary battery with the anode closest to the positive terminal (arrow would be pointing away from battery in a schematic)
cheers
 

markalishousfantastico

Oct 5, 2010
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i was also thinking would it be a good idea to put a fuse before the rectifier aswell is case it fails or a surge occurs?
cheers
 

Resqueline

Jul 31, 2009
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While the PX6007 is relatively beefy it'll still give you a 0.7-0.8V drop which is too much. The accessory battery needs all the voltage it can get to have a chance of recharging.
Schottky diodes have the lowest available forward voltage drop (but can only stand reverse voltages less than 100V, usually 40V). Choose a 15-25A 40V diode.
What will the driving pattern be (run/stop times)? A 23 hour stop period will drain 3Ah, making it neccessary to have an average charging current of 4A during the remaining hour.
A fuse is usually a good idea. Don't choose it too small or it'll blow on startup. It's there only to prevent the wires from melting. 15-25A is probably a suitable value.
 

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