Permanent Power Supply to Car Battery

[munnaz]

Hi,

I'm just seeing if anyone knows of any possible problems I might have with attaching the 12v output of a computer power supply (see attached picture) permanently to a normal car battery (being used in a car normally). I want to use the power supply to charge the car battery anywhere but have it permanently in the car so I just have to plug in 240v to the power supply. I have 3 main questions:

• will it drain the car battery when car is not running?
• could it damage the power supply when the car is running and power supply not powered?
• and of course, will it actually charge a car battery?

Thanks! Munnaz

[tautech]

Welcome to the forum.

Typical charged voltage for a 12V LA battery is 13.2V
Typical charging voltage for a 12V LA battery is 13.8-14.2V.

PC PSU is not suitable.

[Landrew2390]

1.  Depends on the charger topology
2.  Probably because you're effectively feeding up to 14.4V backwards through the power supply.  They're normally not designed to handle that.
3.  12V isn't enough to charge a car battery.  You need a higher voltage to charge lead acid batteries.  The charger also needs to be a Constant Current/Constant Voltage charger.

If you want to learn about battery charging in depth, I recommend this website.  It's a little more complicated than just hooking up a power supply and letting it go.
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

[munnaz]

Thanks guys.

[Rerouter]

Way to hack a solution, Get a 1W load for the 3.3V rail, grab the feedback 3.3V wire from the ATX24 plug, and divide it down with a pot (start at a ratio very close to 1, then slowly edge away with a voltmeter on the 12V rail, this will raise the output voltage as the regulation % follows from the 3.3V rail,

also get yourself a big stud diode, if the atx looses power and there is still power on its outputs, weird things can happen,

[Jeroen3]

1. No. There will be reverse power diodes.
2. Yes. It will not survive automotive transients. ATX supplies are build for a low price, transients are nonexistent in their intended application scenario. Maybe with additional rectifiers it will. But you'll worsen point 3.
3. Only the first few Ah's, then the voltage required to charge is above 12.5V.

Instead, get a charger. A normal power supply will not enjoy the current hungry nature of batteries and will not run at all on CC mode when the battery is low.
Also consider that you do not want your car to go up in flames due to a faulty DIY charger circuit.