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# Charging A car battery with a PSU's 3.3 and -12 rails? Answered

Hello guys, is it safe for my car battery to charge it with a computer psu between the +3.3 (15 amps) and -12 (0.5 amps) rails, the multimeter reads a voltage of 15.2 between them, Thanks

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Ok thanks everyone for your answers and sorry, i had problems with my internet connection, it's an old psu so i think it wont do the job

As you might have noticed from the specs on your power supply, the -12 V rail can only supply a relatively small amount of current, only 0.5 amperes.

By the way, the current*time capacity of a car battery is something on the order of 100 ampere*hours, or so. Which means if your charging current is limited to only 0.5A, then it will take a few hundred hours, i.e. days, to charge. Or half that time for half that amount of charge.

But that's not outlandish. Actually, charging a car battery this slowly is a common method of charging, and it's called a "trickle" charge. So that's what kind of charging you'll be limited to using the wimpy -12V rail of your power supply.

I think the next step is too imagine what amount of series resistance will be necessary to limit the current to not more than 0.5 A.

I start with some naive assumptions. First I assume the 3.3V rail, the -12V rail, and also the battery to be charged, have zero internal resistance, and the only series resistance in this circuit is in the form of an actual external resistor R, put there intentionally to limit the current to less than 0.5 A. (Since the spec on the power supply says current flowing into the the -12V rail cannot exceed 0.5A.)

To find a guestimate for R, I assume the voltage of the car battery, in need of charge, is not less than 10.3 volts.

So the maximum voltage drop across R is: VRmax = 3.3 - (-12) - 10.3 = 15.3 - 10.3 = 5.0 volts. So the minimum value for R is 10 ohm, since R= VRmax/Imax

= (5.0 V)/(0.5 A) = 10 ohm.

It may be the case that 10 ohms, or more, of internal resistances already exist, in your power supply rails. I don't know what the actual values will be. But I think 10 ohms of total series resistance is good place to start. I mean, put your measuring devices on it, an ammeter, or a voltmeter across a small known resistance, so you actually know how much charging current is flowing into the battery, since you don't want to exceed that 0.5A limit.

No. It's not dangerous for the battery, but it probably is dangerous or at least impossible task for the power supply. Car battery, depending how drained and how big it is, can draw up to maybe 8 amperes at that voltage. Or if it's small and relatively charged it still draws 1-3 amperes. That will make the power supply switch off or worse do something it is not meant to and get damaged in the process.

That 15.2V is potential difference, doesnt mean to say you can use it in the real world, what happens when you put a load on it? do you still get 15.2 V and at what current will it charge, 0.5A?