I thought this would work ... but it did not.

Hi all,

I needed an emergency battery charger so I thought to myself what do I have around the house that should work for this; maybe not perfectly but should do something.

I had a spare ATX 300W computer power supply.

I figured that if i took a 12v dc and a 3.3v dc and connected them to get 15.3 vdc and a few black wires as ground I'd have a charger of sorts.

So I did just that and connect the positive side of the battery to the yellow and orange wires and the Negative side to the black wires; plugged in the unit and flipped the switch.

The fan was moving and it seemed that everything was going according to plan.

I was very prudent.

I had the unit outside.

I calculated that i probably needed about 35 to 40 amp hours of charge; it was a rough guess and with a 300W PS running at 15.2 volts it would be close to 20 amps and that would mean I'd need about an hour to an hour an a half to charge the battery.

I figured if I did the charging in 15 minute segments I'd be safe and not have any over heating or any excess hydrogen buildup.

It didn't work. No, I didn't have any explosions or anything it just didn't work.

On the third time that i shut the PS off I noticed that the fan was still turning and it's something that I did not pay attention to the previous two times I shut it off.

Hmmm. That's odd, well at least to my way of thinking (assuming that it was working)

It wasn't until I unplugged the unit that I realized that I hadn't done anything with respect to charging.

The fan on the PS was still running.

So, I disconnected the PS and brought it inside and discovered that the PS does not turn on anymore.

I thought that would have worked.

Did I miss something? I remember way, way back in physics about electron theory and that electrons flow out of the Negative side, and maybe that's how battery chargers are configured internally so that when you attach the red wire you are attaching the 'negative' internal connection.

In my case, I assumed that the postive voltage was supposed to be hooked up to the postive terminal of the battery.

I realize that a computer PS is NOT a great candidate for a battery charger, but i had an emergency situation.

I welcome any comments (please don't chastise me too much :screwy:)

thanks

I admire the macgyverism but there can be many reasons it didn't work. First off is the battery any good to begin with. If the battery has an internal short the power supply would have died almost instantly. Why was the battery drained in the first place? I could go on about the other reasons but that would involve too much typing at 7am.

What you did is shorted the 3.3V output to the 12V output by connecting them in parallel. In theory, you would have needed to connect the positive of the 12V supply to the negative of the 3.3V supply, then the ground of the 12V supply to the battery and the positive of the 3.3V to the positive terminal of the battery. However since the internals of this switching supply probably share a ground, that still would not have worked and still would have damaged the power supply. Additionally, the two powerforms probably are not rated for the same current and with nor real form of current ramp up/feedback for a charging application, the discharged battery likely would cause such a current draw that the power supply would go in to foldback anyway.

As Snshddog mentioned above, you do need to check the condition of the battery and determine why the battery failed before connecting a real charger to the battery. If there's a Harbor Freight near you, you can pick up a charger intended for automotive use pretty inexpensively there along with a multimeter to help you determine if the battery is good to start with.

-Rod

If I recall correctly (its been a month or two), you have to jump a green and black wire together on a standard power supply to use it outside of the computer. I use them to test spare car stereos and 5v sensors.

I admire the macgyverism but there can be many reasons it didn't work. First off is the battery any good to begin with. If the battery has an internal short the power supply would have died almost instantly. Why was the battery drained in the first place? I could go on about the other reasons but that would involve too much typing at 7am.

The battery is still good. I drained it somewhat trying to start the car when I finally realized that my 1991 Honda probably has a faulty Master Relay. When I turn the key I get the engine light but I don't hear the fuel pump anymore and it didn't appear to me that I was getting any spark. I tried a few things and each time I reset something or did something I tried starting the car; after about 15 tries, the battery started getting a little weak.

If I recall correctly (its been a month or two), you have to jump a green and black wire together on a standard power supply to use it outside of the computer. I use them to test spare car stereos and 5v sensors.

Yup, had the green and black jumpered.

What you did is shorted the 3.3V output to the 12V output by connecting them in parallel. In theory, you would have needed to connect the positive of the 12V supply to the negative of the 3.3V supply, then the ground of the 12V supply to the battery and the positive of the 3.3V to the positive terminal of the battery. However since the internals of this switching supply probably share a ground, that still would not have worked and still would have damaged the power supply. Additionally, the two powerforms probably are not rated for the same current and with nor real form of current ramp up/feedback for a charging application, the discharged battery likely would cause such a current draw that the power supply would go in to foldback anyway.

As Snshddog mentioned above, you do need to check the condition of the battery and determine why the battery failed before connecting a real charger to the battery. If there's a Harbor Freight near you, you can pick up a charger intended for automotive use pretty inexpensively there along with a multimeter to help you determine if the battery is good to start with.

and I may be totally wrong about what I just said!

-Rod

Well, I guess that explains that. I live south of the border, very poor ... no that's incorrect dirt poor? extremely poor? Most of the time wondering where the next meal is coming from poor. Friends have helped in recent months with a little cash that pays electric and this internet because I am working on a computer project that should have a decent payoff 9 - 12 months from now. This means I can't afford to go and buy any charger or multi meter. Like I said I don't think that it was as much of a current draw as it was a current push. Fan was spinning with it unplugged. I thought connecting or bundling the leads in parallel would have done it; you are suggesting if I understand it correctly that the power should have been connected in series. Not that I understand where the negative part of the 3.3 volts was
http://www.techspot.com/gallery/data/501/atx-psu-pinouts.gif

I grabbe two yellow wires from the ATX 4 pin Molex connector and two orange wires from the 6 pin AUX connector and 4 ground wires.

It's been too many years since I studied this, but if I rememer my theory a battery doesn't have resistance; it shouln't draw any power; even when dead. I thought that chargers regardless of power output (trickle vs very hot charging ) pushed energy back into the battery.

Someone said you could trickle a charge using a phone charger, but that would only get the battery up to 12 volts. If I recall that it needs to be something like 12.6.

But, a phone charger with its tiny transformer would be what is called a trickle charge right?

...you are suggesting if I understand it correctly that the power should have been connected in series. Not that I understand where the negative part of the 3.3 volts was
http://www.techspot.com/gallery/data/501/atx-psu-pinouts.gif

I grabbe two yellow wires from the ATX 4 pin Molex connector and two orange wires from the 6 pin AUX connector and 4 ground wires.

You understand correctly what I'm suggesting. And since I believe all grounds are common in this ATX supply, it wouldn't work to connect these in series with this supply anyway, hence my prior comment of "However since the internals of this switching supply probably share a ground, that still would not have worked and still would have damaged the power supply."

It's been too many years since I studied this, but if I rememer my theory a battery doesn't have resistance; it shouln't draw any power; even when dead. I thought that chargers regardless of power output (trickle vs very hot charging ) pushed energy back into the battery.

I'm no expert on batteries. With that in mind, an ideal battery will have no internal resistance. A real battery will have some internal resistance though which contributes to "self discharge" and is a large factor in why a battery sitting on the shelf will eventually discharge itself. A typical lead acid battery will use constant voltage, constant current to charge the battery. As the voltage on the battery increases, the current draw of the battery will also decrease. I don't completely understand the theory behind this. A smart charger will actually stop supplying current momentarily and measure the voltage, then depending on the no-load, no-charge voltage, will determine what rate to continue charging the battery at. If you watch the current meter on a standard transformer battery charger you'll probably see the current meter spike when the charger is powered on. That surge of current draw will likely kill your ATX supply, and if not, the connection of the parallel 3.3V supply probably would.

Someone said you could trickle a charge using a phone charger, but that would only get the battery up to 12 volts. If I recall that it needs to be something like 12.6.

But, a phone charger with its tiny transformer would be what is called a trickle charge right?

Many of the current celphone chargers charge off of only 5V (like a USB port would supply) which won't even provide a trickle charge to the battery. If you happen to have another ATX supply and a test light you'd be better off just using one of the 12V supply lines through the test light to the battery. The test light will limit the current draw on the ATX supply. You'll probably see the test light glow very brightly at first and dim over time. Once the test light stops glowing you could probably connect the ATX supply directly to the battery as the battery will have a reasonably good surface charge on it at that point. I'm not sure this regulated 12V will ever put enough charge in the battery to get you very many cranks of the starter though.

Assuming your emergency situation has passed, can you get to the car with some jumper cables and a running vehicle? Good luck!

-Rod

Thanks, everyone for taking the time to reply. Why can't these dumb car manufacturers come up with a "Mr Fusion" reactor that you can throw banana peels into it to get the 1.21 gigawatts necessary to go back to the future. Ahem, that is if you also have a DeLorean and a Flux Capacitor.

You can use any 12V DC "wall wart" power supply as a charger. They put out anywhere from 14 to 18 volts open circuit. Making sure polarty is correct, you can connect it to your car battery. To charge a weak battery would take a few days, but they work fine and there is no risk of overcharging. They make great trickle chargers to keep a battery charged while sitting over the winter etc.