Using an SMPS to charge a car battery

[JonathanAnon]

Went out this morning and the damned car wouldnt start.. I kept trying and dragged the battery down to 12.2V and it would do anything... I cycled to the local motor place and bought a plug in charger, but this got me thinking.

But how about this.. If I'd chosen to be MacGyver this morning,


- take the SMPS out of my PC
- plug it in to the wall.
- jump pin 14 to ground to turn it on
- pull out a 12v DC + and GND - connection (one of the HD/CDROM connections) and hook it up to the battery..
- check the charge periodically with my voltmeter.

Would this not charge the battery with a smaller current, the same as the 8 amp charger that I bought, but obviously more slowly?

[JonathanAnon]

Re: Using an SMPS to charge a car battery

Actually I'm just thinking of the first problem.. The battery in the car needs to be charged to 12.6-12.8V, the SMPS 12v rails wont be that high.

[eccerr0r]

Re: Using an SMPS to charge a car battery

The next problem is there is no current limiting. The ATX PSU is constant voltage, even if you did increase the voltage to the ~13.8V that's needed to charge a flooded lead acid battery, it'd dump 50A and fry the PSU...

[mariushm]

Re: Using an SMPS to charge a car battery

You can adjust the feedback circuitry of an ATX power supply to raise the output to 14-15v, and even above but you have to worry then about the output capacitors being rated for maximum 16v. If you change them to 25-35v you can do more.

I suppose you could use some diodes to prevent current flow going backwards into the power supply...

You'll still have to take care of current limiting and other protections from the power supply though.

Someone was successful in raising his battery voltage a bit: http://www.aussiemagna.com/forums/sh...ad.php?t=75037

[selldoor]

Re: Using an SMPS to charge a car battery

But how about this.. If I'd chosen to be MacGyver this morning,

Youd be at least a week late

[tom66]

Re: Using an SMPS to charge a car battery

If you fly model aircraft (my hobby), try this trick:
https://www.youtube.com/watch?v=6DnLjpoUwDM
https://www.youtube.com/watch?v=1w_K08fwZnA

[eccerr0r]

Re: Using an SMPS to charge a car battery

The nice thing about charging batteries, for the most part you can dump (or at least reduce) the output capacitors, usually batteries don't care about clean power, ripple is OK (and those desulfators... they're all about huge ripples!). Backflow isn't a great problem, the rectifier diodes do prevent some of it but the overshoot protection resistors will drain batteries if the SMPS is left unplugged.

But again, current limiting is the key issue. Some serious mods are needed to the PSU to prevent it from getting fried when a super low ESR device of lower voltage is hooked up to the output.

[tom66]

Re: Using an SMPS to charge a car battery

Most computer SMPS use a simple on/off current limit. When the current exceeds the rated limit, the power supply shuts off. Or, if it has no current limit, it attempts to source hundreds of amps, resulting in rapid failure -- sometimes, even with flames!

Ideally, the supply would be current limited, with a constant voltage. An LED driver is a good possibility: most will not shut down on even a shorted load.

[eccerr0r]

Re: Using an SMPS to charge a car battery

Oh... and my posts seem to imply that protecting the PSU is the only issue... no, also protecting the battery too. The I^2*R (ESR) heating of the battery is not good for it either. Plus charging it too fast may cause heating and coupled with any hydrogen generated....

[larrymoencurly]

Re: Using an SMPS to charge a car battery

The nice thing about charging batteries, for the most part you can dump (or at least reduce) the output capacitors, usually batteries don't care about clean power, ripple is OK (and those desulfators... they're all about huge ripples!).

My ancient Sears battery charger (probably Schumacher) uses just a triac or SCR to regulate current, and the 741 op-amp circuit that controls it has absolutely no capacitors.

How about a laptop charger with a 5-10 ohm, 10W resistor in series? They typically put out 19VDC.

[JonathanAnon]

Re: Using an SMPS to charge a car battery

The next problem is there is no current limiting. The ATX PSU is constant voltage, even if you did increase the voltage to the ~13.8V that's needed to charge a flooded lead acid battery, it'd dump 50A and fry the PSU...

Okay this was the bit I was confused about.. i.e. how much current would it draw... Because the battery isnt a conventional load which defines the amount of current/voltage required, I wasnt sure what would happen when you hook up the SMPS to it.

So I guess the difference between the SMPS and my battery charger (in respect of current) is that the battery charger defines two current ratings, creatively named "Fast Charge" 8AMP and "Slow Charge", and will not allow the battery to draw any more than this. Whereas the SMPS does not limit current, except by the means described by tom above, where it detects overcurrent and shuts down.

If you fly model aircraft (my hobby), try this trick:

My brother is a frustrated model aircraft flyer tom... He's been messing around with them for years, but the only thing I've seen him fly successfully was on the simulator software..

[eccerr0r]

Re: Using an SMPS to charge a car battery

My ancient Sears battery charger (probably Schumacher) uses just a triac or SCR to regulate current, and the 741 op-amp circuit that controls it has absolutely no capacitors.

How about a laptop charger with a 5-10 ohm, 10W resistor in series? They typically put out 19VDC.

Yes, that's one way to do it, this will work. The resistor will prevent the high current rush, but it will not prevent overcharge which is the other issue.

Okay this was the bit I was confused about.. i.e. how much current would it draw... Because the battery isnt a conventional load which defines the amount of current/voltage required, I wasnt sure what would happen when you hook up the SMPS to it.

So I guess the difference between the SMPS and my battery charger (in respect of current) is that the battery charger defines two current ratings, creatively named "Fast Charge" 8AMP and "Slow Charge", and will not allow the battery to draw any more than this. Whereas the SMPS does not limit current, except by the means described by tom above, where it detects overcurrent and shuts down.

Basically if your PSU voltage is lower than the battery voltage, nothing.
If it's the same... nothing.
If it's greater... Remember V=IR, I=V/R. V is the difference of the battery and the SMPS voltage. R is the sum of the ESR of the battery, resistance of wires, and the effective ESR of the PSU (NOT the caps). Typically each of these three are in the milliohms. So, say the battery is 12 volts, the SMPS PSU is 14 volts to be enough to charge it, and the ESR of the battery, PSU, and resistance of cables = 10 milliohms each, then current draw = (14V-12V)/(30 milliohms) = 66 AMPS. While theoretically possible to charge that fast, but if anything is just not right...

Oh... and fast/slow on those "cheap" car chargers tends to be a winding or so selected off the transformer to change the voltage. There typically is some resistance in the wire or an actual resistor to limit the charge current a bit, but typically it will still go over the rate suggested by the switch...

[JonathanAnon]

Re: Using an SMPS to charge a car battery

Yes, that's one way to do it, this will work. The resistor will prevent the high current rush, but it will not prevent overcharge which is the other issue.

Okay so taking your example above, if there the voltage of the SMPS is 14V and the voltage of the battery is 12V, the you could limit the current to 2amps (within the remit of the 12V rail of the SMPS) by a 1 Ohm 4 Watt resistor

I=V/R
2amp = 2V / 1 Ohm

P=VxI = 2vx2amp = 4Watts

(albeit there is still the issue with you needing to shut it off when the battery reads about 13.7V)

[mariushm]

Re: Using an SMPS to charge a car battery

Power dissipated in a resistor would be I x I x R or V x V / R

Yeah, it would dissipate about 4 watts, so to be safe you'd have to use a 7-10w resistor. It would still be hot.

[eccerr0r]

Re: Using an SMPS to charge a car battery

The tricky part of selecting the resistor too is that when the battery is nearly flat, say at 10V, that 1 ohm resistor during the initial part of the charge would be dissipating (14-10)^2/1 = 16W... or worse if the charging connectors were shorted... so that initial 10W resistor will not do so hot, and rather be very hot.

[JonathanAnon]

Re: Using an SMPS to charge a car battery

Okay that makes sense.. So I guess when you're not using a defined current source, and you dont know what current a load will draw of it's own volition, you need to limit the current yourself.. That's answered my question...

[eccerr0r]

Re: Using an SMPS to charge a car battery

hmmm. I wonder if there's any interest in a mod for, say, ka7500/tl494 psu's to current limit so you can use for charging once you up the voltage to 14-odd volts... need some current sense... hmm...

project idea... 2 stage smps charger using a pc psu...

[amzee94]

Re: Using an SMPS to charge a car battery

Whoo awesome idea

[budm]

Re: Using an SMPS to charge a car battery

You can have the power supply feed the 2A constant current source, the output of the current source will feed the 14.5V Voltage regulator, the output of the regulator will feed the Adnode side of a 3A Diode, the Cathode will be connect to the batteries.
The maximum current will always be limited to 2A by the constant current source. When the batteries voltage reach about 13.5V (14.5 -1V Vf of the Diode), the Diode will not be bias on so the charging will stop.

[eccerr0r]

Re: Using an SMPS to charge a car battery

Getting a constant current source is not cheap either. The rub is to make a CC source with the guts of the SMPS... I think all one really needs is an op amp, a transistor, and a few resistors to mod a tl494/ka7500 smps to limit current...

Or quite possibly all the stuff is already there, just need to repurpose...