computer power supply battery charger

googling has provided some info but still confused maybe someone can help

i want to use a computer battery supply as a battery charger--lead acid nicad, etc

it puts out 12v dc and 5 volts dc

advice talks about putting a load resister from 12v to ground to turn on power supply ( power supply is a switching type0

confused at this point as there is 12 v available when i test it--i am assuming at this point the power supply is on--a load across a 5 ohm power resister in series gives 3 volts using ohms law i get .600 amps--am i right so far?

power supply specs say i can get 10 amps at 212 v

what do i have to do to get this? any help really is appreciated

i have no info such as schematic on this power supply

tia pter

Reply to
ilaboo
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Voltage is the force that causes amps to flow. That's Ohm's Law (high-school physics). You don't get to do any electronics without mathematics.

What it means is that the number of amps actually flowing depends on the resistance of the load. Try lighting a 10-amp headlight bulb with your computer power supply -- if it can deliver 10 amps at 12 volts, it will do it.

Now for the bad news. 12.0 volts is not enough to charge a lead-acid battery fully. Fully charged, the battery itself would be at 12.6 volts. Normal output of a float charger is 14.1 volts. Automotive battery chargers drive a large current into the battery by putting out a much higher voltage (something like 20 volts).

Reply to
Michael A. Covington

Perhaps a "charge a dead battery in 30 minutes" charger might deliver

20 volts, at great risk to the battery, but a more normal charge routine will use a maximum of 14.4 volts, or so.

In any case, the 12 volts you get from a computer power supply is definitely not adequate to charge a "12 volt" lead-acid battery.

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Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
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Reply to
Peter Bennett

You'd be best off to get a proper charger for your 12 Volt battery. Chargers are properly current controlled, and have the proper over-head voltage to charge the battery. Chargers are properly designed for charging batteries.

Using a device to charge a battery that is not properly regulated and rated for the application, and meeting proper standards for safety, can result in a dangerous hazard.

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Jerry G.
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Reply to
Jerry G.

Maybe he can use the 5 volt output in series with the 12 to get 17? That ought to be enough with appropriate resistor for a trickle charger at 1 amp or so. But it won't shut off and so needs a timer or somebody watching a clock.

-Bill

Reply to
Bill Bowden

Maybe he can use the 5 volt output in series with the 12 to get 17? That ought to be enough with appropriate resistor for a trickle charger at 1 amp or so. But it won't shut off and so needs a timer or somebody watching a clock.

-Bill

Reply to
Bill Bowden

Maybe he can use the 5 volt output in series with the 12 to get 17? That ought to be enough with appropriate resistor for a trickle charger at 1 amp or so. But it won't shut off and so needs a timer or somebody watching a clock.

-Bill

Reply to
Bill Bowden

+5V and +12V share a common ground wire -- they can't be put in series.

You could use +12 and -5, or +5 and -12, but I think the negative ones only put out a very limited amount of current.

Reply to
Michael A. Covington

You have several problems.

First, you must sort out the computer power supply. Depending on the power supply, it may turn on with a switch (AT) or when you short a certain wire to ground (ATX). Then, it may require a certain amount of load on the 5v line in order to actually turn on, or it may require a certain load on the 5v line in order to regulate properly, or it may not require any load at all.

First figure out which type of power supply you have, then look up the correct wiring colors on the web.

The computer power supply is regulated, but this doesn't help you much, because neither 5V nor 12V are useful for charging batteries. To charge NiCDs, you'll need to control the current flowing into the battery. In the simplest case, you can do this with a resistor and a large voltage drop. Let's suppose you have a 1.2V NiCD or NiMH (they charge very similarly) battery. These are usually slow-charged at C/10, which means that you take the capacity of the battery, divide by 10 in appropriate units, and charge with this current. For example, if you had a 2 amp-hour battery, you'd charge it at 200 mA (and then set your alarm clock for about 12 hours later and take it off the charger). Look up a battery datasheet for more information about this.

Now, suppose you don't want to build any hard circuits. You take your

12v supply, subtract 1.2V for the battery, and have a 10.8v drop. 10.8/0.2 = 54, so you want a resistor close to 54 ohms, with higher being safer than lower. You could do two batteries in series by redoing the calculation, but I wouldn't go to four; at that point, the change in voltage of the batteries as they charge (they'll go from about 1V to 1.5V as they charge) becomes significant and the resistor no longer approximate a current source very well.

This resistor will be dissipating a good bit of power, so get a big one. (0.2 A * 11 V = 2.2 watts)

The situation for lead-acid is rather worse. Lead acid batteries are charged with a constant voltage somewhere in the range of 13.8 to 14.6, depending on whether it's a float charge (left on constantly) or a cyclic or top-up charge (can't be left on permanently without damaging the battery). The only way you're going to get these voltages from your computer power supply is to use the +12 and -5 (or +5 and -12) outputs, and the negative outputs are usually limited to about 500 mA. If you wanted to be cheap and approximate like we were with the "constant current" charge for the NiCDs, you could get into the right ballpark with four 1n400x diodes in series. However, you would then *still* have the problem of making sure you don't load your computer power supply beyond what it can handle and make it give up the magic smoke, and given that even a smallish lead-acid battery is going to draw considerably more than 500 mA to charge, I think you should forget about using the computer power supply to charge lead acids unless you have a 6V one.

Reply to
Terran Melconian

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