computer power supply battery charger

[ilaboo]

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

 

[Michael A. Covington]

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).

 

[Peter Bennett]

On Mon, 16 Aug 2004 11:55:12 -0400, "Michael A. Covington"
<look@ai.uga.edu.for.address> wrote:

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).

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.




--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Jerry G.]

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.

--

Jerry G.
==========================


"ilaboo" <plener@earthlink.net> wrote in message
news:l%2Uc.27273$Jp6.3817@newsread3.news.atl.earthlink.net...
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

 

[Bill Bowden]

Peter Bennett <peterbb@somewhere.invalid> wrote in message news:<7tl2i05tepb1asb52fg5cq4nb1a5vqqlc4@news.supernews.com>...

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.

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

 

[Bill Bowden]

Peter Bennett <peterbb@somewhere.invalid> wrote in message news:<7tl2i05tepb1asb52fg5cq4nb1a5vqqlc4@news.supernews.com>...

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.

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

 

[Bill Bowden]

Peter Bennett <peterbb@somewhere.invalid> wrote in message news:<7tl2i05tepb1asb52fg5cq4nb1a5vqqlc4@news.supernews.com>...

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.

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

 

[Michael A. Covington]

"Bill Bowden" <wrongaddress@att.net> wrote in message
news:ad025737.0408181247.75e594e4@posting.google.com...

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.

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.

+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.

 

[Terran Melconian]

In article <l%2Uc.27273$Jp6.3817@newsread3.news.atl.earthlink.net>,
ilaboo <plener@earthlink.net> wrote:

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

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.