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# amperage for a battery charger

#1
03-11-05, 01:00 PM
jaxx751
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amperage for a battery charger

the last thread this week for me, i swear.

ok, if you are familiar with a iPod, i am trying to make a charger, it will have 3 9Vs, and the iPod is rated at 12V-30V, but i want to make it 12V anyways.

the wall charger is rated at .67A, and i have chosen a 33ohm, 10W resistor, according to my calculations this should give me 12V @ .45A... is this correct?

27-12=15
15/.45=33.3333...
.45·15=6.75

did it screw up some where?

thanks,
-jaxx

I LOVE THESE FORUMS!

#2
03-11-05, 01:58 PM
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Where did the 0.45 number come from? Is this an electrical specification from iPod that it will take 0.45 amps if you connect it across 12 volts? And are you thus inferring the resistance of the iPod from this data?

12 volts / 0.45 amps = 26.7 ohms

So if you add a 33.3 ohm resistor in series with an iPod load of 26.7 ohms, you are putting 60 ohms across a 27 volt supply. In this case, 0.45 amps will flow.

However, all these calculations depend on what the resistance of the iPod is. If iPod says that it draws 0.45 amps when given a 12 volt supply, then the inference may be correct. But if you selected 0.45 amps merely because you want 0.45 amps, then the inference is useless.

#3
03-11-05, 02:01 PM
jaxx751
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i selected .45A because that is what i THOUGHT the 33ohm resistor would be doing... but i think i need something like 22ohm @ 10W.

the ipod is usually taking 12V @ .67A from the wall charger.

i knew i screwed up.

edit: just realized, the ipod doesn't resist anything(i think), i was talking about the wall charger, 120Vac @.4A -> 12Vdc @.67A

Last edited by jaxx751; 03-11-05 at 02:40 PM.
#4
03-11-05, 05:10 PM
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I'm having a hard time trying to understand what are doing. Is this what you have?
You have an iPod that can accept input of 12v-30v. You have three 9volt batteries you want to use to charge the iPod. Just hook the batteries direct. The iPod can accept that voltage.

#5
03-11-05, 06:16 PM
jaxx751
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but i want to limit the amperage to .45A @ 12Vdc.

#6
03-11-05, 06:42 PM
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With the evidence we have so far, there is nothing that would allow us to conclude that this would limit the current to 0.45 amps. Just because you want to provide the iPod with 12 volts doesn't mean that you can just assume it will happen and calculate from there. For example, if the iPod has a resistance of zero (the limiting case), the current will be 27/33 or 0.82 amps. Without further information about the iPod, all we can be sure of is that the current will be 0.82 amps or less.

#7
03-11-05, 06:54 PM
jaxx751
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what info do u need?

* 6-pin FireWire connector
* AC input: 100V to 240V at 0.4 amp maximum
* Frequency: 50Hz to 60Hz
* DC output: 12V at 1 amp maximum

from the apple website...

my mistake, FIREWIRE is rated at 12V-30V, not sure about the ipod.

i want it at 12V... i really hope i got the correct resistor.

#8
03-11-05, 07:30 PM
jaxx751
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#9
03-11-05, 10:10 PM
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You need to know what voltage the i-pod uses. and the amount of current the i-pod pulls.
without knowing that your ohms law will not work.

You cant go by the label on most but not all power packs.
The link explains some problems with power packs.

The best way is,
connect up the Proper power pack to the i-pod, turn on the unit and measure the voltage and current the i-pod uses when playing.

If you only know the i-pod uses 12 volts then go with the voltage regulator.

Don't want to put a break in your fuse But.
If your using standard 9 V battery's, I believe they are rated about 100 ma or less.
I don't think they have enough power to charge your i-pob batteries.

What is the charge rate of your i-pob battery's ?

Last edited by GWIZ; 03-13-05 at 01:29 AM. Reason: Typo
#10
03-12-05, 06:35 AM
jaxx751
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apple's site says ~.7A to start and something like .35A for trickle.

dont worry, i have done a single 9V charger and it worked perfectly.

EDIT: that 12V voltage regulator will output @ 1A, but if the 9Vs are rated at the low rating like you say, will it still do 1A? because i always have this 33ohm 10W resistor.

#11
03-12-05, 07:22 AM
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Posts: 18,497
Until now, I thought this was an academic exercise, perhaps for a science fair project. If you intend to use this for a real application, it's not very practical. Even if it did work, and I don't think it would, half of your power will be spent heating up the resistor. You go to all the trouble to string three 9-volt batteries together and then figure out how to waste much of the energy they produce. Two 9-volt batteries would actually work better than three.

Get the voltage regulator. The 1-amp rating is just the maximum. It won't produce 1 amp all the time. It produces 12 volts. The number of amps is determined by the resistance of the load.

#12
03-12-05, 07:32 AM
jaxx751
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aye, im on the support line with apple to see if 27V is too much.

thanks so much for all of your help, sure somethings i do is a waste, but it's nice to just spend time doing modifications on things...

my pump project is gonna go thru this weekend too.

-Jackson

#13
03-13-05, 01:37 AM
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A standard 9 V battery has 6 AAA battery's in series ( well, they looked like AAA's when I opened up a 9 V battery)
6 x 1-1/2 volts = 9 volts.
I believe the AAA battery's supply 100 ma or less.
That's why I think a 9 V battery can only supply 100 ma or less.

I been able to Pull close to 1 amp from a 9 V battery, But Killed the battery in less then 8 minutes.

"apple's site says ~.7A to start and something like .35A for trickle."
You did not say how long, is that .7A for 3 hours or more.

9 V battery's don't have that power to charge your i-pod battery's.

Just a note.
That wattage you calculate for your resistor, is also the wattage/power that the resistor consumes as wasted power.
The wattage of the i-pod + the wattage you calculated for your resistor = total power consumed from your supply.

#14
03-14-05, 01:30 PM
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Your approach is very inneficient... a DC-DC converter would work much better.

The current rating is a minimum...not a maximum. It's given as a guidance as the current draw determines the voltage drop and efficiency of the supply. Very few devices specify a maximum current, and it's always there for safety only (in case the device experiences a malfunction).

#15
03-17-05, 02:14 PM
jaxx751
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i've gotten it to work fine, i didnt use any resistor, but the battery should last for about 8 hours, 4 hours if there is no battery charge left.

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