General electrical question

cack01 · Sep 5, 2002

If I was to have a fan that needs two Watts, and I hook it up to a resistor to lower the volts does the fan still draw two watts from the PSU or a battery.

Also, off the top of your head, do any of you know how much power it takes to start a car's engine. This is just in general, I don't need anything specific.

thanks.

jokrswild · Sep 5, 2002

watts is volts * amps. A fan will draw a constant amount of amperage (current), so by lowering the volts, it lowers the watts. You can easily lower the fan voltage from 12 volts to 7 volts (5 is probably too low) by hooking up the ground wire off the fan to the 5 volt line. The voltages will subtract, and you'll get a 7 volt fan. good for noise reduction.

cack01 · Sep 5, 2002

ok good so the wattage lowers. So now for a move advanced question, does any body know how to calculate how long a battery will last if it is running a given 12V item.

JigPu · Sep 5, 2002

Well, that depends on two things. First, the ammount of current (in amps) that the item draws. And second, the number of amp-hours your particular battery is rated for.

I think most AA size batteries are measured in milliamp-hours... Not sure. But in any case, make sure your current draw, and your amp-hours are in the same units (both mili, or both real amps, ect), and then simply divide the amp-hours by the amp draw. That will tell you the number of hours it will run.

JigPu

cack01 · Sep 5, 2002

Well... I can't find the rated milliamp-hours for my battery, but I found that its 12V and rated at 5 amps, making it 60 Watts (which I don't know if that matters).

The item I'm going to be running it on, is 12V has a 1.2 amp draw, and so that gave me the rating of 14.39 Watts.

Got any ideas.

OSUmaxx · Sep 6, 2002

You need to know the mAh rating of the battery, or there is no possible way to determine how long it will take to drain. Many batts say right on them...

cack01 · Sep 6, 2002

OSUmaxx said:
You need to know the mAh rating of the battery, or there is no possible way to determine how long it will take to drain. Many batts say right on them...

Well, I won't be able to get that, but its a rechargable battery, so I guess I can do some testing. Just hoping I could get a factual number I could base some things on.

thanks for the help guys.

Carnil · Sep 6, 2002

The other important thing is how fast you're draining the battery. The faster you drain the battery, the less total power you get out of it.

OSUmaxx · Sep 6, 2002

cack01 said:
Well, I won't be able to get that, but its a rechargable battery, so I guess I can do some testing. Just hoping I could get a factual number I could base some things on.

thanks for the help guys.

Ok, well good luck on your project then. What exactly are you doing?

cack01 · Sep 6, 2002

I'm planning an entry for that $50 laptop watercooling contest on the front page. The battery I have I'm planning on using to power the pump and one fan. I just want to know how long it would last.

Veland · Sep 6, 2002

but I found that its 12V and rated at 5 amps

That should be the amp hours right there! So a 1.2 amp draw should last at least 3 hours. I know that equals 3.6, not 5, but the draw is pretty high and that affects the time you get. But for a watercooled laptop this should be long enough.. I read a thread on a norwegian forum about the exact way to calculate how long a battery would last, but I can't find it right now, so this is all guesstimating!

JigPu · Sep 6, 2002

Actually, that isn't the amp hour rating. 5 amps is a measure of the maximum amount of current the battery will push at 12V. (Amps are a measure like voltage. You can't "use" them up) If he uses less than 5 amps, then good for him. Amp-hours say how long the battery will last pusing a specified amperage. It would ovbiously last far less when pumping out maximum current, when compared to an amp or less.

Have you tried going to the manufactuer's web site? They may have ratings somewhere.

JigPu

Veland · Sep 6, 2002

well... that really depends on the manufacturer.. Some may give you amp hours, others may give you maximum draw. It all depends. What kind of battery is it? Small lead-acid or nicd/nimh? Size? Also, it should be possible to find the internal resistans and find the maximum current draw and then see if this is the 5 amps.

To give a small clue as to the relation size/amphours. An AA cell battery may have up to 1500mah, but a low max output.

At least this is my understanding, you are all welcome to prove me wrong, this is an area I really want to know more..

cack01 · Sep 6, 2002

Well... I've looked and looked and looked and only resellers have it, and I can't find a manufactures web site for it. I can only charge this thing when my car is on, so when I drive today I can charge it, then I'll let you know how long its lasts. Don't worry, its not a car battery

General electrical question

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