Understanding the math of electrical loads
(Created page with 'This basic formula P= I X E, just think of that tasty pie. P (power in watts) = I (current in amps) X E (electromotive force volts) So, how much current at 12 volts does a pair…') |
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| − | This basic formula P= I X E, just think of that tasty pie. | + | This basic formula P= I X E, just think of that tasty pie. P (power in watts) = I (current in amps) X E (electromotive force volts) |
| − | P (power in watts) = I (current in amps) X E (electromotive force volts) | + | |
| − | So, how much current at 12 volts does a pair of 55 watt headlights consume? | + | So, how much current at 12 volts does a pair of 55 watt headlights consume? Algebra lets you turn the formula around to find the unknown. 110 watts = I X 12 volts, whip it around you get 110 watts divided by 12 volts = 9.17 amps. If you have a battery rated at 65 amp/hour you can see the time at the above load of 9.17 amps to complete battery exhaustion would be about 7 hours. (Not counting the Puekert effect, which in simple form just means the heavier the current draw the lower the a/hr for the battery). Now comes some fun calculations, hp and watts, both units of work. Let's say some aftermarket, high performance starter motor manufacturer claims he has a 4 hp starter -- can you determine if this is reality? Math again. 1 hp = 746 watts, so 4 hp = 2984 watts divided by 10 volts (your 12 volt battery under the load of the starter) = 298.4 amps. |
| − | Algebra lets you turn the formula around to find the unknown | + | |
| − | 110 watts = I X 12 volts, whip it around you get 110 watts divided by 12 volts = 9.17 amps. | + | |
| − | If you have a battery rated at 65 amp/hour you can see the time at the above load of 9.17 amps to complete battery exhaustion would be about 7 hours.( | + | |
| − | Now comes some fun calculations, hp and watts, both units of work. | + | |
| − | Math again. 1 hp = 746 watts, so 4 hp = 2984 watts divided by 10 volts (your 12 volt battery under the load of the starter) = 298.4 amps. | + | |
| − | For the young ' | + | For the young'uns and their 2 kilowatt stereos, listen up. First and most important, do not try and wire this any distance or use the starting battery. Odysessy AGM batteries are your best bet for monster amplifiers due to their dual personality of being both a deep cycle battery and the highest cranking amp battery per pound of battery weight you can buy. Mount the AGM battery right with the audio amplifier so that your leads are heavy and very short in total lenght. Now you can run that easy-to-work-with 12 ga from your alternator or starting battery back to the AGM for charging. This is why the above method is the ONLY way acceptable and yes it involves math again. Your stereo amp if in fact is a true 2000 watt unit, it is running class A, so efficiency is about 50%. So, 4000 watts input, again divided by 10 volts (remember the voltage drop under heavy loads) = 400 amps. Do you think 20 feet of 8 ga is going to carry this? Nope. |
| − | Back to you adults who don't give a fig about the kid's need to blast tunes, | + | Now, a little 15 lb Odysessy will lay down 680 amps for 5 seconds. If you want to blast your tunes for more than 5 seconds to be truly obnoxious, don't worry ,you can. That current draw is at peak output and that doesn't happen often or for a few milliseconds at a time. With the engine running and sending 15-20 amps back to the AGM battery, you're good to go. Another misconception is large amps burn out your tweeters, but believe it or not it is small amps that burn out the tweeter. When an underpowered amp is turned up so loud that the amp is clipping the wave form, you burn out the tweeter. Clipping is a distortion that muddies the sound and is caused by the amp running out of output voltage swing and clipping the top and bottom of the waveform. Looking at clipping on a scope, instead of seeing a nice completed sine wave the tops and bottoms are a horizontal line which is DC, no longer a frequency. What keeps a tweeter healthy is the fact that its voice coil is moving in and out 8000 to 20000 times a second and that motion keeps the voice coil from burning up due to the moving air keeping the voice coil in an acceptable temperature range. So when the underpowered amp clips at high settings the tweeter is seeing DC, so a lot of heat energy to dissipate but because it is DC, the little sucker isn't moving, so smoked. Radios in new cars no longer will clip because what you think is the volume control potentiometer is in reality just a shaft with a slotted light chopper wheel and as you turn up the volume, each time the light from a led passes through a slot to a light sensor on the other side of the wheel, it requests if you will the ramping up of volume but now can employ control circuitry that stops short of clipping, thus saving the speakers and warranty claims. |
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| + | Back to you adults who don't give a fig about the kid's need to blast tunes, here's a tip on jump starting. Assuming you have a good starting battery that you ran flat for some reason, do your jump this way. You all know how to hook the cables up but after that I see a common mistake and that is cranking the engine right away and when it doesn't spin fast enough you yell out to friend to speed up his engine. Under certain conditions you could damage your friend's charging system. When getting a jump, just let it idle while you chew the fat for about 10 minutes. By doing this, when you go to start your car, better than 90% of the load will now come off your battery. | ||
I don't mind anyone copy and pasteing this article but we all should get in the habit of, if using original material like this for this forum, at least credit the forum it came from or better yet look up at your URL address window and link back to it here. That covers the legal and directs traffic here and possible new members. | I don't mind anyone copy and pasteing this article but we all should get in the habit of, if using original material like this for this forum, at least credit the forum it came from or better yet look up at your URL address window and link back to it here. That covers the legal and directs traffic here and possible new members. | ||
| − | My name is Bob Lemke, known on the automotive forums as "deckofficer", and I am the author of the above article which first appeared here | + | ==Credits== |
| + | My name is Bob Lemke, known on the automotive forums as "deckofficer", and I am the author of the above article which first appeared here: http://tbuckets.lefora.com/2011/02/04/understanding-the-math-of-electrical-loads/#post2. | ||
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| + | [[Category:Electrical]] | ||
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