Battery Math - Page 2 - PeachParts Mercedes-Benz Forum

t walgamuth · #16 03-10-2009, 05:52 AM

So, you thinking of converting your 170 to 12v?

tinypanzer · #17 03-10-2009, 03:19 PM

Okay, this is all screwed up.....

Let's pick this apart bit by bit and then I am going to leave it alone...

"what the heck are you talking about....amperage and voltage are constant in a circuit.."

That statement can be true, but isn't necessarily. First, amperage varies with load and so does voltage to a lesser extent. Ever crank up a stereo so loud that the bass notes cause lights to dim? That's an increase in the amperage demand, and it results in a voltage drop that you can visibly see.

"it's the usage of said circuit that constitutes wattage consumed, they draw off of the circuit."

No, it's not the usage necessarily, though it is often used to describe such. Watts is merely power. Go look it up, the symbol for power in electrical engineering is P and Watts is the unit of measurement for P. All you have to know to calculate watts is two of the three variables I outlined earlier.

"That's why there are these things on power lines/poles called transformers, they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

That statement is just flat out incorrect. The function of a power transformer is to simultaneously step DOWN voltage while stepping UP current. Notice how the wires going from the transformer to your house are several times thicker than the wires supplying it? That's because the current on those wires is very high while the voltage is relatively low. The transformer trades the high voltage for current so that you can have 200A service in your house at a relatively safe 120V. For example: A transformer with a winding ratio of 100:1 would give you 10V@100A out if you supplied it with 1000V@1A. That's how you can get so much power (watts) out of such skinny wires on a phone pole. Wires need to be fat to carry current, but voltage doesn't care as much. So you can put tens of thousands of volts (high tension lines can go up to a half-million or more volts) down a skinny wire and still convert that into current that would burn up a wire 30 times thicker.....

"they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

Again, voltage is stepped down, current is stepped up. The lights won't blow because at a fixed voltage, it's the resistance of the light bulb's filament that determines how much current the circuit will draw. I could put the light bulb on a 1000A circuit, it's still only going to draw 1 amp if it's a 120 watt bulb.

"power plants do not put out wattage, they produce voltage and amperage, not watts...the watts thing is the way the electric companies measure your consumption in order to bill you..."

Again, this is just wrong. Watts = Volts X Amps. So, if they are producing voltage and amperage as you just stated, they are also producing watts, aren't they? And no, watts is not for billing purposes, that's KWh or kilowatt-hours. A kilowatt hour is different from a kilowatt because it adds the factor of time. As I said before, KWh is directly convertible into Joules.

"Now, watts are what actually USE the electricity...get it? Ever hear of a 30 watt fuse? No, 30 amp fuse is something you hear about. Ever hear of a 60 amp light bulb? No, you hear of a 60 watt light bulb, get it?"

Sorry, but it's you that doesn't get it..... Watts are not a measure of electrical use, they are a measure of power. Go read the literature if you refuse to believe me. The formula for watts is P=I*E in electrical engineering abbreviations or W=A*V in standard notation.

Go look it up!!

Matt L · #18 03-10-2009, 04:47 PM

For a DC circuit, P = E*I. But not for an AC circuit unless it has a power factor of unity.

Watts = volts * amps * PF, where PF is the cosine of the phase angle between the voltage and amperage. If there is no phase difference, PF is unity (1).

A (fictional) purely capacitive or inductive load will show a PF of zero. No matter what the current flow or voltage, there is no power consumption.

babymog · #19 03-10-2009, 05:15 PM

I'm fully understanding the past-tense now.

Quote:

Originally Posted by cliffmac

I worked for CommonWealth Edison

tinypanzer · #20 03-10-2009, 05:15 PM

Yes, that is correct, Matt L. I think you can understand why I decided not to complicate my explanation with that tidbit.

For all intents and purposes, what I said is perfectly adequate since we were using a resistive load in our examples. There is no reactive power, and therefore P=I*E

mpolli · #21 03-10-2009, 05:34 PM

Strelnik, where are you???

Ivanerrol · #22 03-10-2009, 08:16 PM

Quote:

Originally Posted by mpolli

Strelnik, where are you???

His eyes lit up like 40 watt bulbs due to overload current after reading these technical posts, the heat generated from his eyes has temporarily fried part of his brain and at the moment he's still confused - he'll be back. ...... Now that's power factor for you.

asuperglide · #23 03-10-2009, 08:30 PM

Quote:

Originally Posted by tinypanzer

Okay, this is all screwed up.....

Let's pick this apart bit by bit and then I am going to leave it alone...

"what the heck are you talking about....amperage and voltage are constant in a circuit.."

That statement can be true, but isn't necessarily. First, amperage varies with load and so does voltage to a lesser extent. Ever crank up a stereo so loud that the bass notes cause lights to dim? That's an increase in the amperage demand, and it results in a voltage drop that you can visibly see.

"it's the usage of said circuit that constitutes wattage consumed, they draw off of the circuit."

No, it's not the usage necessarily, though it is often used to describe such. Watts is merely power. Go look it up, the symbol for power in electrical engineering is P and Watts is the unit of measurement for P. All you have to know to calculate watts is two of the three variables I outlined earlier.

"That's why there are these things on power lines/poles called transformers, they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

That statement is just flat out incorrect. The function of a power transformer is to simultaneously step DOWN voltage while stepping UP current. Notice how the wires going from the transformer to your house are several times thicker than the wires supplying it? That's because the current on those wires is very high while the voltage is relatively low. The transformer trades the high voltage for current so that you can have 200A service in your house at a relatively safe 120V. For example: A transformer with a winding ratio of 100:1 would give you 10V@100A out if you supplied it with 1000V@1A. That's how you can get so much power (watts) out of such skinny wires on a phone pole. Wires need to be fat to carry current, but voltage doesn't care as much. So you can put tens of thousands of volts (high tension lines can go up to a half-million or more volts) down a skinny wire and still convert that into current that would burn up a wire 30 times thicker.....

"they step down voltage and amperage to a more usable form so that you don't blow your light bulbs out."

Again, voltage is stepped down, current is stepped up. The lights won't blow because at a fixed voltage, it's the resistance of the light bulb's filament that determines how much current the circuit will draw. I could put the light bulb on a 1000A circuit, it's still only going to draw 1 amp if it's a 120 watt bulb.

"power plants do not put out wattage, they produce voltage and amperage, not watts...the watts thing is the way the electric companies measure your consumption in order to bill you..."

Again, this is just wrong. Watts = Volts X Amps. So, if they are producing voltage and amperage as you just stated, they are also producing watts, aren't they? And no, watts is not for billing purposes, that's KWh or kilowatt-hours. A kilowatt hour is different from a kilowatt because it adds the factor of time. As I said before, KWh is directly convertible into Joules.

"Now, watts are what actually USE the electricity...get it? Ever hear of a 30 watt fuse? No, 30 amp fuse is something you hear about. Ever hear of a 60 amp light bulb? No, you hear of a 60 watt light bulb, get it?"

Sorry, but it's you that doesn't get it..... Watts are not a measure of electrical use, they are a measure of power. Go read the literature if you refuse to believe me. The formula for watts is P=I*E in electrical engineering abbreviations or W=A*V in standard notation.

Go look it up!!

You have the patience of a saint to explain that. Very well done. I have 30 years in the power generation field generating power with conventional rotating machinery. Your explanation is correct.

cliffmac · #24 03-10-2009, 09:31 PM

OK fine, then explain to me why both my Mercedes and BMW owners manuals refer to the Alternator as 14 V/80 A and 14 V/80 A respectively and the Starter Motor as 12 V/1.7 kW and 12 V/1.1 kW, again, respectively? Alternator produce power, starters draw power, why only kW rating on starters only? What, does the starter produce electricity, that would be great if they did....

Arthur Dalton · #25 03-10-2009, 10:57 PM

German electric motors use metric horsepower [ PS] measures for mechanical power capacity/output.
Power is PS , and 1 PS =735.5 watts.
US electric motors are stated in HP...like a "3/4 HP motor...."
And 1 HP = 746 watts.
So 1 HP is close to a Metric PS.

So , your 1.1 kw starter is approx 1.5 PS [ ISO Horsepower]
You alternator has ZERO mechanical output. But it has 80 AMPS electrical output capacity.
One is a Mechanical power measure and the other is an Electrical measue ..and completely different...
Many German starters will be stamped right on the case " 1.4 SP ..or 1.8 SP, etc "