# Electrical Experts

Discussion in 'The Garage' started by sickquad, Apr 26, 2004.

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Got a question for someone with some electrical knowledge. Say I have a battery which is 12 volts. And to this battery I have a light hooked up to it which runs on 12 volts and draws 12 amps. The battery is not seeing any kind of charge, the light is purely drawing from the battery.

What kind of information do I need to figure out how long the light will run? Is there a formula out there for this? I have a electrical engineering book from undergrad but can't find the info I need. The "battery expert" at Sears had no idea also.

I'd appreciate the help.

-Chris /forums/images/graemlins/dunno.gif

2. ### ramjet gmcCK5 StaffStaff MemberModeratorGMOTM Winner

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[ QUOTE ]
What kind of information do I need to figure out how long the light will run

[/ QUOTE ]

well we need to know the power in the bat is ther any amp hours-or VA volt-amps given

3. ### justinf1/2 ton status

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I agree, the battery should have a rating for amp-hours. This can be used to determine approximately how long the battery will last given a certain amp draw.

4. ### HarryH31 ton statusAuthor

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Yep, with the amp-hour rating you can get a close guesstimate. A battery with a 96 amp-hour rating would power a light drawing 12 amps for about 8 hours. (96 amp-hours / 12 amps). The actual time will vary based on temperature, age of the battery, and manufacturing variances, but that will give you a ball park figure.

5. ### 84_Chevy_K10Banned

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Not to mention state of charge. Contrary to popular belief, an alternator doesn't always maintain your battery in 100% state of charge. You obviously wouldn't get the whole 8 hours if your battery was only 80% to begin with. /forums/images/graemlins/thumb.gif

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Well, that's pretty simple. So another question. Do you think the amount of wire between the battery and the lamp will make a difference? I know wires will hold a capacitence, but will they also have a resistance? I have a chart which rates wire gauge with wire length and amperage, but I think that it just in regards to not burning up the wire.

I think this calls for an experiment.

Thanks for the info.

-Chris

7. ### 84_Chevy_K10Banned

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Wires do not have any capacitance.

Wire does have resistance, however, adding resistance will cut the current flow (amperage). More resistance = less current flow = dimmer lights = battery lasts longer in your situation.

8. ### skelly11/2 ton status

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There's a chart somewhere, but I would suggest using 10 or maybe 12 gauge wire with that 12A. 14 gauge would have too much resistance for that kind of current draw and would melt the insulation and eventually cause a short between the hot and the negative wires, causing a blown fuse.

9. ### justhorsinaround3/4 ton status

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Battery wouldn't last longer but yes the light would be dimmer. The line loss attributed to resistance (which equals heat) is why the light would be dimmer. The battery would be doin the same job just less voltage at the end.

Good luck to y'all in the future.

Allan

10. ### wasted wages3/4 ton status

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[ QUOTE ]
Wires do not have any capacitance

[/ QUOTE ]

Oh contrair,,,,,,,read on,,,,,,,from the belden catalog,,,,

Paired- Low Capacitance Computer &amp; Computer P.O.S. Cable Number of Pairs: 1 Total Number of Conductors: 2 AWG: 24 Stranding: 7x32 Conductor Material: TC - Tinned Copper Insulation Material Trade Name: Teflon® Insulation Material: FFEP - Foam Fluorinated Ethylene Propylene Outer Shield Material Trade Name: Beldfoil® Outer Shield Material: Aluminum Foil-Polyester Tape/TC - Tinned Copper Outer Jacket Material Trade Name: Flamarrest® Outer Jacket Material: LS PVC - Low Smoke Polyvinyl Chloride Plenum (Y/N): Y Non-Plenum Number: 9841 Applications: Low Capacitance Comptuer and Computer P.O.S. Cables

11. ### 84_Chevy_K10Banned

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That is not true. Instead of the light being full brightness, some of that electrical energy would be disipated as heat.

Ohms law states that I = E/R. So, in this equation, 12 volts is a constant. The resistance of the light bulb itself is also going to be fairly constant, in this case it is 1 ohm so that makes it easy to figure this equation.

So:

I = E / R

I = 12 / (1 + resistance of the wire)

Say that the resistance of the wire = 1 ohm.

I = 12 / (1 + 1)

I = 12 / 2

I = 6 amps

So, since the resistance has doubled, current flow has been cut in half. It will now take the battery twice as long to discharge as before. The lamp will now only have 1/2 the current flowing through it as it did before. Either way, the total electrical draw in amps is lower because the resistance measurement is higher, and the battery lasts longer.

Just as I stated before, the lamp is dimmer, and the heat dissipated through the wire is where the rest of that electrical energy goes.

12. ### 84_Chevy_K10Banned

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[ QUOTE ]
[ QUOTE ]
Wires do not have any capacitance

[/ QUOTE ]

Oh contrair,,,,,,,read on,,,,,,,from the belden catalog,,,,

Paired- Low Capacitance Computer &amp; Computer P.O.S. Cable Number of Pairs: 1 Total Number of Conductors: 2 AWG: 24 Stranding: 7x32 Conductor Material: TC - Tinned Copper Insulation Material Trade Name: Teflon® Insulation Material: FFEP - Foam Fluorinated Ethylene Propylene Outer Shield Material Trade Name: Beldfoil® Outer Shield Material: Aluminum Foil-Polyester Tape/TC - Tinned Copper Outer Jacket Material Trade Name: Flamarrest® Outer Jacket Material: LS PVC - Low Smoke Polyvinyl Chloride Plenum (Y/N): Y Non-Plenum Number: 9841 Applications: Low Capacitance Comptuer and Computer P.O.S. Cables

[/ QUOTE ]

All I see is specs there. Capacitance is measured in Farads. I see no spec for how much capacitance that particular wire has for a certain length.

That said, few things in Science are absolute. I suppose you can't say that wire has ZERO capacitance, but it is not a significant factor in most situations if it does exist.

Just like a well oiled bearing has some friction. Same concept--it's an insignificant figure if it does exist.

By definition, capacitance is used to block current flow anyway. Although it is often portrayed as a rapid charge/discharge that capacitors are also useful for, in many circuits, capacitors are used to block current flow (R/C circuits are an example of that, one that I can still remember from school)

So, even supposing wire did have capacitance, it is pretty much insignificant in this circuit anyway.

13. ### spearchucker1/2 ton status

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[ QUOTE ]
By definition, capacitance is used to block current flow anyway.

[/ QUOTE ]

DC current only, it doesn't block AC.

Wires do have capacitance, but negligable in this case. They also have inductance, but again negligable. Every electrical connection also has capacitance.

14. ### 84_Chevy_K10Banned

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Sorry, forgot to mention DC. Since this was a battery I pretty much made an assumption.

Negligable, insignificant......same thing /forums/images/graemlins/whistling.gif

Agree with my fuzzy math otherwise?

15. ### justhorsinaround3/4 ton status

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some of that electrical energy would be disipated as heat.

So you don't factor that into electrical flow coming out of the battery?

Electricity is like water. You try to damn the flow, i.e. resistance, it will spill out in the form of heat. The same amount of flow is coming out of the battery at the same rate. Some of the electrons make it to the lamp and the rest is dissapated as heat.

Good luck to y'all in the future.

Allan

16. ### spearchucker1/2 ton status

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[ QUOTE ]
Sorry, forgot to mention DC. Since this was a battery I pretty much made an assumption.

Negligable, insignificant......same thing

[/ QUOTE ]

Just clarifying and reinforcing what you said. /forums/images/graemlins/waytogo.gif

[ QUOTE ]
Agree with my fuzzy math otherwise?

[/ QUOTE ]
I'm not even going to attempt math at this point. I'm felling a little fuzzy myself. /forums/images/graemlins/doah.gif /forums/images/graemlins/histerical.gif /forums/images/graemlins/histerical.gif

17. ### 84_Chevy_K10Banned

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Yes, exactly. That is why the light is dimmer. Instead of using that energy to keep the lamp burning brighter, it is dissipating as heat.

That doesn't mean that there is more current flow, as I've proven mathmatically above (if that crap makes any sense to you at all). /forums/images/graemlins/rotfl.gif

Ohm's law is just that--electrical law. It is established as proven fact in the sense of the way current flows in a circuit. It is impossible to defy electrical law just like it is impossible to defy gravity or any other scientific law.

If one day you can figure out how to prove it wrong, it will no longer be accepted as mathematical fact. Until then, Ohm's law is just as valid the fact that my 140 lbs can't jump off a cliff and fly. /forums/images/graemlins/rotfl.gif /forums/images/graemlins/thumb.gif

18. ### 84_Chevy_K10Banned

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[ QUOTE ]
[ QUOTE ]
Sorry, forgot to mention DC. Since this was a battery I pretty much made an assumption.

Negligable, insignificant......same thing

[/ QUOTE ]

Just clarifying and reinforcing what you said. /forums/images/graemlins/waytogo.gif

[/ QUOTE ]

Damn, finally someone agrees with me. /forums/images/graemlins/waytogo.gif About damn time. /forums/images/graemlins/rotfl.gif

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Wires definitely have a capacitence. I worked at a R&amp;D medical device company as a college intern. One of my projects was to fix a problem with a device which was not working right and couldn't get past the first round of FDA approval. Turned out the wires had a capacitence. Manufacturer sent us a new wiring harness with different strand material and had less capacitence.

It was very minimal but it did have a capcitence.

-Chris

20. ### 84_Chevy_K10Banned

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It depends on what kind of circuit you're working with. A circuit with wires and a light bulb is far different than a circuit with semiconductors.