Opposite of voltage drop
Submitted: Tuesday, Dec 30, 2003 at 13:17
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Savvas
Hi all,
On Sunday I replaced the power feed to my
driving lights with 50amp cable and a 40amp relay. This was to get over a voltage drop problem. The power feed is taken straight off the +ve terminal of the battery and through the relay to the lamps.
With the old setup I was getting 12.5V at the lamp terminal compared to 13.7v across the battery terminals with the motor running. So there was a 1.2v drop.
On the new setup, I'm getting a weird result and am hoping someone can explain what's happening. Still 13.7v at the battery terminals, but 13.9v at the lamp connector.
How is it possible to get a .2v voltage increase over and above the battery voltage?
Reply By: KG - Tuesday, Dec 30, 2003 at 13:30
Tuesday, Dec 30, 2003 at 13:30
no idea but i suggest you patent the process :)
cheers,
KG
AnswerID:
41337
Follow Up By: Savvas - Tuesday, Dec 30, 2003 at 15:07
Tuesday, Dec 30, 2003 at 15:07
If only I knew what it was I'm patenting
FollowupID:
303866
Follow Up By: Member - Eskimo - Wednesday, Dec 31, 2003 at 13:18
Wednesday, Dec 31, 2003 at 13:18
I'd call it free powerWow! am I cute
If yer ain't fishing, Yer ain't livin
Richard
FollowupID:
303964
Reply By: Bonz (Vic) - Tuesday, Dec 30, 2003 at 13:33
Tuesday, Dec 30, 2003 at 13:33
do you have two batteries?
Laterally Literal
Seriously Cerebral
AnswerID:
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Follow Up By: Savvas - Tuesday, Dec 30, 2003 at 15:07
Tuesday, Dec 30, 2003 at 15:07
Nope ... Just a single 700 ca battery
FollowupID:
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Reply By: tex1972 - Tuesday, Dec 30, 2003 at 13:37
Tuesday, Dec 30, 2003 at 13:37
did you use the same earth with your multimeter maybe it was voltage drop on the earth side.my autolec told me to run
driving lights strait off the alternator.
AnswerID:
41339
Follow Up By: Savvas - Tuesday, Dec 30, 2003 at 15:05
Tuesday, Dec 30, 2003 at 15:05
I had the -ve connector of the multimeter was on the -ve pole of the battery for each measurement. The lights themselves are earthed to the nudge bar.
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Reply By: Savvas - Tuesday, Dec 30, 2003 at 15:10
Tuesday, Dec 30, 2003 at 15:10
Because this had me intrigued, I decided to check the power at the headlight connector too.
The low beam gave the same result, 13.9v. But the high beam gave out 13.7v.
Battery still 13.7v.
AnswerID:
41347
Follow Up By: Bonz (Vic) - Tuesday, Dec 30, 2003 at 15:50
Tuesday, Dec 30, 2003 at 15:50
Well Savvas, I am stumpted, I thought that the problem may be the earthing on the leads too, it could also be that the 13.9v has a better connection to the multimeter than the 13.7, i.e. 0.2 v drop over the meter connections, give it a scratch around and then check on nice bare shiny sweet metalLaterally Literal
Seriously Cerebral
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Reply By: Member - Sam (NSW) - Tuesday, Dec 30, 2003 at 15:56
Tuesday, Dec 30, 2003 at 15:56
you haven't got any excess wiring coiled up in the circuit anywhere?
AnswerID:
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Follow Up By: Savvas - Tuesday, Dec 30, 2003 at 15:58
Tuesday, Dec 30, 2003 at 15:58
No ... all trimmed to length.
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Reply By: Spanner - Tuesday, Dec 30, 2003 at 17:30
Tuesday, Dec 30, 2003 at 17:30
If it's 13.7V at the battery you are measuring with the engine running. The extra 0.2 V you see at the lights is from the alternator. Try measuring with the engine off.
Cheers
AnswerID:
41363
Follow Up By: Bonz (Vic) - Wednesday, Dec 31, 2003 at 08:08
Wednesday, Dec 31, 2003 at 08:08
I would have thought that the battery acts like a big regulator, setting the voltage levels for the whole vehicle, other than voltage DROP, then the battery is like a sentinel watching over the voltage behaviour.
If you have 13.9v at the alternator, and 13.7v at the battery then you should have at least 13.7 at any other point not 13.9 sneaking past the battery to them pesky lights. There should be a big wire running form the alternator to the battery taking the bulk of the "regulated and rectified" supply from ther alternator thru the battery to the vehicle. Still stumpted.Laterally Literal
Seriously Cerebral
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Follow Up By: Spanner - Wednesday, Dec 31, 2003 at 08:44
Wednesday, Dec 31, 2003 at 08:44
True Bonz, but it depends os where the accesories are fed from. If it's closer (electrically) to the alternator then the alternator will have a greater effect. All cables (even the thick ones) and connectors/terminals will have some sort of voltage drop depending on how much current passes through them. The altenator is probably delivering much more than 13.9V too, likely 14.1 to 14.3.
Cheers
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Follow Up By: Bonz (Vic) - Wednesday, Dec 31, 2003 at 09:42
Wednesday, Dec 31, 2003 at 09:42
Eric summed it up nicely down below didnt he? I get about 14.2 v on charge on the second battery, thats how I can tell the contactor is in, otherwise it sits about 13.2v
Great thread though, learnt a bit hereLaterally Literal
Seriously Cerebral
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Reply By: cokeaddict - Tuesday, Dec 30, 2003 at 21:12
Tuesday, Dec 30, 2003 at 21:12
You see guys,
This is the beauty of this
Forum. Great question asked by Savvas, and some even better suggestions by members. Ahhhhh dont ya just love this place.I love it when you talk DIRTY !
AnswerID:
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Reply By: Eric Experience. - Tuesday, Dec 30, 2003 at 23:20
Tuesday, Dec 30, 2003 at 23:20
Savvas.
The wiring on modern vehicles is diferent to the old bangers, befor the days of fusable links and big power loads of accesories the alternator used to be connected directly to the battery and the voltage regulator measured the battery voltage and just kept the battery charged, now the main role of the alternator is to run the accesories and charging the battery is of secondry importance. The result of this rethink is that the output of the alternator is taken directly to the accesory switch and the voltage regulator samples the voltage at the same piont, the voltage is slighly higher so the voltage drop in the loom is compensated for, that is why the wires going to the head lights on a modern car are so thin. The battery is charged via a length of wire that acts as a resistor that limits the current to the battery. all the above have allowed the modern car to have better lights that last the life of the car. Now to answer your question, what you are seeing is the drop in the loom that feeds the battery. If you can understand all the above you will see why it is not good practice to connect any load directly to the battery as it reduces it charge rate. Eric.
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Follow Up By: Savvas - Wednesday, Dec 31, 2003 at 06:59
Wednesday, Dec 31, 2003 at 06:59
Excellent explanation, Eric. Thanks!
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Reply By: Phil G - Wednesday, Dec 31, 2003 at 09:57
Wednesday, Dec 31, 2003 at 09:57
Saavas,
You haven't mentioned about the lights being on. To correctly measure voltage drop, you need current running through the circuit you're measuring. It doesn't appear that you've done that.
So, with the lights turned on (doesn't matter if the motor's running or not), you need to put the pos multimeter tip on the pos terminal of the battery and the neg multimeter tip needs to go on the pos lamp terminal while the lamp is on. The meter reading is the voltage drop on the pos side of the circuit. You may need to drop the scale to 2 volts for a more accurate reading.
Don't forget the earth!! Then put the pos multimeter lead on the neg lamp terminal and the neg multimeter lead on the neg battery terminal, and measure voltage drop on the negative side of the circuit.
Many people get poor driving light performance because of inadequate earthing. Often best to run a thick earth wire directly back to the battery.
AnswerID:
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Reply By: Member - Eskimo - Wednesday, Dec 31, 2003 at 13:20
Wednesday, Dec 31, 2003 at 13:20
.2 extra volts is probably just an abnomaly with the way and where its being measured.
Wow! am I cute
If yer ain't fishing, Yer ain't livin
Richard
AnswerID:
41428
Follow Up By: Mad Dog Morgan (Vic) - Wednesday, Dec 31, 2003 at 15:37
Wednesday, Dec 31, 2003 at 15:37
Yep, I agree. You can't have more than the battery has.I may be mad but I'm not crazy
BEAM ME UP SCOTTY
Hooroo
Ray
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Follow Up By: Spanner - Wednesday, Dec 31, 2003 at 20:48
Wednesday, Dec 31, 2003 at 20:48
There has to be more voltage than the battery has somewhere or it will never charge. The only place it can come from is the alternator.
Cheers
FollowupID:
303999
Reply By: Old Jack - Friday, Jan 02, 2004 at 21:39
Friday, Jan 02, 2004 at 21:39
Just for General interest,
the new Standard for car electrical systems will be 42 Volts! the next generation of cars will be using a smart Buss system to run power & control with digital signal processing at each point ie the headlights, blinkers, stereo & egine managment system can all go through one common cable type, with a huge reduction in wireing thoughout the car(saves a great deal of weight too!). a number of cars have used this typoe of system on 12v DC for several years but not in garden variety cars. the same type of buss systems are used for industrial instrumentation & control applications.
the 42 volt makes for much smaller lighter cabling, the use of voltage inverter's & AC starter motors & drive systems becomes cheaper & more efficient....
more complex systems pack into a smaller space.....
something goes wrong you break out the laptop and the program tells you whats stuffed & where, can even tell if something is strating to breakdown!
AnswerID:
41592
Follow Up By: Savvas - Friday, Jan 02, 2004 at 22:28
Friday, Jan 02, 2004 at 22:28
It's the same reason that we don't use the US standard of 110V for mains power. The higher voltage means we can use a lower amperage. Therefore the wire can be lighter.
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