A question of voltage drop - Fiberglass RV


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Old 12-08-2012, 04:18 PM   #1
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A question of voltage drop

How many volts should you see on the voltage gauge?

At the tow vehicle battery when the vehicle is idling?
At the cigarette lighter socket when the vehicle is idling, but no accessories?
At the cigarette lighter socket when the vehicle is idling, but defrost, heater fan, and other accessories are on?
At the cigarette lighter socket when the vehicle is at highway speed?
At the trailer battery, when the tow vehicle is hooked up and idling?

A fully charged battery should read 13 Volts. A battery that is being charged should be more than 13Volts but less than 14Volts. A 50% discharged battery should be 12.5 volts, and a very dead battery will read below 12.0 volts. Anyone who drains a battery below 12.0 volts should be charged with the crime of "battery abuse."

Saying this, some voltage drop should occur as you get further away from the power source. For the sake of discussion, lets assume batteries are in good condition but not fully charged.

Derek
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Old 12-08-2012, 04:25 PM   #2
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Originally Posted by glamourpets View Post
How many volts should you see on the voltage gauge?

At the tow vehicle battery when the vehicle is idling?
At the cigarette lighter socket when the vehicle is idling, but no accessories?
At the cigarette lighter socket when the vehicle is idling, but defrost, heater fan, and other accessories are on?
At the cigarette lighter socket when the vehicle is at highway speed?
At the trailer battery, when the tow vehicle is hooked up and idling?

A fully charged battery should read 13 Volts. A battery that is being charged should be more than 13Volts but less than 14Volts.

Saying this, some voltage drop should occur as you get further away from the power source. For the sake of discussion, lets assume batteries are in good condition but not fully charged.

Derek
I can only tell you what I've observed. The voltage at anypoint in 12 System when the tow is idling in my case is about 14.1 to 14.3.

Automobile system voltage is 13.8 10%, I believe. It could be 15%

I wouldn't obsess over it, because it will vary and all the electronic devices are specified to operate in the range or a wider range.
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Old 12-08-2012, 04:48 PM   #3
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I can only tell you what I've observed. The voltage at anypoint in 12 System when the tow is idling in my case is about 14.1 to 14.3.

Automobile system voltage is 13.8 10%, I believe. It could be 15%

I wouldn't obsess over it, because it will vary and all the electronic devices are specified to operate in the range or a wider range.
Hmmm. I'm seeing between 12.4 and 13.2 at the cigarette lighter (depending what's turned on) with engine running. I thought my readings were a little low. Any explanations beyond a pooched alternator?

Derek
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Old 12-08-2012, 04:49 PM   #4
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I'm curious why you are asking. It might help in answering your questions.
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Old 12-08-2012, 05:27 PM   #5
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I'm seeing between 12.4 and 13.2 at the cigarette lighter (depending what's turned on) with engine running. I thought my readings were a little low. Any explanations beyond a pooched alternator?
My 2004 Ford Focus wasn't charging the battery properly, and I found that it never reached 14V. The problem turned out to be a bad connection in the wiring harness to the alternator: the Focus charging system is designed to work in two steps, dependent on various conditions, and the failed connection was keeping it from being switched into the higher-voltage stage. A replacement wiring harness fixed that. It runs as high as 14.2 V (measured at the lighter socket).

It doesn't matter how far you are from the alternator or battery if the wire you're using is not carrying any significant current. Voltage drop is due to resistance and current, and a voltmeter uses very little current. The lighter socket is a perfectly good place to monitor a vehicle's charging system voltage.

On the other hand, if you measure the voltage at the trailer battery while that battery is being charged with 20 amps over 10 metres of thin wire, you'll see much less voltage than at the battery of the tow vehicle which is charging it.

(No, the Focus is not my tow vehicle. That's the Sienna, which has never had any kind of electrical failure other than wearing out one battery.)
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Old 12-08-2012, 05:31 PM   #6
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~14 is a typical charge voltage at the battery. Voltages at other places in the car follow the simple rule of Ohm's law, but the variety of resistances (in connections and wires) and currents (from multiple loads) make the answer highly dependent on the electrical architecture and wiring in the car. High end vehicles can have ~80 electrical control units each having many inputs and outputs. Even a mirror can have a connector with 16 wires and a microprocessor in them.

As a rule of thumb, electrical control units in a car typically make their products work from 9-16V. However, car manufacturers have different specifications. With the advent of engines starting and stopping at red lights for example, sometimes your 12V outlet may even go to less than 3V for a short period of time when the engine restarts. Typically the 12V outlet may be around 12V most of the time, 11-14V almost all the time, but may go outside of that in some circumstances. Most aftermarket electronic devices are designed to handle this variability though.

Sorry for the lack of specific answer, but it is not a simple topic when you get into the details.
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Old 12-08-2012, 05:35 PM   #7
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Hmmm. I'm seeing between 12.4 and 13.2 at the cigarette lighter (depending what's turned on) with engine running. I thought my readings were a little low. Any explanations beyond a pooched alternator?

Derek
It's easy enough to check at a battery stores or automobile repair place. If you have a battery charger, check it to see if it has an alternator test function, mine does. Most battery stores that I know of will check it for free as will many automobile repair shops.

Another possibility is a bad cell in your battery. The same places will check the battery. Two very good ways to check battery condition, 1) measure specific gravity of the liquid in each cell, 2) a load test, where they charge the battery to full charge then put a large load on it for a short period of time and measure the change in voltage before and after.

It usually doesn't cost have the test done, the expense is replacing what ever is needed.
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Old 12-08-2012, 06:31 PM   #8
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Wow. This discussion isn't as simple as it appears. We can rule out the battery as a factor as it was replaced today. The decision was based on the fact that the origional battery looked like it installed at the factory (in 2005/6), we have had some sluggish morning starts lately, and the voltage at the cigarette lighter has been lower than expected. The new batt just seemed to be prudent preventive maintenance.

Its entirely possible that I am creating a problem where there isn't one. I just didn't expect the voltage to consistently be dropping below 13 volts.

Derek
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Old 12-09-2012, 08:50 AM   #9
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Whoa on Battery Voltage checking......

1st, what are you measuring with? Anything less than a modest digital meter is more of a crapshoot than accurate.

Next.... Voltage with the engine running or when plugged in isn't battery voltage, it's charging voltage. Unless you are checking that, it's values are of no interest.

Dr. Ohm (From the very "short" lived TV reality series "Ohms Law") sez "Voltage drop only occurs when there is current flow" You can have a 250' #18 wire and voltage drop will be near zero when measured with no other load on it except the meter. If you have a no-load voltage drop, it usually means you have a really bad connection.
For the tekies out there rember, voltage is "Potential".

According to Interstate Battery, battery voltage should be measured with no load present. Any time there is current flow, the battery voltage will drop due to internal resistance of the battery. By using a good digital meter or a Voltminder (voltminder.com & many on line sources) you can read voltage to 2 decimal places.

Even turning on an LED ceiling light will cause a voltage drop, usually about .02 volts, but it immediately recovers when the light is off. Likewise, when we run the electric awning out on the Coleman, the fully charged battery voltage drops to under 11.5 volts, but recovers (minus any capacity used) as soon as the awning motor stops running.

Bottom line: You can accurately measure battery voltage anywhere in the system as long as everything is turned OFF (and you are isolated from the tow vehicle) at the time. Measuring battery voltage with an thing ON will give a false reading.
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Old 12-09-2012, 02:34 PM   #10
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Whoa on Battery Voltage checking......

1st, what are you measuring with? Anything less than a modest digital meter is more of a crapshoot than accurate.

Next.... Voltage with the engine running or when plugged in isn't battery voltage, it's charging voltage. Unless you are checking that, it's values are of no interest.

Dr. Ohm (From the very "short" lived TV reality series "Ohms Law") sez "Voltage drop only occurs when there is current flow" You can have a 250' #18 wire and voltage drop will be near zero when measured with no other load on it except the meter. If you have a no-load voltage drop, it usually means you have a really bad connection.
For the tekies out there rember, voltage is "Potential".

According to Interstate Battery, battery voltage should be measured with no load present. Any time there is current flow, the battery voltage will drop due to internal resistance of the battery. By using a good digital meter or a Voltminder (voltminder.com & many on line sources) you can read voltage to 2 decimal places.

Even turning on an LED ceiling light will cause a voltage drop, usually about .02 volts, but it immediately recovers when the light is off. Likewise, when we run the electric awning out on the Coleman, the fully charged battery voltage drops to under 11.5 volts, but recovers (minus any capacity used) as soon as the awning motor stops running.

Bottom line: You can accurately measure battery voltage anywhere in the system as long as everything is turned OFF (and you are isolated from the tow vehicle) at the time. Measuring battery voltage with an thing ON will give a false reading.
I thought the voltage regulator would detect a low voltage, and would tell the alternator to produce more juice. This should prevent the voltage drops seen when you turn things on because the regulator is compensating for the drop. At a certain point you will draw more amps then the alternator can produce. Only when you have maxed out the alternator will the voltage drop consistently below 13 Volts. Voltage might dip briefly, but it should recover relatively quickly. Perhaps I have this confused? Its not what I'm experiencing on the car either.

Derek
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Old 12-09-2012, 03:17 PM   #11
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There are two seperate subjects here, Battery voltage and alternator output.

In as much as the alternator usually senses voltage at the engine battery it would be unaware of any voltage drops that occured after that point in the TV or the trailer.

There is a provision on some GM/Delco alternators for remote voltage sensing, but that is a whole bag of issues on it's own.
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Old 12-09-2012, 04:09 PM   #12
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There are two seperate subjects here, Battery voltage and alternator output.

In as much as the alternator usually senses voltage at the engine battery it would be unaware of any voltage drops that occured after that point in the TV or the trailer.

There is a provision on some GM/Delco alternators for remote voltage sensing, but that is a whole bag of issues on it's own.
Ok, but couldn't we say that the engine battery and the trailer battery are ultimately wired in paralel. There is really only 30' of wire, a fuse, and perhaps a solenoid separating them. Assuming the solenoid is allowing current to flow, a drop in one battery will ultimately cause a drop in the other.

Derek

Disclaimer: The poster of this message shall not be held responsible for any self-harm caused by the line of questioning. The line of questioning seeks wisdom, and is not intended to offend.
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Old 12-09-2012, 05:36 PM   #13
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I'm curious why you are asking. It might help in answering your questions.
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I guess I'm slow, but I still don't understand what you are trying it figure out.
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Old 12-09-2012, 06:02 PM   #14
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I guess I'm slow, but I still don't understand what you are trying it figure out.
The whole thing started with the discovery that the voltage gauge, when plugged into the car, is showing much lower readings at the cigarette lighter then expected.

I know that a battery wont charge if you give it less than 13 volts. I also know that my truck (with two batteries and dual alternators) always shows between 13 volts and 14 volts with engine running. The only time I ever saw the gauge drop below 13 volts on the truck was was after one of the two alternators had failed. On the car, I'm seeing an average of 12.5 volts to 12.9 volts. The question becomes - why the difference? and Is the difference normal?

Derek
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