voltage drop

Submitted: Saturday, Apr 08, 2006 at 13:32
ThreadID: 32680 Views:3296 Replies:7 FollowUps:8
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as suggested, i have tested the voltage at the rear outlet (for the engel).
it reads 12.4V without the fridge plugged in, 11.4V with the fridge plugged in and turned on.
is this an acceptable voltage drop? engel seems to work fine.
i don't have a problem yet but i am re-wiring to the rear step with big cable for a winch, but cannot find an easy way into the vehicle at the rear without removing the shelf system.
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Reply By: Member - Roachie (SA) - Saturday, Apr 08, 2006 at 13:55

Saturday, Apr 08, 2006 at 13:55
I'm not sure what previous POST you're referring to so I have no idea what sort of vehicle you're trying to get the wires into the rear of......

A full one volt drop is waaaayyyy too much and means (in most instances) that the cabling is nowhere near heavy enough. However, what is the voltage at the battery when the fridge is on? If that too is down to 11.4v then it may mean the battery is on the fritz.

If you have a wagon (which I'm assuming as you talk about a shelf system), then there should be large rubber grommets in the lower rear inside panels that you can run cables up through..........at least that's how I do it in the Patrol.

Cheers

Roachie
AnswerID: 165790

Follow Up By: howie - Saturday, Apr 08, 2006 at 15:01

Saturday, Apr 08, 2006 at 15:01
thanks roachie, it is a patrol, and spurred on by your grommet talk i found the one that is perfect (probably the one you meant, i was looking too far up).
the test i performed was suggested by the shop i bought the cable from, not a post. i will now convert all my rear power outlets to the new cable.

incidently, i have managed to get a draw-wire inside the chassis 'ladder' rail from the rear(near said grommet) and emerges from the rail just behind the front suspension.this involved a metal 'fish' line (telecom stuff) and a lot of cussing, but should be safe in there.
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Reply By: Mike Harding - Saturday, Apr 08, 2006 at 14:17

Saturday, Apr 08, 2006 at 14:17
>it reads 12.4V without the fridge plugged in, 11.4V with the
>fridge plugged in and turned on. is this an acceptable voltage drop?

I wouldn't be happy with it. That's about an 8% loss in the cable and/or fuse. From memory an Engel typically draws about 4A which is not a high current and something like 1.5mm sq. or 32/0.2 should do pretty well for short runs. Also check _all_ connectors and the fuse rating, about 10A should be OK. If you are planning to take more load than just the fridge off this circuit consider a larger cable.

Mike Harding
AnswerID: 165794

Follow Up By: howie - Saturday, Apr 08, 2006 at 15:07

Saturday, Apr 08, 2006 at 15:07
thanks mike, further to my reply to roachie , the cable going in is 0.9mm twin which should sort out any drop. i am just waiting for approval from western power before i connect.
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FollowupID: 420686

Follow Up By: ozdragon - Sunday, Apr 09, 2006 at 02:13

Sunday, Apr 09, 2006 at 02:13
If you are only running 12v cable...why do you need approval from Western Power. I didnt think they had anything to do with 12v.

Peter
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FollowupID: 420799

Reply By: Crackles - Saturday, Apr 08, 2006 at 17:09

Saturday, Apr 08, 2006 at 17:09
Howie. I've run twin 12mm squared cable to the back of the car & get a .2 of a volt drop between the battery & the plug on the fridge when it's running.(bit less than 4 amps)
Do the same. Measure your voltage at the batt & fridge when running not just at the rear. I would be disapointed with any more than .4 volt drop & suggest upgrading wires & fitting better quality plugs.
A full 1 volt drop would lead to a shorter run time with the fridge cutting out well before the battery was discharged.
Cheers Craig...
Trojan deep cycle batts, 30 amp Circuit breaker, 12mm wire, 32 volt clipsal plugs.
Autofridge
AnswerID: 165812

Reply By: Derek from Affordable Batteries & Radiators - Saturday, Apr 08, 2006 at 17:33

Saturday, Apr 08, 2006 at 17:33
I would agree with the above posts and would also like to know what is the voltage at the battery and then at the fridge. Also is the battery fully charged to do this test ?

Regards Derek.
AnswerID: 165815

Follow Up By: howie - Saturday, Apr 08, 2006 at 18:53

Saturday, Apr 08, 2006 at 18:53
aux battery is always fully charged (hopefully) thru the redarc.
lugged the engel outside AGAIN, stops me watching television and drinking beer i suppose, and here are the results.
battery and rear power reads 12.7v with nothing plugged in.(gone up since earliar, hotter?)
battery reads 12.4v and rear power 11.5v with engel running.

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FollowupID: 420714

Follow Up By: Derek from Affordable Batteries & Radiators - Saturday, Apr 08, 2006 at 20:33

Saturday, Apr 08, 2006 at 20:33
Your voltage drop is a bit high on a fully charged battery.

You must upgrade your wiring and solder all joints. Check the fuses as well for any suspect joints.

Go for a 8 B&S cable or 6mm2 tinned twin core. Make sure to run a (+) and (-) all the way battery to fridge.

Bit late to do it now so have a beer and do it in the morning.

Regards Derek.
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Reply By: tonysmc - Saturday, Apr 08, 2006 at 19:30

Saturday, Apr 08, 2006 at 19:30
Howie, It may be a coincidence, however when my engel is running and the low battery cut out switch cuts off the power because the battery is flat, the volt level measured at the battery instantly jumps 1 volt. When I have played around switching the fridge on and off I find that there is a 1 volt difference when the fridge is running to when it isn't. This is with the fridge connected straight to the battery and the battery is FLAT. I was only doing it to set my low volt cut out at a level I wanted and have never checked on a fully charged battery.
Your situation is different as there is a difference between the battery and your plug which doesn't seem right, however I just found it interesting that as in my case you also have a 1 volt difference when your engel is running and when its not.

Tony
AnswerID: 165837

Reply By: Member - Ian H (NSW) - Saturday, Apr 08, 2006 at 19:34

Saturday, Apr 08, 2006 at 19:34
It is worth checking the article by Collyn Rivers regarding cabling sizes. You can probably get it from the archives. My advise is to put in as big a cable as you can. Yoy only ned about 6 metres and it is cheap compared to a warm beer somewhere along the track.
AnswerID: 165839

Follow Up By: Mike Harding - Saturday, Apr 08, 2006 at 19:50

Saturday, Apr 08, 2006 at 19:50
>My advise is to put in as big a cable as you can.

Good point - a few metres of cable won't cost much and you're bound to discover something else you just _have_ :) to run from it in due course.

Mike Harding
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Reply By: Member - Phil G (SA) - Sunday, Apr 09, 2006 at 21:42

Sunday, Apr 09, 2006 at 21:42
This depends a bit on how you are measuring voltage drop. I prefer to have one multimeter probe on the pos battery terminal and the other probe on the pos lead as it enters the fridge (I use a sewing pin thru the cable for this). You'll need one probe thats about 5 metres long - easily doctored up with a piece of wire. With the fridge turned off, voltage drop will be zero (unless you are running other accessories in the circuit) and with the fridge running you'll get an exact measurement of the voltage drop in that cable.

The reason I use this method is because its more reliable than spot readings. Tthe voltage in the battery itself falls while the fridge cycles, and comes up again when the fridge stops cycling.

Also, to interpret your voltage drop will depend on which series Engel you have. The current F-series has an amp draw of only 2.8amps while cycling - a voltage drop of 1 volt would be rather huge. The amp draw of most other engels is 5-6amps when cycling, and one volt might be almost acceptable.

Also, if you have the later engel, it will turn off and not work when the supply hits 10volts. If you have the earlier engel it will slowly chugg away at lower volts until it kills the battery and maybe itself!

Wiring is obviously important, but fuse holders and any other connectors are sources of voltage drop - particularly when they start to corrode. My method is to solder a blade fuse into the wire at the battery end avoid any additional connectors until I get to the fridge - then use something substantial like an Anderson plug. I actually use a thing called a Deans plug which is used in R/C cars. In the unlikely event that I would blow a fuse, I'd need to solder a new one in.

I earth the lead to a bolt on the body which has been cleared of paint and has a smear of vaseline to stop corrosion. And I also ensure that the 2nd battery has a substantial earth lead to both body and motor. To run an earth lead all the way back to the battery is a reliable alternative, but I think its unnecessary if you do the above properly. And a long earth lead will also have voltage drop.

Cheers
Phil
AnswerID: 166056

Follow Up By: roger baker - Sunday, Apr 09, 2006 at 22:48

Sunday, Apr 09, 2006 at 22:48
You need to find out exactly where the voltage drop is occurring.
I have measured 7 ohms between the battery terminal and the chassis, on some vehicles, which can cause massive voltage drop depending on the current being drawn

Roger
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FollowupID: 420997

Follow Up By: Member - Mike DID - Sunday, Apr 09, 2006 at 23:00

Sunday, Apr 09, 2006 at 23:00
You need to ensure that all electrical loads are switched off before taking resistance readings.

I once had technicians call me in because they measured 2 ohms resistance in a busbar drawing 200 amps. The penny should have dropped - there wasn't a 400 volt drop along the busbar ! (50 volt batteries)

You CAN'T use a multimeter to measure resistances if there is any current flowing.

Mike
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FollowupID: 420999

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