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Old 07-05-2017, 11:22 AM   #1
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Testing voltage to batteries when towing

I may be asking the impossible but I want to run my refrigerator on 12 volt when driving. No big deal here. But, is there a way to measure the voltage getting to my batteries while connected.

I understand there are so many variables between tow vehicles, alternators, wire gauge, distance etc there is not a blanket solution. I understand that headlights, wipers, a/c can effect results.

Using a multi-tester can I connect the tow, start the engine and somehow measure what the Escape batteries are receiving for a charge from the tow? Then the obvious would be to turn on the refrigerator and measure.

At this point the only method I see, is to stop every 30 minutes and see what my Trimetric battery monitor is reading for charge percent on the Escape batteries.
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Old 07-05-2017, 12:01 PM   #2
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If you have or can borrow a DC clamp on amp meter, and can get to either the charge wire (Pin 4, typically a black wire) or the ground wire (Pin 1, typically a white wire) running the tow vehicle engine at a normal operating RPM would give you the charging current.

If you can't get at a single wire, this won't work since the meter requires one wire only (positive or ground will work as long as you don't have any other loads on the ground wire). I suppose you could make a short extension cable to break out the wires, but that is starting to get expensive, particularly since, as you mentioned, there are many variables that could make the results questionable.

I've used my Trimetric set on current to attempt to do the measurement, but the 195 watts of solar on the roof made it difficult to believe the result. According to a couple of measurements, the RAV4 produced between 6 - 8 amps with the trailer battery at 90% & the RAV4 battery near full. I never tried it with the truck...
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Old 07-05-2017, 12:02 PM   #3
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Paul, I replaced my Trimetric with the Victron 700 and Bluetooth dongle. This allows you to monitor your batteries while towing. Very easy to use and setup. We always run the refrigerator on propane and on our previous camper the flame blew out, fridge switched to battery. By the time we arrived at our stop for the night our batteries were depleted and we spent a chilly night without heat. Scott

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Old 07-05-2017, 01:55 PM   #4
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This charge wire would be at the tow vehicle or inside the Escape? I am assuming these wires may be in a bundle, thus making it impossible to apply the clamp?
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Old 07-05-2017, 02:13 PM   #5
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Paul, what brake controller do you have? My Prodigy P3 brake controller shows voltage going to the trailer. You could compare that reading with one at the batteries at the same time and get a close estimate on % voltage drop.
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Old 07-05-2017, 02:45 PM   #6
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Net gain or net loss?

Paul
My son in law Todd asked the same question yesterday. I have a jump starter pack with me so I told him to check volt.tage on D.C. Before starting out, put the reefer on DC starting out. Drive today only about 80 miles, then Check battery voltage when we stop. We will have electric tonight so we can recharge battery on camper with converter or jump him with starter pack and let the vehicle alternator recharge the car battery. I think we're good to go on that. Both ways and if not I'll put a tow strap on his pathfinder and tow him across Ontario. The Highlander will do it. We will be at the Pinery Park in Ontario in a few minutes.
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Old 07-05-2017, 03:19 PM   #7
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A trailer brake controller sees (and in the case of the Prodigy P3 can display):
  1. tow vehicle supply voltage (to the black wire of the controller),
  2. voltage from brake light switch (to the red wire of the controller), and
  3. output voltage to trailer brakes.
It has no way to see the voltage supplied to the trailer at the towing connector, or any voltage in the trailer. If you just want to see what is available in the tow vehicle (and don't have another convenient display) this is useful, but it doesn't answer the question of what is getting to the trailer.

Quote:
Originally Posted by fudge_brownie View Post
This charge wire would be at the tow vehicle or inside the Escape? I am assuming these wires may be in a bundle, thus making it impossible to apply the clamp?
Although the question was about monitoring voltage, monitoring current would tell you more about how well the charging circuit is working, and that's what Jon provided. For current, it doesn't matter which end of the circuit that you monitor.
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Old 07-05-2017, 03:29 PM   #8
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Quote:
Originally Posted by Brian B-P View Post
A trailer brake controller sees (and in the case of the Prodigy P3 can display):
  1. tow vehicle supply voltage (to the black wire of the controller),
  2. voltage from brake light switch (to the red wire of the controller), and
  3. output voltage to trailer brakes.
It has no way to see the voltage supplied to the trailer at the towing connector, or any voltage in the trailer. If you just want to see what is available in the tow vehicle (and don't have another convenient display) this is useful, but it doesn't answer the question of what is getting to the trailer.

Probably why I suggested comparing the voltage displayed by the P3 with a simultaneous reading at the battery...
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