Tow vehicle charging batery

Discussion in 'Wiring' started by McMike, Sep 27, 2021.

  1. McMike

    McMike Member

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    How much juice should the tow vehicle be sending to the battery? Steady 12-ish?
    Is it possible to overcharge the battery?
    Thanks
     
  2. eoleson1

    eoleson1 Well-Known Member

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    If you check the voltage at the charge wire in the trailer plug, you should see about 13.5 volts with the vehicle running. You will not overcharge the camper battery using the TV.
     
  3. theseus

    theseus Living the Darkside... Silver Supporting Member

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    @eoleson1 is right. There is no way you could over charge your battery while towing. If you are worried, turn on your fridge to DC while driving.
     
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  4. jmkay1

    jmkay1 2004 Fleetwood/Coleman Utah

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    . Just make sure you turn it off or disconnect the pigtail if you stop for lunch it could kill your car battery if you left it on.
     
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  5. Patrick w

    Patrick w Active Member

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    You might see 13-14. If your lucky. Longer the run the higher the voltage drop.
     
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  6. BikeNFish

    BikeNFish Well-Known Member

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    Mine produces 13.6 volts.
     
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  7. theseus

    theseus Living the Darkside... Silver Supporting Member

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    This depends on if your tow vehicle doesn't turn the relay off when the car is off. My Ford Expedition is designed to cut power to the line when I turn off the vehicle.

    So another rule of thumb is know when that line is charged and when it is not. On all the time? You will need to unplug when you park for a bit.
     
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  8. davido

    davido Well-Known Member

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    I run the fridge on 12v when towing. I've parked the tow vehicle for up to an hour while leaving it connected. I think my Bronco keeps power to the trailer engaged even when the vehicle is off; definitely the rear power window moves more slowly when the fridge is in 12v mode and the vehicle is off.

    Voltage drop is a function of source voltage, wire gauge, and wire length. The longer the wire run, the thicker gauge you need to maintain the same voltage at the destination end. If you're finding too much voltage drop consider just tracing the charge line back to its source and replacing it with a thicker gauge wire. Otherwise you're just leaking power in the form of heat in the wire (small amount of heat; not to worry about, just a loss of efficiency).

    This website has a good length to voltage drop chart that recommends wire gauges. I think that for typical tow vehicles 10AWG is a nice upgrade, but for most tow vehicles, I imagine even going to 12AWG would be an upgrade over stock:
    https://astrolabesailing.com/2017/0...signing-an-electrical-plan-sizing-wire-fuses/
     
    Last edited: Sep 28, 2021
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  9. McMike

    McMike Member

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    Thanks for all the replies.
     
  10. Rusty W

    Rusty W New Member

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    There are nuances to this (and I'm talking only of tow vehicles in combination with a PUP). All your vehicle's alternator can "see" is your vehicle battery. When your vehicle batter is fully charged, very little charging makes it's way to your PUP house battery. Therefore, while your vehicle's start battery isn't fully charged you'll likely measure significant voltage/current making it's way to the PUP battery. When your vehicles battery is fully charged, you will see much less making it's way to your PUP.

    There are special DC to DC chargers on the market made exactly for this purpose. They "present" the PUP battery as a load to your alternator so it can "see" the battery and manage the charging of both your tow vehicle battery and the PUP house battery.

    Hope this helps.
     
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  11. Mark Jarosz

    Mark Jarosz New Member

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    If you are not seeing 13-14V take apart and inspect the wiring in the connector. Corrosion can lead to high resistance and it wont charge the battery. Just inherited a 20 yr old PUP and got everything working except fridge on DC while towing. Had 2 issues - Bad battery and 7 pin connector to truck was corroded inside and partially melted at the + terminal. Replace and connector and now back to 14v at battery while towing.
     
  12. lifespeed

    lifespeed Active Member

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    The DC-DC charger is an impressive addition to a camper, I have one myself. It can charge the battery at 40A, I use the model from Redarc. A slightly more technical description is these are DC-input battery chargers with a boost converter, increasing the voltage to whatever is appropriate for the stage of charging the battery needs; bulk, constant voltage, or float. It is still important to have decent gauge wire (use a voltage drop calculator) as the Redarc won't draw more current than whatever drops the voltage below 10.5V.

    Depending on your electric requirements, a DC-input charger can allow your tow vehicle to replace a portable generator as a battery charging source. Obviously it can't run an air conditioner.
     
  13. McFlyfi

    McFlyfi Well-Known Member

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    This is THE answer to the eternal "will my tv charge my pup battery". It should be a sticky somewhere.
    It won't matter if your tv is producing 13.5 volts if it's only sending half an amp or less down to a battery it can't "see". Voltage drop isn't the culprit-there isn't any current (amps) being sent.

    I've measured this twice, and an 8 hour drive has added less than 10 amps to a 150 ah battery at around a starting SOC of 80%.
     
  14. McSkippy

    McSkippy Active Member

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    Ten amps for 8 hours is 80ah, more than half of your battery's capacity.
     
  15. McFlyfi

    McFlyfi Well-Known Member

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    You are not reading this correctly...Less than 10 amps TOTAL in 8 hours of driving. The vehicle produced less than 1 amp per hour, for 8 hours.
     
  16. lifespeed

    lifespeed Active Member

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    Voltage drop is the culprit, and is the reason the camper battery doesn't draw much current. The voltage drop on the wire lowers the voltage such that current into the battery is low. It reaches a steady state determined by the voltage drop along the wire.

    If you want your TV to charge your camper, install a DC-input battery charger. It works great, but isn't free. Upgrading the wiring from the TV battery all the way to the camper battery to something large like 8G will help, but by itself will not do what a DC-input charger will.
     
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  17. TSQ

    TSQ Active Member

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    100% this.

    For the situation being discussed (discharged lead acid battery, charge rate <1C), voltage at the battery determines charge current.

    The alternator does not "see" different batteries/loads. It only measures voltage at one point (traditionally the output terminal of the alternator itself) and will regulate its output to try and maintain a specific voltage at this measurement point, regardless of what is connected to it.

    You will never fully charge a trailer battery by just linking it to your vehicle starter battery. Your deep cycle lead acid battery will need to be driven up to ~14.2V to fully charge (liquid, gel, and AGM have their own preferred charge profiles). If not stratification and sulfation will eventually destroy your battery.

    Back in the days when vehicles used non-sealed starter batteries they were regulated to 14V+. This would boil off a fair bit of water, but you would just keep adding more distilled water.

    When "maintenance free" (sealed) starter batteries started being used the regulated voltage was dropped down to ~13.5V to minimize water loss.

    Now AGM batteries are becoming popular and more vehicles are being equiped with "smart" regulators (ie: Euro 6) that will charge at ~14V for a limited time and then drop down to ~13V (or less).

    The only way to fully charge a trailer battery with a modern vehicle is with a DC-DC charger.

    For people that spend most of their time plugged in it matters less (ie: only use the vehicle charging to run the fridge while driving).
     
  18. jeepster04

    jeepster04 Active Member

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    Mine wont even run the fridge without pulling down the trailer battery. Once the battery voltage drops to match the voltage coming in from the TV, then it will run off the TV.

    I have a spare 12 AWG wire that runs from my battery to the back of my Jeep that Im pondering using to run to the 7 pin battery +. I'll use the stock trailer wiring to control a relay.
     
  19. bobinfleet

    bobinfleet Well-Known Member

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  20. jeepster04

    jeepster04 Active Member

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    Just a note on the DC to DC chargers, they are not magic. The amps going into the unit will be the same if not more than the amps going out, so you need the proper gauge wire going into the unit. The only thing the DC to DC charger does is control the output so the battery is properly charged... This means it will up the voltage as needed so the battery charges, which means the incoming amps will be even higher.

    You cannot simply put the DC to DC charger inline with the battery + wire (mine is 18 awg) coming from the factory 7 pin wiring. The 7 pin plug wouldnt be able to handle amps the dc to dc charger needs even if you did have the proper gauge wire going to the 7 pin.
     
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