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Battery charging - two (related) queries

deucedeuce Member Posts: 47
edited October 2020 in Battery/Electrical & Solar
1. I recall someone in the past month or two saying something like "I think the alternators on the more recent autos/trucks only charge the vehicle battery, and not the towed camper battery."  Is this true?  I distinctly remember the (very young) set-up guy at my T@B dealer saying, "you'll get a charge, but you may have to pull from state line to state line (in the West).  I don't seem to get any kind of boost.  Thoughts?

2.  Having recently completed my double 6-volt set-up, can I leave the cable connecting them to the camper (permanently) on when I attach my charger, with the ON/OFF switch off, or should that connecting cable be taken off the terminals before attaching the charging cables?  (

And, would it damage the battery or charger if the connecting cables were in place and the switch was in the ON position?)
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    Sharon_is_SAMSharon_is_SAM Administrator Posts: 9,528
    If you are charging your battery with a smart charger using alligator clips, the batteries will charge faster if you isolate the batteries by turning the battery switch off.  Just leave your terminal connections in place.  

    Apparently many modern TVs complicated electronics limit the ability to charge the TaB battery.  
    Sharon / 2017 T@B CSS / 2015 Toyota Sienna Minivan / Westlake, Ohio
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    4ncar4ncar Member Posts: 1,072
    I believe it depends on the TV and what options it has and how the ECU is programmed.  In my case, the ECU knows when I turn on “tow/haul” mode and I can see a significant change in the the charge voltage going to my trailer batteries (twin 6v). Even with the 12 volt fridge on, btw.
    TV- '16 Chevy Colorado LT Crew Cab-DuraMax
    2018 320S Outback
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    VictoriaPVictoriaP Member Posts: 1,494
    A lot of modern cars can barely manage to charge their own starter battery, let alone a trailer deep cycle. One trick that sometimes works is to turn the headlights ON (not to auto on or daylight mode) when towing. That apparently causes the alternator to increase output. 

    But basically, unless you install a DC to DC charger, don’t expect more than a top off of your batteries from driving. Your 6v pair would take a long time to fill from even a partial drain via the tow vehicle. And if you have a fridge running on 12v, don’t expect even that much, it may consume everything the alternator provides, and even more if it’s a 3 way running on 12v.

    As 4ncar notes, vehicles with specific towing modes are more likely to produce better results on this score without needing aftermarket help to make it happen.
    2019 320s BD Lite, white with blue (“Haven”)
    2015 Subaru Outback 3.6r (unsafe 200lb tongue weight limit until 2020 models)
    2020 Subaru Outback XT
    Pacific NW
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    deucedeuce Member Posts: 47
    ECU Is acronym for what? Electric console Unit?
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    N7SHG_HamN7SHG_Ham Member Posts: 1,261
    ECU is an engine control unit, also commonly called an engine control module or powertrain control module. Basically engine computer.
    2019 T@B 400 Boondock Lite
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    4ncar4ncar Member Posts: 1,072
    VictoriaP said:

    As 4ncar notes, vehicles with specific towing modes are more likely to produce better results on this score without needing aftermarket help to make it happen.
    Screen shot of my BMV SOC screen while driving at Hwy speed, w/3 way Fridge set to 12v and my truck in “Tow/Haul” mode. Next time out I’ll take a screen shot of when tow/haul mode is off for comparison. I have viewed it, but did not take a screenshot.


    TV- '16 Chevy Colorado LT Crew Cab-DuraMax
    2018 320S Outback
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    BrianZBrianZ Member Posts: 1,763
    Here's our battery monitor & SCC showing 11 amps of charging current being delivered to our T@B battery, 5A (70W) from solar panel on the roof of our 2005 Sienna van, and 6A (80W) from the car battery.  These screen shots were taken just before leaving the campground.  I should mention that the 12V charge line in our 7-pin socket at TV hitch is wired directly to the TV battery (via a 30A self-resetting circuit breaker).

    -Brian in Chester, Virginia
    TV: 2005 Toyota Sienna LE (3.3L V6)
    RV: 2018 T@B 320S, >100 mods 
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    tphaggertytphaggerty Member Posts: 49
    7amps from my 2019 Ridgeline when towing. There are folks installing DC to DC chargers, some that add a separate charging cable and some (20 amps max) that use the charging line on the 7 way.
    2019 Honda Ridgeline RTL TV
    2018 T@B 400, 300Ah Renogy LiFePo batteries, 350W Renogy rooftop solar
    Poughquag, NY
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