Forum Discussion

towpro's avatar
towpro
Explorer
Sep 18, 2020

Why do I need a DC to DC charger?

I came to this group because of the tendency for truck campers to be more off grid oriented with knowledge about coach batteries.

Unless your using Lithium, why would you need a DC to DC charger between truck and coach? I know there will be some loss in the factory wiring (which gets smaller every year) but do I really need to limit current or voltage?

On my Ram it was not switched, and I knew to unplug the cord when camping to not discharge the truck batteries. Is the DC to DC charger only to prevent the truck battery discharge?

I was tempted to put a bluesea ACR on this line but they will conduct either direct either directions at a given voltage, IE my Camper solar would start to charge truck batteries which I don't need.

Really this question comes from I am wiring a new Toad. My Jeep Liberty I have the 12V feed wire from RV connected (fused) direct to the battery. Now I am wiring a new toad, a Ford C-max and I am reading about everyone using these special DC to DC chargers. But why do I need it? The truck alternator is only going to put out a max voltage, by the time it goes through the thin wire to the toad, its going to be even lower, so why do I need to regulated it?
Truck Campers
  • BB_TX's avatar
    BB_TX
    Nomad
    Sep 18, 2020
    Curiosity leads to google. And there I found this;

    A DC-DC charger is effectively a smart-charger for your 12V system. It isolates the house battery system from the alternator, so that the vehicle's computer management system sees it as something like a set of lights.
  • Kayteg1's avatar
    Kayteg1
    Explorer II
    Sep 18, 2020
    You don't.
    If your truck doesn't have charging circuit switched off, check if your camper has build-in battery separator.
    If not, battery separators sell for like $50 on ebay.
    On vehicles with smart charging system that gets complicated, but to my knowledge no US build truck to date has such system.
    So unless you have batteries with higher voltage, or drive Mercedes with smart charging, the B2B charger is just dead weight.
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Sep 18, 2020
    towpro wrote:
    ...why would you need a DC to DC charger between truck and coach?


    Kayteg1 wrote:
    You don't.
    .
    .
    .
    On vehicles with smart charging system that gets complicated, but to my knowledge no US build truck to date has such system.


    YOU ABSOLUTELY DO NEED A DC-DC CHARGER !

    Prove it to yourself. Start your truck. Check the voltage at the battery. Immediately after starting it should >13.8V. Let the truck idle for about 10 minutes and check the voltage again. Probably <13.2V ! Have some one hold the engine at a high idle. I'll bet it is still <13.4 !

    13.2V - 13.4V at your house battery bank will not let it discharge but it will NOT re-charge the house battery bank !
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Sep 18, 2020
    BB_TX wrote:
    Curiosity leads to google. And there I found this;

    A DC-DC charger is effectively a smart-charger for your 12V system. It isolates the house battery system from the alternator, so that the vehicle's computer management system sees it as something like a set of lights.


    You are partially correct ! A DC-DC charger actually BOOST the voltage that is being applied to the house battery bank to a proper multi-step "smart" charge.
  • Kayteg1's avatar
    Kayteg1
    Explorer II
    Sep 18, 2020
    theoldwizard1 wrote:


    YOU ABSOLUTELY DO NEED A DC-DC CHARGER !!


    Now I wonder how could I survive 34 years of RVing without one? :S
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Sep 18, 2020
    Kayteg1 wrote:
    theoldwizard1 wrote:


    YOU ABSOLUTELY DO NEED A DC-DC CHARGER !!


    Now I wonder how could I survive 34 years of RVing without one? :S


    Because vehicle "smart charging" system did not show up until after 2001 ! (Said the guy who actually wrote some of the software before retiring !)

    If you just want your house batteries to not run down (further than they are), you don't need one.
  • time2roll's avatar
    time2roll
    Nomad
    Sep 18, 2020
    towpro wrote:
    The truck alternator is only going to put out a max voltage, by the time it goes through the thin wire to the toad, its going to be even lower, so why do I need to regulated it?
    The DC-DC charger will BOOST the voltage and provide a proper charge at the correct voltage and then float.
    The DC-DC is not installed to limit the voltage from the alternator being too high.
  • Ed_Gee's avatar
    Ed_Gee
    Explorer II
    Sep 18, 2020
    Regarding the Smart Charger on the Ford Transit 350 HD diesel RV platform......it is not so smart. It was designed to charge the engine/chassis battery 'smartly'. However when RV manufacturers hook up the house battery system to it ( through the battery solenoid ), the Smart charger becomes less smart......it does not know when to drop the high charge rate down to normal float voltages.....it continues to pump out 14.7 volts all day long. On long trips this is harmful to the batteries which are being constantly subjected to Bulk charge voltages long after they had reached full charge. Personally I installed a switch to my Battery solenoid to disable connection of my house battery bank from the alternator.....to protect ALL my AGM batteries.
  • Kayteg1's avatar
    Kayteg1
    Explorer II
    Sep 18, 2020
    theoldwizard1 wrote:


    Because vehicle "smart charging" system did not show up until after 2001 ! (Said the guy who actually wrote some of the software before retiring !)

    If you just want your house batteries to not run down (further than they are), you don't need one.

    My Sprinter conversion has smart charging and I have aux battery directly connected to van battery (with battery separator).
    System powers 260 W refrigerator when on the road and with smart charging operating at 13.2-13.4 v it works well.
    In conclusion >> what's your business in promoting B2B chargers to people who don't need them?
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Sep 18, 2020
    Kayteg1 wrote:
    System powers 260 W refrigerator when on the road and with smart charging operating at 13.2-13.4 v it works well.
    In conclusion >> what's your business in promoting B2B chargers to people who don't need them?


    You are changing my words ! I said :

    theoldwizard1 wrote:
    If you just want your house batteries to not run down (further than they are), you don't need one.

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