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Adding second 12 volt battery

Discussion in 'Gen 3 Prius Technical Discussion' started by Revell, Aug 3, 2023.

  1. Revell

    Revell Junior Member

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    2012 Prius
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    I know there are lots of threads on this, but I haven't been able to find the answers for these specific questions. I have a 95Ah AGM battery that is charged with a 20A DC-DC charger (CTEK). The charger is connected to the Prius 12 volt battery. The battery is used to power a refrigerator when camping. This has been working well for a couple of years. However lately we have started spending more and more time in the camp and less on the road. So the battery gets less charge time. To solve this I am considering adding another CTEK charger that works as a relay allowing a max charge current of 120A. But I am afraid of the risk of overloading the DC-DC converter. People on here have done similar solutions and have reported no problems. But that big 95Ah AGM will draw a lot of current if it's half full. Also I am a very cautious person :) So I'm trying to figure out what's safe.
    Its a 2012 Prius.

    1. That signal called "IDH", that the DC-DC converter send when it's almost overloaded, has anyone been able to measure that signal on the cable that goes to the HVAC? Anyone know if it sends just basic 12 volts or some kind of digital signal?

    2. There is a custom PID that I have added to the Torque app called "Prohibit DC/DC converter signal". What is that? I was thinking maybe it is the IDH.

    3. How much current can I load the cable between the fuse box in the engine compartment and the 12 volt battery with? I can't find the size or length of it anywhere. If no one knows, can the max load be calculated if I measure the resistance in the cable?

    4. The black cable in the attached picture, is that from the DC-DC converter? Perhaps I can measure current on that to know if the converter is overloaded. The picture is from this video:


    Thankful for any input. Cheers
    Anders
     

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    #1 Revell, Aug 3, 2023
  2. CR94

    CR94 Senior Member

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    You might get suggestions for more sophisticated solutions, but here's my crude one:
    How 'bout connecting your added battery to the Prius 12-volt system through a resistor of value chosen to limit charging current to a safe level when the battery is "half full"? Of course, it would also have to have sufficient heat-dissipation capacity.

    A fuse connected in series with the resistor and battery wouldn't be a bad idea.
     
    #2 CR94, Aug 3, 2023
    Last edited: Aug 3, 2023
    Revell likes this.
  3. Revell

    Revell Junior Member

    Joined:
    Nov 11, 2017
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    Location:
    sweden
    Vehicle:
    2012 Prius
    Model:
    N/A
    I don't know about a resistor, but limiting the current can be done with a DC-DC charger. Victron Energy has DC-DC chargers that can be paralleled to charge at whatever rate I wish. The issue is that I don't know what a "safe level" is, really.
     
    #3 Revell, Aug 5, 2023
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