1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Maximum Amps from Battery

Discussion in 'Living Life in a Prius' started by richard1540, May 16, 2022.

  1. richard1540

    richard1540 Junior Member

    Joined:
    Aug 20, 2019
    17
    8
    0
    Location:
    Australia
    Vehicle:
    2011 Prius
    Model:
    i-Tech
    I want to install a DC to DC charger and a lifepo4 135ah battery into my 3rd Gen Prius. I looking at a Renogy 40 amp charger.

    12V 40A DC-DC On-Board Battery Charger | in-vehicle dual battery chargers | Renogy

    I have 2 questions.

    Would 40 amps be too big a current draw from the 12v AGM Prius system?

    I need an ignition connection to tell the charger the motor is running. Would a tap on the centre consoles 12v cigarette lighter socket be suitable? Or is there a better way?
     
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    23,073
    14,982
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    How much current the system is able to supply can vary quite a bit depending on which of the car's own electrical things are operating. The car's DC/DC converter is rated to supply 120 amps maximum. They did not gratuitously oversize that, so it's fairly well mated to what the total draw of the car itself can be when lots of the power accessories (HVAC, defogger, lights, seat heaters, etc.) are in use, so at those times, it hasn't got much capacity left for added uses.

    On the other hand, a lot of the time, not many of those things are operating, so there's plenty of capacity left for you. At such times, 40 A would be no problem, or even 80. Probably best not to plan on exceeding 80, as the car's basic operations still amount to 40 amps or so even with most optional stuff not in use.

    The car's a hybrid, so telling the charger when the engine is running wouldn't really be something the charger needs. What you would want is to know when the car is in READY mode (whether the engine is running or not), because that's when the car's DC/DC converter is supplying power.

    There aren't really convenient circuits to tap in the car that identify READY mode for you. Any "IG" circuits you find will be powered either in READY or in IG ON mode, and that's definitely not what you want, because in IG ON the car does not produce power.

    On the other hand, in any mode except READY, the voltage at the battery will just be the voltage of the battery (generally something below 13), and in READY it will be the voltage from the DC/DC converter (generally above 13.5). So an easy way to tell when you are in READY is to buy an off-the-shelf gizmo called a "low-voltage disconnect" that recognizes that voltage difference, or build an equivalent yourself.
     
  3. PriusCamper

    PriusCamper Senior Member

    Joined:
    Mar 3, 2012
    10,775
    4,372
    0
    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    I suspect Renogy has some kind of BMS in their lifepo4 135ah battery... And a low voltage disconnect is another battery management tool that would be wise to have for existing 12v.

    However, I'm skeptical that a DC to DC charger is the way the to go. Why not instead take the charging burden off the car and use a 120V AC to 12V DC charger you plug into your house electricity to charge the lifepo4 135ah battery? Then simply add a 1K or 2K DC to AC inverter into the system when you can't plug your charger into an outlet?

    Of course, you've not said what you need so much 12v battery power for yet? Perhaps that would clarify how to design the best system?