3rd Gen Prius Battery Charger - Power/Amps?

Discussion in 'Gen 3 Prius Technical Discussion' started by andreimontreal, Oct 20, 2025 at 7:49 PM.

  1. andreimontreal

    andreimontreal Active Member

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    Say I add an aux 12V lifepo4 battery isolated with a battery isolator. Anybody got a clue how many amps is the Prius capable of, what's the power for charging the 12V batteries? I'm trying to figure out how fast I could charge a 100Ah lifepo4.

    PS: I have seen in this old thread that someone measured 1Kw - so 83 Amps. Looking for a confirmation. I know the main DC-DC inverter has a 100 Amp fuse. Is the charging capability directly related to that minus the needs for the car's system?

    Andrei
     
    #1 andreimontreal, Oct 20, 2025 at 7:49 PM
  2. ChapmanF

    ChapmanF Senior Member

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    The Prius 12-volt system doesn't have any direct control over battery charging current. The DC/DC converter is rated 120 amps, and it produces a voltage sometimes closer to 13.6 V and sometimes more like 14.7.

    The car chooses those different output voltages in a kind of rudimentary scheme to control the charging of the factory-supplied 12 V battery, but of course all that really happens is that whatever current flows is just whatever that battery will draw when faced with 13.6 or 14.7 volts. The same will happen with your added battery, unless you add some extra circuitry to have more control.
     
    #2 ChapmanF, Oct 20, 2025 at 7:54 PM
  3. andreimontreal

    andreimontreal Active Member

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    So, in theory, accounting for the car's needs, the system could give something like 100 amps for charging?

    You think I should add a voltage regulator? From what I read, very loose research, 14.7V for charging is fine; and I immagine the Toyota engineers knew what they were doing.

    edit: Is there a way to dermine how much to limit the aux battery charging? I heard 3 numbers for that fuse: 100, 120, 125 amps. Whatever it is (and it matters) I'm not sure how to determine how much the car and the main 12V battery needs so the aux (lifepo4) charging would not draw too much and blow the fuse. From reading about those with inverters, I recall that 1000W is a "safe draw" ... but then again, correct me if I'm wrong.

    edit2: if I want to add an inverter, I suppose the proper/safe way would be to put it on the aux battery/ies?
     
    #3 andreimontreal, Oct 20, 2025 at 10:59 PM
    Last edited: Oct 21, 2025 at 1:17 AM
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