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Obviously the traction motors are driven by the battery - that's the whole purpose. I suspect that the power steering and air conditioner compressor are driven by the traction battery (vs the 12V battery). What else?

Edit: Since my question was tagged as too general, I'll rephrase to ask about a 2020 Toyota Rav4.

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  • Welcome to Motor Vehicle Maintenance & Repair! Your question is too broad. It depends on what type of system you're talking about. There are a bunch of different approaches to how a hybrid vehicle works, which includes the accessories you've mentioned. One system may use the hybrid battery only for propulsion, while others may utilize it for everything ... then there are the systems which range everywhere between.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 20, 2021 at 18:33

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I'm not sure if you mean hybrid or plug-in-hybrid. That should have been specified.

I have a 2016 Toyota RAV4 hybrid (standard hybrid, plug-in wasn't offered back then). My car powers everything including the power steering and windshield heater from the 12 volt battery, with these exceptions:

  • Traction for motors is provided from the high-voltage battery, and as a result of this because the car does not have a traditional starter motor, the engine is started from the high-voltage battery (so if your high-voltage battery doesn't have any charge, charging the 12-volt battery won't make the car start)
  • The air conditioning compressor is operated from the high-voltage battery
  • The DC-DC converter providing 12 volts is operated from the high-voltage battery, so your 12 volt battery charge is kept topped up by the high-voltage battery
  • The car lacks alternator, so the motor-generators are used to charge the high-voltage battery. The 12-volt battery is charged by the motor-generators and the DC-DC converter so there is no direct 12 volt charger.

Usually the idea is that 12 volts is a safe voltage but the high-voltage battery is operating at an unsafe voltage. Thus if something can be powered by the 12 volt battery, it will be powered by it. There are cars with 12 volt power steering, so 12 volt power steering is used. There are cars with 12 volt heated windshield, so 12 volt heated windshield is used.

However, the traction motors require so much power that good luck powering them with 12 volts. Similarly, the air conditioning compressor is quite powerful and the 12 volt battery is not beefy enough for it.

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  • Thanks for your answer. My car is not the plug in version. The plug in version is called "Prime". You mention that the engine is started with the high voltage battery. If thats the case, why is there a 12V connection point under the hood for jump starting?
    – Les
    Jan 25, 2021 at 1:34
  • Wanted to edit my other comment, but can't. I figured out that the 12V battery can get drained because I left something turned on. Without the 12V battery, systems critical to the starting and operation of the car won't work. That still leaves the question of how to start the car if the high voltage battery goes dead. Maybe that just doesn't happen?
    – Les
    Jan 25, 2021 at 12:42
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    The 12V connection point is under the hood, not for jump starting, but for booting the computer. If your 12 volt battery gets flat, you cannot boot up the car computer. Thus the car won't start. With the 12V connection points, you are able to start the car systems, but unless the high voltage battery has charge too, the engine may not be able to start.
    – juhist
    Jan 26, 2021 at 17:10
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    It is extremely rare for the high voltage battery to go flat (requires storing the car unstarted for extended periods). If this happens, the only option is to haul the car for a Toyota service where they are able to charge the high voltage battery. Toyota doesn't sell the high voltage charger for private individuals, probably out of fear that the ability to charge the small high voltage battery might alter the taxation of the car (because it would then be a plug-in hybrid of sorts).
    – juhist
    Jan 26, 2021 at 17:12

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