How the electrical system works in Prius Prime?

The HV is the engine starting battery, the 12 volt battery just powers up the computers. Be very carefull jump starting the 12 volt battery, hundreds if not thousands of dollars damage if you do it wrong.

This video explains Gen 2 Prius electricals, but is not Prime specific.

12 volt charging starts about minute 15 to 18

Unless the 12 volt battery can power the computers, they never let the HV Battery start the engine.

Everything but the Motor/Generators in the transaxle and the Air conditioning compressor are 12 volt. Never use electricity in the Prius except in Ready, so the HV Battery can charge the small 12 volt battery.

You do not have a battery charger for the HV Battery. Neither does the dealer. Expect Toyota to take weeks to get one to your dealer. Do NOT let the HV Battery get low.

 

I remember reading that our plug-in Prius Prime has a built-in charger that also has a 12V power supply. The TIS documentation reports it can provide enough power to boot the 12V system and then recharge the traction battery. However, this is not something I've verified and I'm having trouble getting back on to find the source.

The J1772 plug depends upon sharing a "pilot" signal before a relay in the EVSE closes to provide 110/220 VAC to the car. How this happens without 12V from the car isn't clear. I don't see the nominal pilot signal circuit providing enough 12V power so let's keep this aside for now.

It is likely this mode only happens AFTER the car starts charging with a nominally working, 12V battery, and the secondary 12V DC-to-DC charger is just to make sure the car remains 'stable' while the traction battery charging is going on.

Bob Wilson

 

It’s been years since Toyota offered printed shop manuals, but electronic versions of the Repair Manual, New Car Features, Collision Repair Manual, and Electrical Wiring Diagram books for the 2017 Prius Prime are shown as available by subscription to Toyota TIS (techinfo.toyota.com). The prices start at $15 for two days, if you just want access to the publications and not the Techstream diagnostic software, which is useless without a Mongoose or other compatible interface module.

You may find some of your questions answered in the New Car Features book. Don’t let the title mislead you; it’s a system-by-system review of the vehicle’s technical characteristics, not an advertising brochure.

I don’t know about the Prime specifically, but other Prius models (and more exotic vehicles, like the Mirai) have a fairly conventional low-voltage electrical system for the lights, ignition, entertainment system, power outlets, etc., but there is no alternator. The auxiliary (12 volt) battery is charged by a DC-DC converter powered from the hybrid vehicle battery; this converter operates only when the hybrid system is in READY mode. If the auxiliary battery voltage is too low, the hybrid system can’t start, even if the hybrid vehicle (HV) battery is charged.

In my Prius, with the hybrid system in READY mode and the car parked, the engine will run for a few minutes at a time, and then stop, to recharge the hybrid battery, but the hybrid system will stay in READY mode. There are two other modes (ACCESSORY and ON, similar to key-on/engine-off in other cars) in which the car will turn off by itself, and I believe the Prime is similar, as your Owner’s Manual describes on page 326:

◼️ Auto power off function
If the vehicle is left in ACCESSORY mode for more than 20 minutes or ON mode (the hybrid system is not operating) for more than an hour with the shift position in P, the power switch will automatically turn off. However, this function cannot entirely prevent the 12-volt battery discharge. Do not leave the vehicle with the power switch in ACCESSORY or ON mode for long periods of time when the hybrid system is not operating.​

Isn’t the whole idea of the Prius Prime that the HV battery can be charged externally, in everyday use?

I realize the sense in which you meant this, though, and by way of explanation for @Marinna: as on older Prius models, the HV battery charging system built into the car, whether it’s charging from the engine or from a plug-in source, requires 12-volt power to operate, and the HV battery itself must have at least a minimum state of charge. If the HV battery charge is too low, it can only be recharged by a technician using a special service tool (SST), such as the Midtronics GRX-5100 (SST 11413-00002) or the THS Charger Set (SST 09880-10021).

This is an extraordinary situation, however. If the auxiliary battery is discharged, but the HV battery has at least the minimum charge and there are no other failures, the auxiliary battery can usually be recharged with any high-quality car battery charger or by jump starting from another vehicle (with due care, as @JimboPalmer warns; see pp. 694–699 in the Owner’s Manual).

Last I checked, it wasn’t an essential SST for all dealers, but those with substantial Prius service business are encouraged to buy Toyota’s hybrid electric vehicle workstation, which includes a GRX-5100 and other high-voltage paraphernalia (see page 24 of the SST Storage System Master Index).

 

in the pip, not only does the hybrid battery keep the 12v battery charged while 'ready', when you plug in to charge the hybrid battery, it also charges the 12v. i wonder if prime is the same?

 

Any Prius hates not being used. I do not recommend a Prius to an owner who will not start it for 3 weeks.

If you, even for moment, reverse polarity while jump starting from another car, you will burn up at least hundreds of dollars of fuses and fusable links, if you are lucky. If you are unlucky parts of the inverter are damaged. So few mechanics can repair the inverter, they most often replace it for $4000. Many jump packs protect against reverse polarity, a feature I recommend.

 

gen 4 prius, including prime, has a new larger 12v battery under the hood. driving it every 3 weeks should not be a problem.
i leave my pip with the tiny 5 year old 12v in the hatch for 5 weeks without driving it, and it's very robust when i fire it up.
as far as using accessories whilst waiting for your car to charge, all you have to do is restart it every time it shuts off. the charger is also charging the 12v, so it's no biggie.

 

car uses the hybrid battery after it boots up, so it will keep running until you shut it down. yes, a small battery pack will start it. some carry a rechargeable jump pack. but i think you are over worrying, the new 12v battery is much improved.

 

What is interesting with the Prius is, after being in ready you can disconnect the 12V and drive off. Any and all voltage levels are sourced from the HV. After this test drive remember to connect the 12V again.

 

I bought a small recharge device, about 8X 4 inches, supposedly can provide up to 60 amps, I have only used it on my computers, also charges in car. Neat. Cost ~ $35

That's neat.

Sits in my spare tire. That's just too neat.

TY

 

I will sometimes be using a CPAP machine in my 2017 Prius Prime (it plugs directly into the cigarette lighter plug, with no inverter, via an adapter made by the manufacturer of the CPAP machine). Note that for this purpose I have had an extra cigarette lighter plug installed wired directly to the 12V battery with 10 gauge wire and an inline fuse. I want to make sure that I understand what is being said in this thread so that I don't drain my 12V or HV battery while using it. I have a few questions I hope you'll be kind enough to answer:

1. From what I've read in this post and others, if I'm using my CPAP, I want to be sure to leave my Prime in "READY" mode, not the "ON" or "ACCESSORY" modes. If I leave it in either of the latter modes, the Prime will eventually shut down and I risk draining the 12V battery in the meantime. True?

2. Apparently, in the "READY" mode, the HV battery will periodically automatically charge the 12V battery when the 12V battery starts to get low, thus preventing the 12V system from discharging. But, if the HV battery starts to get low from charging the 12V battery, will my Prime also start itself periodically if necessary so that it charges the HV battery?

3. I see warnings here and in other threads to never discharge the HV battery as doing so requires servicing by a Toyota dealer with a special tool (the SST). (A) I know the Prime has multiple high voltage batteries and a different plug in mode charging system, so is this risk/warning also true of the Prime? (B) How might one end up discharging the HV battery (so I can avoid doing so)?

4. If you leave the Prime in "READY" mode and walk away from it (removing the electronic key from the range of the Prime), the Prime beeps, but it seems to stay in "READY" mode. If you keep the the electronic key out of range, does the Prime end up shutting down at some point?

Thank you for any explanation you can offer. I've read other threads about this, but I'm still not clear about the above questions, especially as related to the Prime.

 

1. True that you want it in Ready to power for continuous power, but most modern cars will shut off the power in Accessory after a time period to protect the battery and save charge to start the car. You could drain the 12 volt in it by repeated restarting of the mode and if the battery is already low on charge. So On is the mode that can result in a drained 12 volt.
   
2. Yes, the engine will come on to top off the battery when the state of charge gets low.
3. People have driven the Prius without gas in the tank. This leads to discharging the battery too much. So the car needs to be in Ready and have no ICE fuel in order for there to be a risk of the battery draining too much. I doubt your CPAP is a heavy enough load to drain the HV battery in the event the car ran out of gas while you were sleeping.
4. I can't say for current models, but the gen2 could be driven off without the key in that case.

 

That's correct. Many people have found out the hard way.

Close. The HV battery continuously maintains the charge on the 12V battery.
Yes, the engine will start as needed to maintain charge in the HV battery in any Prius. That's why they are so popular for camping in the car. Air conditioning all night and you only use a very small amount of gas.

As @Trollbait said, running out of gas will do it. But really, any time the ICE won't start and you keep trying, you're running down the HV battery cranking a dead engine. That will eventually run it flat.

Not sure about that since I don't have a Prime. I'm not even sure on my PiP or Gen 2 because I've never done that.

 

Thank you for the answers.

Yes, as regards question #4, I think that if you do walk away with the electronic key leaving the Prime running, the Prime could still be driven until it is shut off, though I haven't tried. My thought is that if I am camping or somehow outside my car using the CPAP, I really don't want to leave the electronic key in it in case someone does hop in and drive it off. At least that way they wouldn't have the key and be able to restart it once they shut it off.

 

Assuming the 12 volt was able to boot up the computers and engage the HV battery first.

 

what is CPAP ?

 

1. From what I've read in this post and others, if I'm using my CPAP, I want to be sure to leave my Prime in "READY" mode, not the "ON" or "ACCESSORY" modes. If I leave it in either of the latter modes, the Prime will eventually shut down and I risk draining the 12V battery in the meantime. True?
True. If there is any chance you will do so, a jump start pak may be in your future.

2. Apparently, in the "READY" mode, the HV battery will periodically automatically charge the 12V battery when the 12V battery starts to get low, thus preventing the 12V system from discharging. But, if the HV battery starts to get low from charging the 12V battery, will my Prime also start itself periodically if necessary so that it charges the HV battery?
Yes the 12 volt battery is charged continuously while in Ready, and the engine will start to keep the HV Battery charged if it can. Nothing can start (or stop) the engine in N, so avoid N.

3. I see warnings here and in other threads to never discharge the HV battery as doing so requires servicing by a Toyota dealer with a special tool (the SST). (A) I know the Prime has multiple high voltage batteries and a different plug in mode charging system, so is this risk/warning also true of the Prime? (B) How might one end up discharging the HV battery (so I can avoid doing so)?
A) still true B) the most common ways to discharge the the HV Battery are driving with an empty tank and operating the car in N. In N the engine cannot start to recharge the HV Battery. Toyota is required by law to have an N, you never want to use it.

4. If you leave the Prime in "READY" mode and walk away from it (removing the electronic key from the range of the Prime), the Prime beeps, but it seems to stay in "READY" mode. If you keep the the electronic key out of range, does the Prime end up shutting down at some point?
It will run out of gas eventually.

This video explains the gen 2 high voltage system

This video compares and cotrasts the Gen1, Gen2, Gen3, Gen 4 Prius and a Ford Fusion and Chevy Volt.