Most decent way to run your 115V essentials out of Prius Prime

How are you connecting the inverter? The prime has a current sensor on the battery, and the DC-DC converter that powers the 12V system uses that to control the battery current (presumably to 0A, or slightly charging the battery) when the car is running. If you put a load directly on the battery that the current sensor doesn't sense, you could drain the battery and the car won't know it. That would quickly ruin the 12V battery since it isn't designed for deep discharge. Even discharging it once could ruin it (which seems like a poor design in my opinion, especially for a car without a traditional starter). Once the battt is discharged and/or ruined, the car won't be able to keep it charged well (another poor design, but whatever). Putting a good external charger on the 12V battery with the car off (or better yet with the battery out of the car) could help temporarily.

I think you need to hook up to the battery on the output of the current sense element, not directly to the battery lugs, but I haven't done it myself so I'm no sure, and I don't know what the battery connector looks like. The current sense element is on the negative side, iirc.

 

I didn't intend to get hit from behind while sitting at a traffic light and sending my wife to a hospital. But things don't always happen the way we intend because we only THINK we're in control. Life is full of distractions and timers get not set or not heard, for example. What you're doing is probably fine while you're experimenting. But when a storm comes and the power goes out, you really don't want to put yourself in a position where a minor brain fart can turn your car into an expensive and useless decoration in your garage.

Running appliances with an inverter is a great advantage in the Prius, especially a Prime, but the way you're doing it with the car in Neutral is a recipe for disaster and you're not listening to the people who are trying to help prevent the disaster, so I'll just back out now since you seem bound and determined to proceed in spite of the cautions from the experienced.

 

The way Priuses work since 2017 means that while in DRIVE the 12V won't be charged very much because the output of the DC-DC converter drops to 12.0V (seems to vary quite a bit from car to car). To charge a 12V battery to full requires something over 13V. A 12V battery that's charged to 12.0V is only about 25% full. I suspect this explains some of Toyota's recent problems with the 12V batteries in these cars.

 

I don't know about your PRIME, but my PRIME charges the 12v battery fine. Look at the 12v battery monitor graph on my previous comment. Most decent way to run your 115V essentials out of Prius Prime | Page 2 | PriusChat
For ~7 hours of PARK and READY mode, the 12v battery was continuously charged at above 14v entire duration I experimented and the resulting resting voltage was 12.9v up from 12.6v. I see the similar but slightly less charging from just driving my PRIME ~1.5 hours once a week.

 

It did that while in P, right? The DC-to-DC converter outputs 14V in P. It's in DRIVE that the output drops to 12.0V. If you drive a long distance using 12V power your battery will be pretty dead when you get there.

EDIT - I should say a long time rather than a long distance. It can take me 45 minutes to go 2 miles in downtown Los Angeles, so distance isn't the real factor. I think they key is that you have to be using 12V power while in drive. I am usually charging a laptop or doing something that draws a couple of Amps from the 12V system and my commute is 2 hours. My 12V battery routinely reads 11.8 - 12.0V when the car is off and after I bleed off any surface charge.

 

Well, if the 12v battery is low enough, my PRIME charged it for the duration of my ~1.5 hour drive without dropping the charge voltage.

 

Something's not right. I've loaded the diagnostic screen in 5 different Priuses while driving and ALL of them report 12.0V for the 12V system while in drive no matter how long the drive (in fact a regular hatchback was even worse than my car, reading something like 11.6V). I've confirmed 12.0V on my car with a Fluke 88V, whether measured at the battery posts or the fuseboxes. Toyota sent a regional tech to the dealer to check out my car and they reported that is the designed behavior.

Have you tried loading the system voltage screen in diagnostic mode while driving to compare what it says versus the app?

 

No. How do I do that on PRIME? The graph is from Bluetooth Battery Monitor directly connected to the 12v battery. It records the voltage read at the posts every 2 min. It has no connection to the car's EUC. The fact that the 12v battery is receiving above 14v at the posts for the duration of the drive and the fact that the resting voltage increased after the drive proves it was charged by merely driving the car for 1.5 hours isn't it?

 

I have the small screen and it's just pressing the up button on the display 5 times and then the down button 5 times.

See this post

If your DC-to-DC converter is keeping the 12V system above 12V while driving it would be different behavior than all the 2017-2019 Priuses I've tested and Toyota's statement that 12.0V in DRIVE is normal operation.

 

So I can do this even while driving the car?

(c) Method 2

(1) Turn the power switch on (IG).

(2) Press the seek/track up panel switch 5 times and then press the seek/track down panel switch 5 times with the screen and audio turned off.​

I am not sure what Up and Down buttons you are referring to, but from the description, are they those two buttons (SEEK and TRACK) in the picture?

 

I never checked my previous 2017 PRIME's 12v battery for I never had to park my car longer than a few days and I never had any trouble with the 12v battery. I only started to be concerned about my current 2020 PRIME's 12v battery due to COVID-19 stay in shelter order. I am only driving once a week now, but the battery monitor is clearly showing that the car while being driven charges up the 12v battery at above 14v.

After weeks of not charging the 12v battery but just driving once a week for ~1.5 hours, the lowest the 12v battery I have seen is 12.3v at which point I used an external charger to charge-up the 12v battery to full.

 

They may have fixed it for 2020, which would be great news. You can see the behavior in the 2017 for sure by loading up that screen and switching the car from P to DRIVE and back. The voltage will go from ~14V to ~12V and back as you change "gears."

As you know, a battery cannot be charged to a higher voltage than the charger is supplying, otherwise current would start flowing the opposite direction (without a diode). So when the 12V system is being held to 12.0V, using the 12V battery will drain it until the voltage dips below 12V. After dipping below 12V the battery will accept a charge, but only up to the 12.0V being fed to it.

 

That is bizarre. If what you are telling is true, I don't see any 12v battery in Prius lasting very long. How many Prius drivers would let the car sit in READY mode on PARK for hours long at least a few times a week regardless of how often he/she drives. Without doing prolonged PARK in READY, the battery would be dead in a few days if it is not being charged while in DRIVE. I never PARKED in READY with Gen3 Prius or 2017 PRIME and I never externally charged the 12v battery either. But never had any problem with the 12v battery, despite the fact that I owned those two cars less than 3 years each.

 

It is being charged while driving, but only to 12.0V (about 25%). That's why there's a lot of people reporting dead 12V batteries after a few days parked.

 

Well, as I said, I never checked my Gen3 nor 2017 PRIME's 12v battery, so I have nothing to compares to the behavior of 2020 PRIME. But this is the first time I am reading about the pre-2019 PRIUS DC-DC converter charging the 12v battery ONLY to 12.0v during DRIVE. On the contrary, I have read in some other threads that reported the voltage reading is higher than 12.0v while in READY (and I think during DRIVE as well as PARK) using a cigarette lighter module type of monitor on pre-2019 PRIME. I would want to hear from others confirming this.

@Mark57 installed BMII on his 2017 PRIME. I hope he chimes in here to comment his finding on the 12v battery charge state during DRIVE.

 

I have checked mine and, as I said earlier in this thread, it hovers around 13 volts while driving once it finishes replacing the charge lost while sitting unused. But I checked it with an actual meter plugged into the 12V socket. The reading on the car's display is not reliable. I wish they didn't have that there because it has misled so many people.

If the converter only put out 12V while in Drive, I never would have been able to put in those 700+ mile days last year. In fact, no Prime would even be useful as practical transportation. They have dead 12V batteries after a few days parked because they didn't drive them long enough to get fully charged and every time you do that, the charge gets a little lower.

 

I thought I remember seeing a paragraph in the owners manual about what happens if you leave the car in ready mode for more than 20 hours. Something to do with how the 12V battery is charged changes after 20 hours. But I searched through the 2018 owners manual for a while, and I couldn't find any. Does anyone know what I'm talking about?

I don't think anyone here is running into the 20 hour limit yet, but it could give us some insight into how the car charges the 12V battery.

 

[QUOTE="Salamander_King, post: 3045333, member: 140530"/]

(c) Method 2

(1) Turn the power switch on (IG).

(2) Press the seek/track up panel switch 5 times and then press the seek/track down panel switch 5 times with the screen and audio turned off.​

[/QUOTE]

That worked for me. The key was the audio had to be off and the screen had to be off. I used the seek buttons.

Mine started at 14.0V while parked and stayed at 14.0V on a short drive. By the time I got home it started to drop. It dropped to 12.4 and then went back up to 12.6 and stated there.

My Bluetooth dongle reads 0.3-0.4V higher. So when the car was at 12.4, the dongle said 12.7. I suspect the higher number is more accurate.

When I put it in park the voltage went back up to 14.0 on the display and stayed there (it was hot, so I only waited a minute).

I think there are two modes of controlling the 12V system. The simplest mode is more or less constant voltage around 14V. That charges the 12V battery, but once the battery is full it wastes energy. That works like a car with a traditional alternator.

The other mode checks the battery voltage (I think that is what it was doing when it dropped to 12.4), and if it is high enough to be considered charged then it uses the negative side current sensor on the battery to control the current to (presumably) nearly 0, so the DC-DC converter is powering up he 12V system, and the battery is maybe only slightly charging.

In Park mode it defaults back to constant voltage control. I think after 20 hours of continuous driving it also goes back to constant voltage control, but I must be going crazy because I can't find any reference to that in the owners manual or online.

So if this display is not totally accurate, @PiPLosAngeles might actually have a battery voltage of 12.3V (or possibly even higher if other people have more of an offset error than I measured), not the indicated 12.0V. still not great, but maybe high enough that the car thinks it doesn't need to charge it anymore.