12v Battery Is Discharging, Re-Charge Now !

I don't know the details of the electrical system, but it seems like if you have a battery that's charged to ~13V in the same circuit as the DC-to-DC converter running at 12V, what's going to happen is the battery is going to supply all of the current for the 12V systems until its voltage drops to match the output of the DC-to-DC converter. If that's the case, what's essentially happening is that the car is cutting the 12V system off from the traction battery and running everything off of the battery while in drive until the battery has discharged about 50%.

 

dc converter runs around 14v i think. it is also variable based on the 12v reading iirc.

 

When in drive it's always ~12V.

 

I don't know about the DC-DC converter voltage for sure, but the 12v system at the battery post is definitely staying above the resting voltage of the battery much higher than 12v. As you can see on the graph below, during my ~4.5hr non-stop traveling the terminal post voltage reading was more like 12.8v - 12.9v with occasionally higher spikes above 14v. The 12.8v - 12.9v steady-state voltage during the trip while the car is in DRIVE is quite a bit higher than the resting state before the trip or after the trip.

 

Interesting. That hasn't been my experience in the 2018 PP at all. The 12V system was *always* ~12V in drive and ~14V in park, whether measured at the battery posts or the 12V outlets (with some small loss for wiring path). The diagnostic screen showed the same.

I need to check with the 2021.

 

Compared to during the driving, the car parked in READY mode keeps the 12v battery being charged at above 14v continuously.

 

if it is always 12v in drive, doesn't the battery die quickly?

 

Just FYI, the data above were from my 2020 PP. But it should be the same system as your 2018 PP.

 

On my 2021 I get 12.8V in drive. Much better than the 2018. That's probably why the 2018 battery used to hang out around 11.7V.

On a side note, when I went to test the battery I discovered some rodents had already crawled into the engine compartment and chewed out all the insulation on the underside of the motor and used it to build a nest on top of the intake. The car is parked outdoors, so that's a new one for me.

 

Had that last year when we got home from Florida.
Problem solved, haven’t seen a trace since.

 

It's identical to this post:

Part number for insulation for lower shield | PriusChat

I've never seen such a thing on all my years. I wouldn't think that would happen in a car that drives almost every day.

 

I never checked the 12v battery on my 2017 PP for ~3 years of ownership. The car never sat longer than a few days at most. Never had a problem with the 12v battery for 3 years 40K miles. It's strange if your 2018 PP had different 12v system than 2017 or 2020 PP models.

Similarly, I never had rodent problems with my cars always parked outside. It's not that we don't have mice. In fact, we have plenty of rodent infestation inside of our garage where our cars are never parked but the feeds for our poultry are stored. I credit the lack of damage to cars for the fact I am providing a better place for them to nest and live with plenty of food and no feral cats roaming inside of the garage.

 

And just to throw some more confusion into the mix, here's my 2020 PP on a 2.5 hr. drive. Notice that the battery was quite low to begin with, so that may have something to do with the results. Basically, the car outputs ~14V3 whenever the car is in Ready mode regardless of whether it's moving or not. The only time things change is when I'm charging the traction battery. Then I typically see ~13V5.

 

Interesting. Yeah, I don't know how my car would have behaved if the 12v battery was lower to start. I never let mine go below 12.4v while I had the car mostly parked on my driveway. Well, I no longer own the car, so I can't test the hypothesis anymore. In any case, my data is also only n=1 case for both Ready mode parked and Ready mode driving. Certainly, more study needs to be performed to make any final conclusion.

 

Interesting. My 2021 battery was 12.2V when I tested this afternoon. Put the car in ready and it measured 13.99V. Drop it into drive and let my foot off the brake enough to roll a foot or two and the voltage dropped from 13.99V to 12.8V. It's important to let the car roll in drive to see the voltage drop.

I was wondering if the DC converter was programmed to adjust the voltage to be just a bit higher than the 12V battery's voltage, but EdPalmer42's data suggests otherwise. It seemed to fit as both of my cars ended up outputting about 0.5V above the 12V battery's initial voltage when in drive.

 

I just put my Gen4 on a 4v battery tender. Read 12.0v and will condition and charge it to 12.8v which means the battery is still very good. The Prius needs to be used regularly or the charge will go down, doesn't mean the battery is bad or going bad but a battery tender on a lightly or rarely driven Prius is a must have.

 

unfortunately, prime behavior is different, even when driven every day.

and i would recommend a 12v tender.

 

I thought that I read someplace that when the traction battery is being charged; the 12vdc system also receives a charge to ensure available power is on-hand to run the battery pack cooling fans, if needed. I plugin daily; unless I'm locked-down and run 5 miles on gas for my daily commute.
Any of my cars that sits for 3 weeks, either get driven or a few hours on the battery charger while I do yard work.This normally applies to my camping/fishing 4x4. This usually means it's time to go fishing again.

 

I have found the various voltage reporting on this thread interesting.
Update on voltage readings since my lithium battery installation in a 2017 Prime-
I measured between 11.6 and 12.2 with the OEM 3 year old battery before starting in the morning and then 13.5 when running.
When I received the Ohmmu lithium battery it measured 13.2 volts out of the box.
Since installing and driving the car the past weeks I measure 13.2 in the morning before starting. This draws down to 12.96 with the headlights on high beam with car not energized. 14.2 when car is energized. This does not seem to change whether the car is in park or in drive. Same goes for loading up the 12 volt system - headlights on high beam and fan at full speed and I measure the same 14.25
When the car is plugged in and charging I measure 13.22

 

I am having this happen to my plug-in Prius and I didn't know what was happening. It doesn't happen to my Avalon hybrid, so I guess I never expected it would happen to the Prius. I will have to drive it more often and for longer periods of time!

Mike