Does charging the drive battery charge the 12 volt battery?

Thanks for all the contributors in this thread. I was kinda waiting to see the final answer to the title question. It looks like with @jb in NE video, in fact, charging the drive battery charge the 12 volt battery.

 

stone cold 12 volt = 12.57 volts

plug in evse = 13.71 volts

maybe that's why my 7 year old battery is 12.57v, and your 1 year old is only 12.5v

 

I don't think 0.07V is a critical amount. The comparison is probably not valid between different models and designs. There are most likely lower electrical loads on the Prime than on the PIP (i.e. LED lights, improved HVAC, etc.). Just the fact that the voltage is 13.71 at some point is not an indicator of charge. According to the Weber video, the wall charge converter for the 12V puts out 14 V, but it must be regulated to prevent overcharging the 12V battery. If my battery is nearly fully charged, as it appears to be, then I would not expect 13.71 volts at the battery from the charger. As the battery nears full charge, the voltage applied should drop to taper the current.

I'm sure that all the smart engineers at Toyota know the expected electrical loads on the 12V and sized it appropriately and also designed a charging system that can meet all those loads.

 

This extends the longevity of the auxiliary battery. Older Priuses, like the gen3, put 14V on the 12V circuit all the time which makes the battery wear out prematurely.

There's another thread somewhere where people discussed this behavior more.

 

if there weren't so many dead prime batteries, i would agree with you. in fact, this thread (and others) might not even exist)

 

I got almost 9 years out of our 2010’s original 12 volt.

That seems good to me.

 

Do you think they are failing from lack of charge or overcharging?

 

i have no idea, but something is wrong. haven't seen this many bad 12 volts since the 2010's sat in the dealers lots for a year.

 

@bisco, thanks. You beat me to it on the PiP charging voltage. That's about what I remember measuring.

I wouldn't say the PiP was over charging the 12V, but it was probably charging more than needed. There's no point in pumping electricity into the battery once it's charged. That's just wasted heat at best and boiled off water at worst. So it makes sense to me to reduce the voltage to match the fully charged voltage once the charger detects that full charge has been reached. The Prius is all about reducing energy use, after all.

As to why so many Prime batteries are failing, aren't they the same battery as the other Gen 4s? Those don't get ANY charging from the HV charger since there isn't one. Are they also having a high failure rate? I do not know.

What we're perceiving as a high failure rate may be accurate or may be an illusion. I do not know. But there are lots of possible explanations other than the fact that the HV chargers aren't wasting energy overcharging the 12V. Correlation is not necessarily causation.

Question for @jb in NE on that excellent video. You said you had it in ready mode and the 12V battery level was dropping. I was under the impression that the inverter's DC/DC converter was maintaining the 12V battery any time the car was in ready. Also, you said it was in ready while the wall charger was charging the car. My 2017 doesn't permit that. Which bring up the question: was in actually in ready or was it in acc?

 

agreed,, not sure on causation, or on the number of bad 12 volts, or if gen 4 is also having troubles.

but there is no question that a percent of prime 12 volts have gone bad, it's easy to find the threads here.

 

I misspoke in the video - the car was in Ignition On mode, not ready and not in accessory. Plugged into the wall, not actively charging.

 

Thanks for clarifying. I was pretty sure that was the case. Makes perfect sense now. (BTW, I'm the announcer for a recorded radio program. Every once in a while I have to go back to the studio to re-record something, so don't feel bad.)

I love how clear the video made it.

 

Or excessive current draw while shut off?

A dead battery in not necessarily a battery failure. A perfect battery will lose its charge if overdrawn or undercharged.

I think our Prime batteries are Group 140R. One I've found is made (or marketed) by DEKA, their 5140RMF, sold by NAPA as the 75140R, $139, or sold as the O'Reilly 140RPRM, $135, also available with other labels. I have not visually checked these to be sure the terminals are in the correct locations for the car's cables--I think right-hand-positive is when the terminals are close to the viewer and the positive is on the viewer's right, so these are OK for our cars' cables.

 

This may be the issue. On one thread on this forum, somebody measured the phantom loads in the car. Having a Carista OBD dongle plugged in was quite a load. There is the key fob sensor, any anti-theft features, any idle computer or ECM loads, etc.

From my experimentation, I don't have any doubt that the charging system is properly designed and will keep the battery charged. The issue seems to be that when not charging (i.e. not in Ready mode or actively charging the traction battery from the wall), the battery is depleting from internal loads.

 

all prius 12v deplete from loads when off. someone would have to get the spec and take a measurement.

i left mine on for 10 weeks while away and it dropped from 12.7 to 12.4

 

you mean the 12v. all the best with your experiment, sounds like a good learning test. will this be a yearly snowbird thing?

 

i always told my father who was a 6/6 snowbird that a hybrid wasn't a good choice. i hope i'm wrong!