Charging 12V step change - 14.0 to 13.5 volts.

I am not able to enter the "hidden" MFD diagnostic screens with my 2007 (although I can do this with my 2004.) However, after a 30 mile drive in the 2007 this evening, I left the car READY and hooked up my Fluke DMM to the positive dedicated jumpstart terminal.

I measured 14.0V when in P. Then I shifted into D and the voltage dropped down to 13.4. When electrical accessories are turned on (like lights or the cabin ventilation fan) then the voltage comes back up to 14.0.

If the reason to drop voltage down to 13.4V is due to concern about overcharging the 12V battery, then I do not see why the voltage comes back up when additional electrical load is applied. The DC/DC converter has the capacity to provide all 12V power to the car when READY and the 12V battery is just along for the ride.

 

Hi Patrick,

I can't check these days because I don't see the voltage dropping in the cold weather. I'm pretty sure, when this 'feature' is working, that as long as the car is in drive and the cabin fan is off, the voltage stays low and if the rear window defroster is turned on, the voltage drops further to something like 13.2 volts.

Thanks for checking.

 

The only exception to that is the voltage remains high for a short period right after startup in drive even if all accessories are off. In my test (see graph Post#2) that was about 3 minutes.

 

Even when fully charged, the 12V battery continues to consume energy while 14 Volts are supplied to it. Reducing the voltage to 13.5 V slightly reduces the energy consumption of the battery, thus also reducing fuel consumption of the car by a very tiny amount.

Switching off all unnecessary electrical loads is common practice by hypermilers. Since this is a prerequisite for the reduced charge voltage, obviously this is a feature that Toyota developed especially for hypermilers.

Oh, and, uh, by the way, just purely coincidentally, in many of the official fuel consumption test cycles, it is allowed to switch off all unnecessary electrical loads.
.

 

Keep in mind, at full-tilt, the Prius DC/DC converter only puts out about 100A. This is less than 2 horsepower which is trivial. It's almost impossible to max out this converter even with everything electrical you can possibly turn on. There is more total mechanical loss in the car than this! Now you might just possibly be able to notice a 1.4kw difference in your mileage, but anything less than that, I would say not.

Now for the 12v battery: I have tested this, when floated at 14v and 70 degrees F, it consumes less than 100ma. This is less than .2 percent of a horsepower! Assuming a best case of 250 watt-hours per mile, (ev mode) this would only save you about 10 feet every mile, which means you'd have to go 520 miles to gain one extra mile! That would only be valid in electric mode, once you add gas to the equation, it goes to something like 1 mile for every 2000.

The reduction of voltage has nothing to do with efficiency, it has to do with preserving the life of the 12v battery.

 

If that is the motivation, then why does the voltage come back up to 14.0 if you turn on accessories like the exterior lights or the cabin fan?

 

I suspect they are watching the float voltage, and when it dips, it resets the cycle. Some testing would have to be done to determine the exact conditions. I know for sure there's no reason they would put such a thing in for efficiency, as the energy saved is so miniscule.

Floating a lead-calcium battery at 14v is a bit too high if you want it to not lose water, and as it is it seems like many of them still lose water after a few years in hot climates. Keep in mind just the various LED's alone that come on when the car is ready use more energy than the 14v float! (I won't even mention what the VFD pulls down in the combination meter!)

A lot of modern cars, such as BMW's, implement a proper temp-compensated 3-stage charge algorithm in the ECM (that controls the alternator). Why Toyota didn't do this is a mystery.

 

Quick update: the other day I just observed the drop in charging voltage for the first time. Turning the fan on (at the lowest fan speed) or turning the headlights on would raise the voltage back up. I suspect not many drivers out there would keep the fan off most of the time. So this battery conserving mechanism won't help most drivers.

 

I have been watching the 12V charging voltage and haven't seen the drop except one time. I wonder if it is temperature dependent. The only time I saw it was when it was unseasonably warm, around 50F.

 

I never see the voltage drop during the winter months.

It's a pretty regular thing when the temps warm up -- say above 55 or 60.

 

I have only tested a 2019 rav4 hybrid (logging with OBDLink, while testing an LFP auxiliary battery, at around 24ºC), but some things are likely to be relevant to prii. Probably most important is the difference in charging profiles between driving and parking, shown below (other interpretations are discussed and updated elsewhere).



The rav4 does differ from the prius in some aspects: use of the blower or other accessories has no effect on charging.

 

I was beginning to post a question about this until I saw this thread.

After replacing my 12V battery, at which time the weather also turned warm, my charge voltage started to drop from 14.2 VDC to 13.6 VDC within a few minutes of driving. It always goes back to 14.2 V when in placed park and ready.

The battery was at 12.2 volts last week and I charged the battery back to +90%. There was no difference in the Prius DC/DC converter charging behavior afterwards.

With the last battery, the charge voltage was consistently 14.2 V. I don't remember ever seeing it any lower. That battery was replaced due to age, not failure.

I have had no issues since replacing the battery.

I must admit I have become rather fixated on the voltages, temperatures, and other performance measurements of Prii innards, one of those being the 12 V buss. (Spent too many years chasing problems due to power supply issues!)

Tom

 

That behavior you observed is perfectly normal. It indicates you have a new battery in good condition.

A great idea to keep your battery lasting a long time.

Conversely, this is also normal behavior and indicates that it was an old or worn battery that was in less than stellar condition.

 

Thanks for the reply!

I would think the battery ECU would call for higher charging voltage/current if battery is sitting at 12.2 volts. It doesn't seem to matter...battery can measure 12.8 volts after charging and I still see the DC/DC converter drop from 14.2 to 13.6 volts. It drops to 13.6 volts after just a couple of minutes of driving. 14.2 volts in park. These voltages have been verified with a DVM.

I have to charge the battery with a charger every couple of weeks. I'm retired now but still drive the Prius at least every other day for 30-45 minutes but this doesn't seem to be enough driving to keep the aux battery healthy. The average commute in American cities in 2019 was 27.6 minutes.

Why did Toyota not fix this issue in all the later generations? My car is a 2005...they knew this was an issue since the beginning. I have owned dozens of cars and have NEVER had to trickle charge the car's battery after a few days of not driving!

 

You are assuming WAY too much here.
The operation of the charging system you have observed is perfectly FINE.
As a matter of fact, back around 2000 most all car makers dropped the "float" voltage down to where it is now to keep from "overcharging" the batteries.
Toyota did not and does not "know this is an issue" because it is NOT.

NOW.....it is possible that your specific car is not charging right but that doesn't mean that they ALL are not working right.

How OLD is your battery now ?
The drop while sitting that you have observed can indicate that it is nearing the end of it's useful life.
It can also indicate that you have some "accessory" running all the time and draining it down.

 

Thanks for the reply!

The battery is 2 months old. Load tested by shop up the street - nothing wrong with the battery. I do have SKS but have that turned off. I measure around 20 ma current draw on battery after all the pumps and CPUs settle down.

There are numerous threads about the deficient Prius charging system for the aux battery...IF...you do not drive far or often. I bought the trickle charger that so many folks on this board use. I don't think we're talking about a problem that only affects my car.

If a battery is discharged to 50% (I think that is what 12.2 equates to on an AGM battery) wouldn't you think the *smart" Prius charging system would supply more charge? Maybe I have a converter/inverter issue?

It seems that an automobile would include recharge capabilities that suit common use. Average commute times in European Union are 25 minutes and in Japan, 41 minutes. Neither are long enough to keep the battery healthy, according to many here on this board. What about those who live in small towns and drive 2 miles to work every day? Was this the first car made that requires a certain lifestyle and activity schedule to function properly?

 

Yes. And they DO.
If your "common use" doesn't really conform to what the WHOLE INDUSTRY is designing for, then you have the right solution.
Do you leave your charger connected long enough to get a full charge ?

I have: A Prius C. A Ford Fusion Hybrid. A CanAm Spyder. A riding lawn mower. A boat.

The C is the main hauler. It gets driven about 5 days a week for maybe an hour a day.
It NEVER needs any supplemental charging.

ALL of my other vehicles commonly sit for 2 weeks or more with NO use at various times; sometimes much longer.
I attach my tender to each of them after about 2 weeks because I know that when they ARE used the onboard system might not put a full change back. That keeps the batteries near enough to a full charge to extend their life.

Millions and MILLIONS of owners have no problem with the present design.
Because YOU and a few others do does not qualify it as a "problem".

 

Know what? Qualifies as a problem for me...

The previous owners never charged it either because they never had a problem. Neither did I until I lost the traction battery and started reading about the various Prii problems on this site. Listening to people who know nothing but anecdotal stories is a mistake.

 

In my reply back in post #54, I was trying to say I don't think you have much of a problem if any.

I do think you are misinterpreting the data and coming to the incorrect conclusion.

The trouble with a lot of the anecdotal stories is that they suffer the same fate and the next thing you have is an urban legend.