Parasitic battery drain?

dang, i thought i was in the backyard with some friends

 

The Gen 5 new-car features manual clearly details that the 12-V battery health takes a backseat to fuel efficiency, and it's typically not fully charged for that reason.

I have two weeks of data to support this. The 12-V battery is rarely charged, regardless of how long the trip is. By not charged, I mean the charging voltage stays at a float voltage of 12.86–12.92 V. In the last three days, it only charged when the gear was in park, which aligns with @Mr.Vanvandenburg's observations. So, if you are driving the car as soon as you turn it on and turning it off as soon as you are finished driving it, chances are that it will hardly charge.

If you have a Prime PHEV, yes, when plugged in, it drains continuously faster.

I don't think it helps if you are in Canada or Alaska and if the battery heater is operating with the car plugged in. From what I've seen, when the traction battery is charging, the 12-V battery only gets a float voltage of ~ 12.90–13.00 V, not enough to charge it. After the charging (and/or battery heating) is finished, it doesn't get a float voltage.

In my case, I have an AGM battery, which helps because it charges much faster.

My battery was still OK, save some battery degradation, after almost four years, despite a recent full drain when I didn't drive the car for four weeks, but that didn't mean it saw optimal charging. Chances are that it wasn't fully charged when I left the car, which is the typical issue with this 12-V charging system. You can argue if you want that it is only statistics, but it's more than that, as some of us actually have data of the charging system, and we are clearly seeing what is happening.

And the Gen 4/Gen 5 12-V charging systems are identical on Prius HEV and Prius Prime PHEV. Both generations have had decent shares of 12-V-battery problems. Again, this is not to say that you will experience an issue soon, but the charging system does not try to optimize the battery health, and it is unlikely that you will get longer than about four years of life from the 12-V battery even in the best-case scenarios.

And I am sure this detailed data will soon follow with statistics, user-error, defective-battery, etc. comments from some posters.

 

These cars have a sensor with a shunt to measure the current. I wonder if there is a way to access this reading? There is the negative battery post, this connects to the sensor through a lug, then another post is for the battery cable to sensor. Two small wires plug into the sensor.
I wonder what would happen if the neg cable was put directly to the battery post?

 

Of course, the shunt resistor is in plain sight. You can see its top lead in the photo below, which is of shiny copper, and its bottom lead is sitting on the negative battery-terminal connector. Simply clamp a voltmeter between the two nuts (the large one and small one).

However, the problem is that you don't know the value of the shunt resistor. The best way to find its resistance would be to get two multimeters to measure both the current through and voltage across the shunt resistor at the same time. Or, if you have a four-point ohmmeter, that would work, too. You can’t measure it with a regular, two-point ohmmeter because its value is too small, and the contact resistance will be dominant.

 

I am not impressed with this engineering.
Keeping a 12V battery topped up does not take kW of power. I can't imagine a measurable 'Fuel Efficiency' hit by just adding a few Wh of charge to this poor 12V battery.
And your battery took a hit to its life expectancy when it drained to ZERO. No way around that.

Good Grief... A shunt resistor in this day and age?? Why not a Current Transformer like other modern cars?

It's not Rocket Surgery to keep a 12V battery alive and well. Allowing it to just sit there and drain to ZERO is unexceptable.
Again,,,, I am not impressed with this engineering.

For reference: My '17 Bolt @ 117k miles has its original AGM battery in it. I can see the date of manufacture sticker.
(But, I should probably be ready for it to die... I carry a jump pack for that day.)
GM EV's have a 'Desulfation routine' the 12V gets at times. Maybe that helps a lot.
I don't baby it. I leave it plugged in all weekend at times. Many times we can go 5 days without plugging in at all. I definitely don't trot out to the car with a second cord to plug in a 'battery minder'.

 

Yeah I know the sensor is in plain sight. I asked if there is a way to access the current reading. It was a joke to say what would happen if bypass the sensor, like who is going to experiment with it to find out?

 

There should be a way to access it digitally through a scanner, I guess.

You can shunt an ammeter on the positive terminal and a millivoltmeter across the large and small nuts on the negative terminal to calibrate that voltage reading to the current reading. That would avoid the contact-resistance issue on the negative terminal when you obtain the millivoltmeter reading.

I see they also sell Hall-effect battery ammeters on AliEpress. They are probably not very precise though.

 

Ha ha, I am not impressed with the engineering of the 12-V charging system either. In the last three days, the 12-V battery virtually never charged.

The fuel-efficiency issue is not about the charging energy at the battery. Toyota wants to minimize the DC–DC-converter voltage and keep it at 12.86–12.92 V to reduce the total power drain on the 12-V system, which is perhaps around 100 W or more, as that is proportional to the DC–DC-converter voltage. This is stated in the Gen 5 new-car features manual. (While the Gen 4 new-car features manual does not go into as much detail, the Gen 4 and Gen 5 12-V charging systems are identical.) So, they save about 5–10 W or perhaps up to about 20 W on the 12-V-system power drain by not charging the 12-W battery. LOL

The shunt resistor is tiny, like perhaps about 10 mΩ. It is standard on all battery sensors. Hall-effect current sensors are not precise, and that’s why they are not used.

Also, it would be no brainer to have the DC–DC converter periodically kick in when the car is plugged in to maintain the 12-V battery. Instead, Toyota chose to continuously drain the 12-V battery after the 12-V-battery charging/heating was finished.

 

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the hall effect things are all over amazon too. It gets to be, well will it change anything I have to do by knowing more? No. A lot of cars have battery sensors on the 12 volt, gas cars, even the Bolt. I don’t know why really except efficiency. My truck has simple alternator and battery no sensor, works great. Never have to do anything unless just being obsessed by keeping it 100% charged.

 

I agree. 20W is nothing.
When I put my high end battery minder on a 12V it initially charges at 2A / 24W. Then once it has reached a higher voltage, ~15V it drops back to a 'floating voltage of ~13V and holds that, but does periodic tests where it stops and sees what the battery does.
In a way your floating voltage is still charging the battery some amount. It's higher than an open 12V battery.
Back in the day,,, 13.8 to 14.2 VDC was what the car's '12V system' was at when the alternator was online.
Are you saying the voltage at your power outlets is only that floating voltage you talk about?

Also, I'm not sure about the term 'Hall-effect' sensors for current measurements. I know about Current Transformers. They go around the outside of a conductor and measure current passively. They are a ring of windings with a 2 pin connector.
They are accurate and used extensively in modern cars and aircraft. Much older aircraft use the shunt method of current measurement,, and the current gen Prius.