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.

 

"Hall effect" devices work on magnetism and the pinching of electrons creating a very small voltage across the width of a conductor. Long story short, it's a means of inferring current amperage through a wire by measuring this small voltage. It's sometimes used because it avoids the need for a current sense resistor which can be inefficient and get hot. Hall effect devices "work", but it can be inaccurate due to the voltage signal being so small compared to the electrical/magnetic noise around it.

Good thing there are hardly any electric or magnetic fields in a Prius...

 

In my case, it’s an AGM battery; so, a 12.89 V float voltage is equal to the open-circuit voltage. People have reported their 12-V battery SOC dropping. Toyota also wants to minimize the 12-V battery cycles to extend the battery life, and that's another reason why they don’t want to charge it all the time.

Current transformers only work for AC currents, as only AC currents can induce a voltage through Faraday’s law of induction. For DC currents, you need to measure the magnetic field. The magnetic field inside a ferrite ring around the wire is measured using a Hall-effect sensor in the ferrite, from which the current through the wire can be approximated through Ampère’s circuital law. Even smartphones have three-axis Hall-effect sensors to measure Earth’s vector magnetic field.

 

I hooked up my scan tool this afternoon. You can read what the battery current is. Of course this is with car in READY mode and battery being charged. I had HVAC and lights in OFF position. Current was a little above 3 amps and voltage 13.6 volts. You can’t read parasitic drain because scan tool needs car to be in READY mode. That said, I did install an ammeter several days ago in series with negative lead. After car went to sleep ( waited 30 minutes), parasitic drain current souls cycle between 20 and 50 milliamperes. Average it out to @35 milliamperes.

Cheers

 

Ha, what is the 12-V-battery current when the DC–DC converter pulls back to 12.86–12.92 V? And how about at 14.10 V? It has about four voltage settings: 12.89 V, 13.05 V, 13.53 V, and 14.10 V.

 

And how/why does the Prius 12v Battery management system choose between the 4 different voltages you itemized here ?

 

It is loosely described in the Gen 5 new-car features manual. 12.89 V is the default. 14.10 V is usually only briefly when the car first starts or at occasions, 13.53 V when the gear is in park (verified by observation), and 13.05 V float instead of 12.89 V float on some days depending on the charging history and SOC (observed).

 

How do I get a 'Gen5 new-car features manual' ?

 

You can subscribe to TIS for two days for about $20. That section I mentioned was posted in one of the battery forums here though.

 

OK, here's some data to digest. I hooked up my Topdon Phoenix Lite 2 scan tool this morning. The data streams I recorded were AUXILIARY BATTERY VOLTAGE (ABV) and AUXILIARY BATTERY CURRENT (ABC). The car had not been driven since yesterday morning. Starting battery voltage before READY per my BM read 12.41 volts.

READY mode enabled, scan tool (ST) ABV 14.219 volts and ABC 7.160 amps. ABV stayed constant at @14.2 volts but ABC started to slowly decrease with time to @3 amps at which point the ABV decreased from 14.2 volts to 12.8 volts As the ABV decreased, the ABC decreased and went negative -4.345 amps when the ABV read 12.795 volts. The ABV held steady at @12.8 volts and the negative ABC started to decrease (lower negative current). The negative ABC held at negative 0.5 amps for a time period. Then the negative ABC dropped off and would pulsate from plus/minus current of about 0.1 amps. With time, the ABC stabilized positive @ 0.2 amps with ABV holding steady at 12.8 volts.

I was recording while driving the car. Came to a stop and noted ABV increased to 13.599 volts and ABC now 3.803 amps. ABC would slowly decrease but ABV stayed stable at 13.6 volts. ABV deceased to 12.8 volts and ABC now negative 2.041 amps. With time and ABV stable 12.8 volts, the ABC negative current slowly became less and went slightly positive before I shut the car down.

NOTE: I am unable to correlate times with the data stream frame shots unfortunately. I don't use this ST often enough to be that knowledgeable on how or if it can be done.

OBSERVATIONS:

The ABV as read on the scan tool data stream was in exact correlation with the voltage meter I have plugged into the 12 volt outlet on the center console.

The scan tool shows a stabilized 12.8 volts but a pulsating ABC. See snapshot below.




One other observation. On the upper right corner on the scan tool is a voltage display. This voltage would constantly fluctuate typically plus/minus 0.3 volts or possibly more. Hard to capture. I did note seeing @14.5 volts as the highest on occasion. This voltage reading is from pin 16 on the OBD2 connector which I confirmed with Topdon.

My takeaway is that the charging system (BMS) for the aux battery seeks out and stabilizes at 12.8 volts. The pulsating ABC is most likely from the pulsating voltage that the ST sees on pin 16. I don't have a WD to see how pin 16 taps into and where. I haven't been able to find a data stream that monitors pin 16 voltage. Pin 16 is typically described as battery voltage. So why the difference between ABV and pin 16 voltage ???

From what i saw and recorded today, short trips will definitely drain the battery over time.

I'm charging the traction battery and plan to leave cable plugged in to see how my aux battery reacts. After that, plan to install the AGM battery and do a similar data stream recording.

Cheers

 

Today I brought my 2023 Prime in for an unrelated recall, and I printed out this TSB and brought it along. The mechanic at the dealer had not been aware of it, but then he realized that it was appropriate for my car, so they upgraded the car's firmware as specified in this TSB. Your dealer may need similar prompting.