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On 18 Feb 2021 we took delivery of a 2021 Toyota RAV4 Prime Plug-in-hybrid from Westcoast Toyota in Pitt Meadows BC . The vehicle has performed flawlessly, with one major exception.

One of the reasons for selecting the RAV4 was Toyota’s reputation for reliability and relevant experience with hybrids. Reliability is very important to us as the vehicle travels on forest access roads well beyond cell phone service which means a long walk to help. Even within cell coverage or via Toyota Roadside Assist, help for a dead vehicle is hours away, probably not until the next day.

On two occasions the 12 volt battery of the RAV 4 PHEV has died overnight. After prompting, the dealer checked that “the system has the latest software in accordance with Service Bulletin. “T-TCI-5389 DCM Reset and Firmware Update (LG)”. The dealer staff, on both occasions of failure, accused us of “not driving enough” yet did not even ask if the vehicle had been driven the day prior or if it was plugged in overnight (yes to both). Attempts at further discussion made it clear that the dealer staff have no interest whatever in finding a solution.

A request to Toyota Service produced another legalistic evasion and a request that we deal through the dealer. So, we have asked the dealer:

1. To remain operational, must the RAV4 be driven a minimum distance per trip/day?
2. Must it be driven in HV mode?
3. How many days can the RAV 4 remain inactive then reliably start?
4. If/when the 12 volt battery discharges again, what would you like us to do:

No answer yet to these questions. So, the present situation is that we have a Toyota RAV 4 PHEV which has its reliability compromised by an unknown defect which is aggravated by a Toyota dealer with no interest in finding a fix.

In the hope of helping find a fix, I am maintaining a log of voltage of the 12 volt battery at the start and end of each driving day. When the RAV4 PHEV is in the garage a 1.5 amp battery maintainer is connected to the 12 volt battery and the key fob is disabled.

Has any RAV 4 owner found a fix for overnight discharge 12 volt battery.


Ron
Do you have the MyT app? That's your proof from the trip recordings that you are in fact driving it enough.

My understanding is that the 12V battery charges off the traction battery so all you need to do is actually switch the car on and it is charging from that point onwards. The UK forums suggest that you just need to have the car 'on' or drive it in any mode fir about 20 minutes a week to maintain a good charge on the 12V battery.

From own experience I've left the car several times for 2 weeks or more and not had an issue with starting. Car is outside in the wet, ice, snow.

Sounds like your dealer is just a c***. Our dealer bends over backwards for us over here in the UK. You could have a fault somewhere like a short or poor earth bonding. Basically they need to put an ampmeter in series from the 12V battery to determine current draw when the car is off and supply when it is on, then compare these amp values with what it should be. They should also check via resistance measurement all of the earth bonds. This is the absolute minimum they should be doing, or they are the baby of two village idiots from two seperate villages that met one night.

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Thanks for the suggestion. Yes, I have a GPS and a dashcam plugged into the "cigarette lighter". Both devices have an accelerometer which shuts them off if no motion - and I check when the vehicle is out in the garage.

To discharge a 100 amp-hr battery in, say 12 hours would require an average current of 100 amp-hr/12 hr = 8 amps, far beyond the draw from these devices.
Ah. This could be your issue. There will be residual current draw or more depending how the device works. Think I read somewhere in the manual but I may be mistaken and thinking of my last PHEV that you should not leave devices plugged into the cigarette lighter for risk of draining the 12V battery.

You don't have to flatten the battery. You only need to take enough energy to lower the voltage below a particular threshold that it causes issues with some electronics.

I'd try unplugging at switch off to see if that fixes it for a week. If not then I'd try unplugging the devices permanently for a week because they may be drawing more current in operation than is being supplied to charge the 12V battery?

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Hi, keep in mind that on a 12V system 1A = 12 watts. It ain't much. Even if you have a small-ish parasitic load that is low wattage you're cycling the battery over and over when it's not in ready state. It's impossible to tell without seeing your setup and the load, but if these devices aren't very very asleep when not in use the cycles could very well toast your battery, where it will charge to full voltage but it's capacity is compromised. A load test may or may not reveal this.

I'm placing my bet on the accessories and a compromised battery. In your use case it would be worth having a boost jump kit or spare battery as reserve. Hope this helps-

Oh and if you have a Scanguage, that'll suck it down too (pretty quick) if you leave it plugged in and don't have the secret firmware update.
Spot on.

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Discussion Starter · #25 ·
Hi, keep in mind that on a 12V system 1A = 12 watts. It ain't much. Even if you have a small-ish parasitic load that is low wattage you're cycling the battery over and over when it's not in ready state. It's impossible to tell without seeing your setup and the load, but if these devices aren't very very asleep when not in use the cycles could very well toast your battery, where it will charge to full voltage but it's capacity is compromised. A load test may or may not reveal this.

I'm placing my bet on the accessories and a compromised battery. In your use case it would be worth having a boost jump kit or spare battery as reserve. Hope this helps-

Oh and if you have a Scanguage, that'll suck it down too (pretty quick) if you leave it plugged in and don't have the secret firmware update.
Appreciate the comments. Will check the current draw when the devices are on, but will also pull the plug overnight. Scangauge is connected only to check for codes etc. Would appreciate a link to the "secret firmware update". We carry a Li-ion jump kit, as backup, but I resent having to do so with a new Toyota vehicle. Cheers
 

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Also, you mentioned that the car was plugged in overnight. If the time from full charge, to unplugging can allso drain your 12v battery. Suggest looking in the book for the reasoning on this.
 

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Discussion Starter · #30 ·
Not sure that I understand your suggestion. Can you identify location in the book (Owners Manual).
The RAV4 was plugged in on both occasions when the 12 volt battery mysteriously discharged overnight. The 12 volt battery is charged by a DC to DC converter from the traction battery - but ONLY when the "power" is on. It seems odd to me that the 12 volt battery is not "maintained" when the vehicle is plugged in.
I wonder if there is as simple hack to turn that function ON.
 

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Not sure that I understand your suggestion. Can you identify location in the book (Owners Manual).
The RAV4 was plugged in on both occasions when the 12 volt battery mysteriously discharged overnight. The 12 volt battery is charged by a DC to DC converter from the traction battery - but ONLY when the "power" is on. It seems odd to me that the 12 volt battery is not "maintained" when the vehicle is plugged in.
I wonder if there is as simple hack to turn that function ON.
I think you’ve stumbled into one of the quirks of the Prime. The 12v bus is not maintained when the traction battery is charging. As unintuitive as that may sound. I think this is probably due to the fact that the Prime is a “plussed” cousin of the hybrid and that feature just didn’t make the cut.

There is some recent discussion that certain modes may maintain the 12v system (such as when traction battery is being heated or cooled), but general consensus is that no charging of 12v takes place when it is plugged in (as crazy as it sounds).

And to add even further to the enigma of the whole situation it’s been suggested that charging actually drains the 12v battery. Thus the advice to not leave it plugged in for long periods of time. Normal overnight charging shouldnt hit it too hard though.

My bet is still a weak battery. See if you can get yours load tested through a full cycle and see what the ampacity comes out to.
 

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Discussion Starter · #32 ·
I think you’ve stumbled into one of the quirks of the Prime. The 12v bus is not maintained when the traction battery is charging. As unintuitive as that may sound. I think this is probably due to the fact that the Prime is a “plussed” cousin of the hybrid and that feature just didn’t make the cut.

There is some recent discussion that certain modes may maintain the 12v system (such as when traction battery is being heated or cooled), but general consensus is that no charging of 12v takes place when it is plugged in (as crazy as it sounds).

And to add even further to the enigma of the whole situation it’s been suggested that charging actually drains the 12v battery. Thus the advice to not leave it plugged in for long periods of time. Normal overnight charging shouldnt hit it too hard though.

My bet is still a weak battery. See if you can get yours load tested through a full cycle and see what the ampacity comes out to.
Appreciate the comments
Battery may have been a part of the problem as it was replaced on the second occurrence of overnight discharge of the 12 volt battery.

However, my suspicion is that the on board computer system connected to Toyota home office then got stuck part way through some install or reset.
 

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There is some recent discussion that certain modes may maintain the 12v system (such as when traction battery is being heated or cooled), but general consensus is that no charging of 12v takes place when it is plugged in (as crazy as it sounds).
When the prime is placed in My Room mode, the 12v Accessory battery is charged. If you turn off all accessories, such as heated seats, steering and A/C system, most of the power available from the charger will charge the traction battery, but some of that is also going to the 12v ACC battery. If too mnay accessories are ON, then the demand for power is greater than what the charger can supply and the traction battery will slowly discharge.

So, if one wishes to top off the 12v acc battery without any fuss, just put your prime in My Room mode with all the accessories off. Do this for a few hours and you should be good.

On a side note, I think many of the 12v ACC battery issues stem from the charging voltage being too low. A smart charger will charge a 12v SLA at 14.5v (Some will do over 15v for a deep cycle). But the Prime only charges at around a measly 13.5v. Not enough in my opinion. Resulting in the 12v battery almost always sitting in and under charged stated. I predict that many prime owners will have premature failure of the 12v ACC battery. Lasting only 1 to 3 years instead of 5 or 6.

dp
 

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When the prime is placed in My Room mode, the 12v Accessory battery is charged. If you turn off all accessories, such as heated seats, steering and A/C system, most of the power available from the charger will charge the traction battery, but some of that is also going to the 12v ACC battery. If too mnay accessories are ON, then the demand for power is greater than what the charger can supply and the traction battery will slowly discharge.

So, if one wishes to top off the 12v acc battery without any fuss, just put your prime in My Room mode with all the accessories off. Do this for a few hours and you should be good.

On a side note, I think many of the 12v ACC battery issues stem from the charging voltage being too low. A smart charger will charge a 12v SLA at 14.5v (Some will do over 15v for a deep cycle). But the Prime only charges at around a measly 13.5v. Not enough in my opinion. Resulting in the 12v battery almost always sitting in and under charged stated. I predict that many prime owners will have premature failure of the 12v ACC battery. Lasting only 1 to 3 years instead of 5 or 6.

dp
Interesting. I could have swore I put a meter on mine and read 14.5 or so. But you’ve given me something to check next time im in there. I agree 13.5 isn’t really where it needs to be if that’s all we get. Love the my room tip.
 

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Interesting. I could have swore I put a meter on mine and read 14.5 or so. But you’ve given me something to check next time im in there. I agree 13.5 isn’t really where it needs to be if that’s all we get. Love the my room tip.
Well, I'll correct my previous statement. I just went to check, and I did get a charging voltage of 14.25V (This is the highest I ever seen it). Perhaps it is because my 12v Acc battery was sitting at 12.8 just before I started the car. It is possible that the lower charging voltage the few other times I checked was because my 12v was only at 12.2 or 12.3 to begin with and the charging voltage was set lower. I just had a charger on my 12v all day yesterday just to top it off so it is pretty much full charge now anyway. I really wish I knew what the charging amperage the circuit is capable of.

Thanks for making me look again.

I have a Bluetooth battery monitor permanently installed on my 12v battery so it makes it very easy to check voltage as long as my phone is in range.
dp
 

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When the prime is placed in My Room mode, the 12v Accessory battery is charged
Was this written somewhere? I looked and never saw any direct statements to this effect, It maybe begs the question why was this not alluded to when they gave the precautionary statement re the 12v battery discharging after prolonged periods of non-use and not being able to start the car, and I think their example was 2 months. I don't think I read the manual suggesting to connect the charger, then after sufficient charge activate My Room mode to help restore the 12v battery?
 

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Was this written somewhere? I looked and never saw any direct statements to this effect, It maybe begs the question why was this not alluded to when they gave the precautionary statement re the 12v battery discharging after prolonged periods of non-use and not being able to start the car, and I think their example was 2 months. I don't think I read the manual suggesting to connect the charger, then after sufficient charge activate My Room mode to help restore the 12v battery?
I doubt you'll find anything in the manual as a strategy but it has to be the case. When you're in My Room Mode all of the accessories are live, and if it didn't charge the battery you'd suck it down in no time. If I was going to store one for long periods I'd probably just bite the bullet and put a battery tender in. Or just put it in ready once in a while, because you don't want the charger to top the traction battery if you're not using it (and I don't think you can NOT charge if you're in My Room). Store at mid charge level I think is the guidance.
 

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I doubt you'll find anything in the manual as a strategy but it has to be the case. When you're in My Room Mode all of the accessories are live, and if it didn't charge the battery you'd suck it down in no time.
But...if all accessories are turned off, which was the recommendation I believe, why is it assumed the 12v battery is charging when there is little to no demand? Is it possible then that this strategy only maintains the 12v battery when accessories are actually turned on?
 

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But...if all accessories are turned off, which was the recommendation I believe, why is it assumed the 12v battery is charging when there is little to no demand? Is it possible then that this strategy only maintains the 12v battery when accessories are actually turned on?
I don't have the wiring diagram of the vehicle to prove how it's set up, but it's how the 12V system on cars is typically engineered. The battery is on a bus with that has contactors for the charging circuit and contactors for the fuse blocks (which distribute the load). The battery is in the "middle." You can run battery only in accessory mode (which will discharge the battery because the charging circuit contactor is open), or you can run in ready / my room mode which closes both the charging and accessory load contactors. I would be very, very surprised if the R4P has a mode where it isolates the battery and runs accessories directly on the charging circuit. I don't think cars do this, because the battery is good to have on the bus to smooth out the power as loads are added and shed. (think of it as a pizza stone in an oven that acts as a heat sink to stabilize temperature- the battery does the same thing).

And if the battery is on the bus it's going to be charging, because the charger will float the system at at least a full charge state.

We have split master switches in our aircraft that will allow us to isolate either the battery or alternator (the charger), but they are there for emergencies, and pulling the battery out of the bus is an opportunity to fry a very expensive avionics stack.
 

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Let's say the 12v battery happens to be discharged to say within 30% of too low to be able to start the car. We now, with the traction battery charger connected, activate My Room mode. All accessories are OFF. I guess it is being suggested the 12v battery is now being charged as a consequence of being in My Room mode. Is this allegedly because it is designed to anticipate accessories will be used so will attempt to 'top off' the 12v battery? If this is what is being said here, I'm puzzled why Toyota failedl to suggest this maneuver for when the car has sat for 2 months or more and the hybrid system won't start because of a discharged 12v battery?
 
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