The 12V Battery Question

[kevin h]

Having horrible thoughts and wondering what would happen.

Gen iii prius is 6 years old in a few months and still owns the original 12v Battery.

I've seen a diagnostic check you can carry out with the Gen2 by pressing display and going into the diagnostic screen, lights on and off a few time, but can't find anything like that for the gen 3.

Anybody aware of a similar check on the Gen 3

Anyway, if shes parked in the garage and the 12v Battery has died, whats the procedure for getting to the Battery and/or getting the car out of the garage?

Following on, now and again i get a message to say that the key fob battery is getting low but it only happens when both fobs are in the car, Never, just one.

I now carry 2 spare batteries around for the day i need to use my manual key.

i know i could replace both straight away, but I'd love to know which one is at fault as only one is ever used. (Thank god the wife can't drive)

Anybody had a similar issue? (not the wife, the key fob)

[MEP's Yaris GS]

Sounds like its all related to a failing 12V, it'll cause other phantom faults.

To jump start you need to pop bonnet and remove fuse cover in top right hand corner of the engine bay, there is a +ve terminal to connect to.

Or connect directly to the Battery in the boot.

[Grumpy Cabbie]

Just be VERY careful jump starting a Prius!?! Seriously, you don't want to mess it up.

The 12v is about £95 fitted by Toyota.

[kevin h]

Know what you meen GC. Thats what Toyota assist is for. Thought they might need access ro 12v Battery, but apparently not. Think I might just replace the 2 key fob batteries though, just to play safe on rhat side.

[johalareewi]

Having horrible thoughts and wondering what would happen.

Gen iii prius is 6 years old in a few months and still owns the original 12v battery.

I've seen a diagnostic check you can carry out with the Gen2 by pressing display and going into the diagnostic screen, lights on and off a few time, but can't find anything like that for the gen 3.

Anybody aware of a similar check on the Gen 3

I don't on my original gen3 T3. Not sure if there is an option on the TSpirit touchscreen.

If you have an ODBII analyser like scangauge or an app on your phone, the Battery voltage is available there.

[sushd]

Diagnostic menu - Try the following, and see if it works:

Put the car in READY mode.

Press CAR + TEL buttons together for 3 times. Everytime you press these two buttons the "sidelight" indicator should come on.

or

Press CAR + INFO buttons together for 3 times. Everytime you press these two buttons the "sidelight" indicator should come on.

[Grumpy Cabbie]

There isn't the option on the t3, and probably the t4 either. But do you need to know what the car says anyway? The 12v lasts 4-5 years or 80k miles generally (sometimes more, some less). Don't abuse it in leaving the car in Acc when using a 12v powered vacuum etc. If you need to listen to the radio or use a car vacuum or air pump, just pop the car in Ready and quickly select EV.

But if you replace the 12v now you shouldn't have issues between now and 2020.

[kevin h]

Bit confused about "car" .

What button is that?

70k miles on clock, 5 yrs 8 months old and I never ever run anything on Acc only. Never know, the Battery might be as new. Think the next service can include a check.

[kithmo]

IIRC to put the Gen3 in diagnostic mode I think it was, press and hold the info button and turn the headlights on and off 3 times.

[MEP's Yaris GS]

IIRC to put the Gen3 in diagnostic mode I think it was, press and hold the info button and turn the headlights on and off 3 times.

Neither seem to work on my PiP :(

[sushd]

Bit confused about "car" .

What button is that?

Oops! The German Prius + rental I had during a trip there last year had the "CAR" button on the MFD. And that's the steps I followed to get into the diagnostic mode! Assumed that all Gen 3s would have the same MFDs ... wrong!

Do you have SETUP and DISP buttons on the MFD? Try with these.

[MEP's Yaris GS]

So to clarify are we talking the physical buttons down each side of the screen or a touch button on the screen?

[sushd]

The physical buttons.

[sushd]

Try this:

1. Press POWER button twice (without foot on brake pedal) to get into ACC mode.

2. While holding down the INFO button, toggle the headlight button one step for 4 (FOUR) times.

3. You should see "FAULT Diagnosis" icon .... select that!

[MEP's Yaris GS]

Right, managed to finally get it to work. Sidelights on and off 3 times while holding the car button. Also worked with setup button and the phone / internet button :)

Rather interestingly also, discovered the DRLs don't come on in READY until the parking brake it taken off. I never really use it, but noticed when playing and they came on when I released it. Handy if you want to sit in READY at night with DRLs off, just make sure parking brake is on when you enter READY - not sure if this is common knowledge or not. Only 10 months of ownership to discover this ;)

[PeteB]

Right, managed to finally get it to work. Sidelights on and off 3 times while holding the car button. Also worked with setup button and the phone / internet button :)

Rather interestingly also, discovered the DRLs don't come on in READY until the parking brake it taken off. I never really use it, but noticed when playing and they came on when I released it. Handy if you want to sit in READY at night with DRLs off, just make sure parking brake is on when you enter READY - not sure if this is common knowledge or not. Only 10 months of ownership to discover this ;)

Found the park brake/DRL thing in the manual which I read before taking delivery of the car.

I use it often if parked in READY, even in daylight, just need to power off and on again with the park brake set.

[sushd]

Right, managed to finally get it to work. Sidelights on and off 3 times while holding the car button. Also worked with setup button and the phone / internet button :)

[kithmo]

IIRC to put the Gen3 in diagnostic mode I think it was, press and hold the info button and turn the headlights on and off 3 times.

Neither seem to work on my PiP :(

Do you have a "CAR" button on the NAV system ?

I found this for the Prius+

Put the car in accessory mode - Press power button without pressing the brake pedal.

To put the MFD into service mode - Press and hold the "CAR" button and turn the headlights on and off 4 times (i.e. from off through sidelight to headlight and back to off, 4 times)

select "Function check/setting"

select "Vehicle signal"

This works on the Prius+ with Touch & Go Plus.

I remember doing something similar with the 2010 Gen3, but that didn't have T&G, it was holding the "INFO" button in accessory mode, lights on and off 3 times. .

[kevin h]

Wow, got as far a fault diagnostic but can't see how you get to the 12v Battery state. So many scary looking options to press

[kithmo]

Wow, got as far a fault diagnostic but can't see how you get to the 12v battery state. So many scary looking options to press

Function check and/or vehicle signal

[kevin h]

Great, thanks all.

Battery reading 11.8V after 70k miles over 6 years.

Drops to 11.3V if I turn the headlights on.

Think that's all fine isn't it.

[Joseph D]

Great, thanks all.

Battery reading 11.8V after 70k miles over 6 years.

Drops to 11.3V if I turn the headlights on.

Think that's all fine isn't it.

That voltage drop is acceptable, especially if it does not significantly drop further over, say, a 5 min period. Having said that any voltage 12.1 or less (for and AGM battery) is essentially fully discharged. You need to put this Battery on a charger and fully charge. After fully charging, check the voltage again (as above), the voltage should be 13.5 immediately after disconnecting and, after ~2 hours, settling to ~12.9-13.1 after the surface charge has dissipated. After this, repeat the load test. If the voltage is not sustaining above 12.3, then your Battery is showing signs of damage. If, under load, it drops down to any voltage under ~12.0 then your Battery is pretty well history.

You don't need to rush out and get a new battery at this point until the load test (as above) shows that the battery cannot sustain over 11 V under the load of the headlights. At some point after this you should notice that red triangle (with ! in the middle) briefly flashes on start up. This is is your signal that battery failure (and starting issues) is imminent. At this point you do need to swap in a new battery.

Just keep monitoring and charge as necessary.

Al the best.

HTH.

[timberwolf]

You can't reliably, or even have a wild stab in the dark, predict the health of your 12V Battery by measuring its voltage.

The on-board diagnostic are an even more unreliable indicator because you have the car electronics/electrics drawing a load. Anything about 10.5V or above will start the car. I suspect the 12.1V figure is meaningless because it is most likely an Open-circuit measurement taken at room (or higher) temperature?

Then we have the often repeated advice of winding down the windows or turning on the headlights. It just flattens the Battery a little more (possibly leaving you with a call-out to the breakdown service, how smart is that?), and you still won't be able to predict whether the Battery needs replacing, or whether it just had a low state of charge.

Snake-oil battery "health" testers at the dealer won't predict if the battery is healthy, so how do you expect to predict it with the on-board computer?

[Joseph D]

Wow, TW, a bit harsh. While I agree with you to a point, I'm going to comment about what you've said.

You can't reliably, or even have a wild stab in the dark, predict the health of your 12V battery by measuring its voltage. ...

While it is true that a voltage measurement in isolation will not tell you the state of health of a Battery, taking voltage measurements under different conditions and monitoring over time, will. It is possible to build a picture of Battery condition and that is why I have posted as I have above. However, some people do not always have the wherewithal to see to process through to completion.

The on-board diagnostic are an even more unreliable indicator because you have the car electronics/electrics drawing a load. Anything about 10.5V or above will start the car. I suspect the 12.1V figure is meaningless because it is most likely an Open-circuit measurement taken at room (or higher) temperature?

While I'd agree that it not the best option, it is still useful as long as it is consistently used; all measurements will be relative if not definitive. Yes, you are correct, as long as the voltage does not drop below 10.5 V at the critical time, the car will go READY. Given that fact, it is hard to convince people that, at 12.1 V, their Battery is flat. The sad fact is if you continue to leave a battery in a discharged state, it will get damaged and you will be replacing it sooner rather than later. But I guess, if a person is happy to part with £95 every 2-5 years, then good for them. With good monitoring and proactive maintenance it can last 10+ years.

Then we have the often repeated advice of winding down the windows or turning on the headlights. It just flattens the battery a little more (possibly leaving you with a call-out to the breakdown service, how smart is that?), and you still won't be able to predict whether the battery needs replacing, or whether it just had a low state of charge.

While the window method is questionable, it still can be of use to some who can't be bothered doing a more scientific test, for whatever reason. If turning on the headlights for 2-5 mins is going to render the battery unserviceable, it is pretty much a goner. So in that regard it has done the job! Using the headlights is a good way to put known sufficient load, which when in conjunction with monitoring of the voltage, is a very useful metric.

Unless the battery is totally shot, as soon as the load is removed, the voltage will return to the level it was, another useful metric (if it does or doesn't do this). I can have my headlights on in ACC mode for 30 minutes and voltage will hold steady on ~12.3-12.5 V, and then return to 12.7 V after the load is removed.

Snake-oil battery "health" testers at the dealer won't predict if the battery is healthy, so how do you expect to predict it with the on-board computer?

These are probably as good as the operator's knowledge. It seems that most don't know what they are doing.

---

At then end of the day, you can only do so much, but if people do not want to help themselves, what can you do?

I don't know how many times I have offered the advice to charge a battery, only to be ignored.

Oh well, C'est la vie.

[Grumpy Cabbie]

This sums it up;

"At then end of the day, you can only do so much, but if people do not want to help themselves, what can you do?

I don't know how many times I have offered the advice to charge a Battery, only to be ignored."

This is not aimed at the OP. The failing 12v issue is reported on here (and other Prius forums) all the time. There is only so much you can spoon feed someone, especially when they ignore the 'advice' they've been given. They're told their 12v is toast, yet they just don't want to pay £95 for a brand new one.

They've just bought a 5 year old/80k mile car showing symptoms of a failing 12v. Go buy a new 12v, "ah but I can't afford it", then test your 12v "ah but I've not got a meter", then wind your windows down "ah but that could run the 12v totally flat and leave me stranded", well buy a Battery monitor., "ah but....." and so it goes on.

And that's before I winge about them not using the search function on the forum or just reading back a couple pages to see similar subjects. It gets a bit boring for members who try to help, repeat themselves about the same issues and then get slated for offering this advice.

A bit of harsh advice; if you've got a fault with your Prius take it to Toyota and pay for a diagnostic.