At what point do you replace the 12v battery?

A Prius isn't anywhere near THAT cheap.

I know it isn't rocket science, because this stuff was part of my electrical engineering career. BTDT. Praying isn't good enough, because too many prayers are answered only temporarily in good conditions on a room temperature workbench, then frequently revoked in the harsher conditions of environmental test chambers.

Design blunders still do happen at all levels, but that doesn't make the ones at THIS level excusable.

 

Per service instructions from Toyota the DC-DC converter, the Prius equivalent of an alternator, has gone bad.

moto g(7) power ?

 

After reading so many 12V battery failure incidences here on PRIUS, especially premature failure on PRIME, as preventative measures, I started to purchase battery related items. I got a jump start battery pack and a new automatic battery charger/maintainer. I am now looking for a battery tester. It seems the Solar BA5 is now discontinued. There are BA7 and BA9, but are they substantially different from BA5? Do I need a dedicated battery tester like this one, or good multimeter is all I need?

 

For the Prius the Toyota instructions require simply a multimeter. Turn off car or charger, turn on high beams for 30 seconds I believe, to remove surface charge, then measure voltage.

I have an old battery tester, EZ-SCAN 100 that links to an Acton OBDII scanner. The instructions note it should not be used for AGM/deep cycle batteries. I just use a multimeter.

moto g(7) power ?

 

Do you have a copy or link for this instruction? Prime and I believe Gen4 uses a regular wet lead-acid battery. I don't know the compatibility with various battery testers, but most of newer testers I find on Amazon say it is "compatible with many battery types, including Flooded, AGM, Spiral and Gel Cell".

 

WIRELESS and Salamander_King, I don't usually do this because a tech info subscription to Toyota is so cheap, but I do not want to mislead you in any way. I am uploading the exact directions.

Please note it may not be so simple to diagnose the DC-DC converter because I think you need to wait until the battery is fully charged. BUT a new battery shouldn't be a concern...

Also, this is specifically for a 2012. I do not have the later model instructions...

 

Thanks @jzchen . I will hold off purchasing a battery tester for now. I don't have any problem with my PRIME now, but it seems to be a battery issue comes up suddenly without warning on PRIUS. I just wanted to be prepared.

 

@Salamander_King i think the Solar BA7 etc are just improvements. The buttons on the 5 are a little touchy, so maybe better build quality. I see some other entries in the field too.

 

I have been driving for 50 years, and I KNOW when a battery is dying. It makes that ugh ugh ugh sound when starting.
Sadly, in a Prius there is no 12 volt starter and the battery never makes that sound, no matter how weak the 12 volt battery is.
If I compare the speed of the power windows when in Ready to the speed in Acc, I might notice my battery was on it's last legs, but honestly, I avoid Acc all I can.

This leaves a mature driver with very little to go on but voltmeters; not a traditional means of deciding if the 12 volt battery is dying. Most cars need starting current, not voltage. I have a ScanGauge II and it shows Voltage if I ask it, but it is not usually one of my top 4 interests while driving.

Some of you ask why we do not have a built in test for a bad 12 volt battery. What would power such a test? Why the very battery we are testing; that can't be a reliable test.

We are lucky some bureaucrat has not forced Toyota to make it go Ugh Ugh Ugh.

"The fault, dear Brutus, is not in our cars,
But in ourselves" Julius Caesar (I, ii, 140-141)

 

@jzchen, Thank you for the link to the battery system test info. I understand what is going on, but unfamiliar with the DC to DC converter.

I have been driving for 51 years, and been a ham radio operator for 45 years, so I am familiar with microvolts to kilovolts.

That said, the weather is changing around here and I'll probably let the local Toyota dealer tackle this job, I have to travel to Florida on business, so I don't have any extra time now for troubleshooting & repair.

I will report back and let you guys know what the diagnosis & repair ended up being.... fingers crossed.

 

If you really want to learn about the Prius DC to DC converter and how to test it, check out this youtube--- very informative.

 

I'm sorry that this did not fix your problem.

I don't think your battery has anything to do with your warning lights unless it's the red triangle.

Your car was designed to run on 12V. The fact that your battery put out 12.2V meant it is doing what it was designed to do.

Warning lights should not be left to guesses (mine included). Hook up a scanner and read the codes. There have to be codes in order to get a warning light. If you can't do this yourself then take it to a trusted mechanic to read the codes. Undiagnosed fault codes could lead to serious damage to your car.

 

While that might sound logical, it isn't necessarily true.

The "12 V system" is really designed to "run" at about 12.6 volts or a bit higher
and each tenth of a volt below that indicates a rather significant loss of battery capacity.
At 12.0, it is around 50% discharged.

So.....all of the computer boards are powered from that 12 V supply.
If they don't get enough power to boot up properly, then the codes they produce can't necessarily be trusted.

That is why the 12 V battery is the FIRST thing you should check when a bunch of codes suddenly show up.
You can't properly diagnose the actual problem with a weak battery.

 

Can anyone confirm this with evidence, like service manual pages or someone who is a toyota tech.
I ask this because this is crucial for diagnosing my Prius. It has one bar on the traction battery, brand new 12v battery and more than half tank of gas. And the issue is that the ICE engine would not start.

 

One bar on the traction battery is not good, did it get down there due to starting attempts?

How brand new of 12 volt: just wondering if it's a loose connection?

 

Did you test the voltage of the brand new 12v battery?

And do you have the "Ready" light on the dash?
Any codes?

 

From what you quoted, I wasn't quite sure which part you were asking for evidence for.

The Prius has a 12-volt electrical system, with lots of the usual 12-volt stuff (lights, heater, wipers, all that jazz). And all the computers that control all the stuff.

But it doesn't have a 12-volt starter motor. The engine is already connected to a transmission that has two honkin' 500 volt motors in it, which kind of ought to be enough to crank over a 1.8 liter 4 cylinder engine. So that's what it does.

When you get in the car and you want to start it, there needs to be enough power in the 12 volt battery to power the computers that are deciding whether you have the right key, have your foot on the brake, etc. And it has to be enough to power the brake lights (which are going to be on, since your foot's on the brake), and also run the brake pump, which is usually running from the time you opened the driver's door, and might be the biggest electrical load going at that time.

Over and above that, it needs enough to pull in the set of relays inside the traction battery to bring that on line and make the car READY. Once that has happened, the 200 volts from the traction battery are feeding the inverter/converter under the hood. The converter part takes over powering the car's 12 volt equipment, so from that moment, the 12 volt battery is being charged rather than supplying power.

Meanwhile, when the car elects to start the engine, the inverter part will take the 200 V from the traction battery, step it up by a variable amount with a max voltage of 500-ish, and send that to motor-generator 1 inside the transmission to crank over the engine.

The most recommended source I know of for this kind of info is in the New Car Features Manual, which is one of the titles in the manual set that you find on TIS (more info). It has the broad overview explanations, which the other repair manual volumes usually assume you have already seen.

There are also lots and lots of further, in-depth treatments of particular subjects available on TIS, including course materials used in Toyota's technician training.

There are some old posts around here from members like hobbit who datalogged the current drawn from the 12 volt battery during starting. It could jump to around 30 amps briefly before the high-voltage system took over, if I remember right. Maybe more if you had other accessories turned on. Nothing like the few hundred amps that would go to a 12-volt starter.

 

Not sure, it was like that when it was towed to my house

 

10 days tops. It was the first thing I addressed when the car was delivered to me