At what point do you replace the 12v battery?

I’ve got a 2012 v five with 92,000 miles on it. I’m going on 8 years on the original battery. On screen diagnostic showed 12v for battery which someone above said I’m on borrowed time with a reading that low.

 

Ok, so my 2010 has a genuine Toyota battery in it brand spanking new not six months ago. As an experiment, I walked out to the car just now, opened the door, and pressed the button on the ScanGauge to show its voltage reading.

It was 11.3 while the brake pump was running, down to 11.2 in the final seconds of the pump run, bounced back to 11.8 once the pump shut off. This is, of course, after several days of no trips in the car.

When I do have an errand day, it will get charged again. Happily, Gen 3 is able to charge the 12 volt battery a bit faster than Gen 1 or 2 would.

My favorite answer to the "at what point do you replace" question is to have a little jump pack in the glove box so I don't have to obsess over it, and then replace the 12 volt when it's clearly dead (which is what I did six months ago when the ten-year-old one finally bit the dust).

 

I would change it out now. IF......that 12.0 reading really was with no load, then you are a bit past "borrowed time".

Seems like I remember that the scangauge reading tends to be a bit off on the low side.
I'd be more interested in seeing the voltage measured with a voltmeter while the battery is really "resting".
IF the reading comes up below 12.3 or so when doing that, then you are abusing your new battery by not keeping it charged up better.

 

Ran the car briefly earlier this morning while blowing off the mower deck, so the voltage is back up a little.



Simultaneous reading, car off, doors closed, window open to reach the ScanGauge. Brake pump last ran maybe twenty minutes earlier.

So you could say the ScanGauge reading is maybe a tenth, less than two tenths low compared to the Fluke. The ScanGauge was blipping between 12.1 and 12.2 when I shot this, so we could say it was maybe "thinking" around 12.15, or about 0.08 low compared to the Fluke.

The ScanGauge's voltage reading is a live reading of the volts on pin 16 of the DLC3 connector where it plugs in. The other readings shown above are just remembered from the last time it communicated with the ECUs, which of course only happens when the car's on.

The reading that's chiefly known for being dependably lower than others, by more than just a tenth, is the one on the Vehicle Signal Check screen of the MFD. On mine that's ordinarily about 0.6 or 0.7 low compared to the ScanGauge right next to it, which I've noticed in multiple cars.

And yes, I am non-obsessive about the 12 volt batteries in my Prii, and my abuse may have contributed to the demise of my last one at ten years, for which waves of remorse occasionally wash over me, and then I dry my eyes and move on.

 

I know that you won't care but for the benefit of others reading this:
If that really is taken at a "no load" situation......and it is an AGM battery......then 12.2 V is a tad bit LESS than 50% charged
and leaving it at or below that resting voltage is NOT good for the battery's health.

 

Does anyone here really know what the 12v battery does and how a Prius works? Like I said, I’ve got the original battery and now 8 years old and 92k miles, with car off reading of 12v. BUT, does the 12v start the actual gas engine or does the hybrid battery start it and the 12v just starts the computers? When you press start, you don’t hear the gas engine turn over, you just get the electric motor to come on and computers to come on. You don’t hear an engine turn over. If you start to drive immediately, the electric motor powers the car before the gas motor comes on. What happens if it gets low? Will you drive in electric mode and the gas engine just doesn’t come on? Will the car just not give the “Ready” and so you can’t even get the electric motor on? You’ll never hear the standard car sound of the gas engine trying to start and just not turning over.
I think I once heard the 12v just starts the computers and the hybrid battery starts the engine but then it wouldn’t make sense if someone had a bad hybrid battery, how could they start the gas engine. Maybe either the 12v or the hybrid can start the engine if the other is low. Has anyone ever heard of anyone driving with the electric motor and the gas motor failing to come on bc of a weak 12v battery?

 

If you read this forum much, you'll see that many many times.

Without the hybrid battery, a Prius can't start the gas engine. Period.

Honda IMA hybrids have a different system architecture that can start the gas engine without that battery.

 

Yes. A LOT of people who frequent here really know.
It seems that you are NOT one of them.

If you spend a little time reading the previous threads on here, you might learn a lot.
Might.

 

So, if the hybrid battery starts the gas engine,why does the 12v need to be so high on voltage, above 12.4v? It sounds like it doesn’t start the gas engine. Why does it need to be replaced as often as a regular gas engine car battery?

Also, you say the gas engine won’t start without the hybrid battery. I had an inverter failure on my 2012 Prius v, and only the gas engine was working, no electric motor. (But maybe the hybrid battery can start the gas motor with an inverted failure).

 

Yes, I am not one who frequents here often and I don’t know. That’s the reason I posted. If I knew, I wouldn’t be posting. It also seems a lot on here don’t know. I see many saying the battery needs to be replaced every 3, 4 or 5 years. But then I also see some with 12v batteries lasting 9 years and still going.

 

The 12 volt battery isn't used to start the car's engine, powering a conventional starter motor. It is however, needed for the car to "wake up", provide initial power to the brake accumulator, various computers, relays and so on.

It's also used whenever the car is off (similarly to any modern car) to maintain volatile memory: data like trip meters for example. This sort of use will slow-but-sure run down the battery with a car having protracted down time and only short trip usage.

 

The 12v battery powers all the control circuitry, including the circuits that connect the HV battery to the "starter" (not that there is a separate starter motor).

As for the life of the 12v battery... I've seen people here reporting that theirs are 7 or 8 years old and thinking that maybe it's time to replace them just in case. Prius 12v batteries have different construction/chemistry from regular car batteries (AGM vs. "wet-cell" -- safer in an enclosed space, and spill proof) and a lower Ah capacity.

How many times were you able to start that Prius V with a failed inverter? My guess is that it was only while the HV battery still had sufficient charge left from before the inverter failed -- or maybe the inverter was still charging it sufficiently to start the gasoline engine and not much more.

If you're planning to replace your 12v battery, give serious consideration to getting a genuine Toyota TrueStart battery, which has a long (prorated) warranty and a price not significantly different from that of an aftermarket one. Do NOT buy an Optima battery. And if you do buy an aftermarket battery anyway, make sure that it is (a) AGM (needs different charging circuitry from a "wet-cell" battery), and (b) has the necessary vent connection for the hose that leads the emitted gases outside.

 

You've asked an insightful question. There is a reasonable story for why that would be recommended, as a consideration to maximize the life of the battery; the principle is to try to keep the battery at a substantial fraction of full charge most of the time, to slow chemical processes that degrade it over time. The Prius charging system does not charge very aggressively, and in a lot of driving scenarios such as short trips, the battery in a Prius tends to read lower most of the time than what would be considered well charged, and there's nothing unreasonable about taking measures, such as a trickle charger, to keep a better charge on it, as an effort to wring the longest lifespan out of it.

By the same token, I've never obsessed over mine to that degree, and the one I just replaced this past winter was ten years old, which doesn't seem like a bad run. I've had it 'splained to me in this thread that the current voltage on my new one doesn't ensure I'll get the longest possible life out of it, and the theory in the 'splanation is sound. On the other hand, my car is now ten years old and a lot of things could happen before it needs the next 12 volt battery, whether I obsess over it or not, and in practice I have other things deserving my attention.

The other thing you may have read a lot of is that you need a voltage reading like that before you can do any diagnosis when the car is acting up. That's a different claim, and you're right to wonder about it. The Toyota manuals are clear in what they recommend you check before proceeding with diagnosis; they have frequent and consistent instructions like this one:



That's a pragmatic and reasonable step. It's fairly conservative; what really needs to happen when you start a Prius involves the (rather heavy, but brief) load of running the brake pump, followed by the (even more brief, and light) load of clicking three relays. After that, the traction battery is powering the car, and the 12 volt is getting a nice relaxing recharge.

If you watch the voltage while "starting", naturally the loads will pull it lower than what you measured while resting. Usually it will be lowest while the pump is running, and bounce up some when the pump is done. If it is pulled below 7, 7½ or so while the pump runs, you might not get a successful start. I have, in a pinch, started Prii that were in that condition, by pulling out the pump relays first, starting the car, then putting the relays back.

Some of the computers in the car are programmed to log trouble codes to let you know if they measured a voltage below about 9½ at any point, and you may find that you have some of those (correct) codes after you have started a Prius under those conditions. That's to be expected, you can look up what those codes mean, and confirm they were correctly logged.

A lot of the "ZOMG don't even try to learn what's wrong with your car until you get your battery to read over 12.428571 volts!" that you can read on PriusChat is based on some oddly persistent misconceptions about how the 5 volt operating voltages inside the computers are derived. Essentially, you have to be assuming that onboard VRMs haven't been invented yet. Even without taking the cover off an ECU and looking, you can learn something about that from the passages in the repair manual that refer to the VRMs.

Then there are fallback explanations that sound something like "well, but you don't know how low the voltage got during any instant of the start attempt!" Which could have been reasonable in the days before common multimeters had a button to measure the highest/lowest 1 millisecond peak and dip. Mine is 30 years old.

You might persist with "well, but how low did it get during some fraction of a millisecond?!" but if you do, you are assuming that not only VRMs but also capacitors have not been invented yet.

So yeah. If your concern is about getting the longest possible life out of your battery, keeping it in the higher ranges of charge as much of the time as you can is reasonable advice. If you have a current issue with your car that needs to be diagnosed, it is sound advice to charge or replace the battery if you're seeing below 11 (on a regular, not peak, meter setting), as Toyota recommends, and also if any of the "hey, your voltage got low" codes are logged. It is not necessary to send your problem solving off on a battery detour if you happen to see, you know, 12.2 rather than 12.4.

 

Ours see's very sporadic use, lots of short trips. It's garage stored, so I wall-mounted a smart charger by the left/front corner, and installed a quick-connect cable to 12 volt positive (where it enters under-hood fuse box) and a ground point.

Any day the car's going to be idle, I connect it, and leave it connected till the car's next use.

It's an Optima Yellow Top, coming up on 5 year this September. Testing with an electronic load tester it has like-new numbers, for voltage and Cold Cranking Amps.

I'm not sure how much this'll extend it's longevity; just my ongoing experiment I guess.

 

And now you have MORE information than you need to know.

A 12 V battery lasts.......as long as it lasts. No way to accurately predict.
But not keeping it fully charged (sitting unused for a LONG time) is hard on it.
So is VERY high temps; think Arizona in the summer time.

Average life: maybe 4-5 years but your mileage WILL vary.

P.S. 12.4 does not indicate that the battery has failed but that it is a bit low and might be failing.
Tenths of a volt are important.

 

Very informative thread here. I've owned lots of cars and almost all battery failures happen when the weather first starts getting colder in the late fall.

I've heard there's a difference with jump-starting a hybrid versus a regular car but don't know what it is. (I have a old portable jump start pack that's always worked great to get cars running when the battery goes toes up.)

 

Thanks for all the very informative responses. They were excellent.
One thing I don’t understand is the only way I can test the voltage reading is if I do it through the car’s dash display inside the car, with some pattern of turning the lights on and off. I tried a voltage meter connected to my 2012 Prius v’s + battery port under the hood and to many unpainted spots under hood and got zero reading. I took it to O’Reilly’s and to Autozone and they each tested it and said they got zero reading. Any ideas why i get zero reading?
Also, I often leave the car in the first power button press mode, with radio off, but with cell phones charging when I go surf. I think it charges for like 15 minutes before shutting off. Does this put much strain on the battery? What about being accidentally left in the second power button press (with no brake press to actually start the car) as this is the only way to move the windows up and down.

 

I'm not part of the "12.4V" camp, so won't address that. Others have written plenty, and I don't want to step into that technical vs religious war right now.

You don't give a location. I'll remind that 12V battery life in traditional cars varies considerably with local climate, with mild climates getting longer life than hot and cold climates. I live in a comparatively mild area, and replaced mine last year at 7 years, similar to my household's non-hybrids. Others in harsher climates have noted significant failure rates beginning much earlier.

Just what do you mean by "the electric motor"?

Prius does not have just one electric motor, it has two motor-generators. Both are required to make the car operate normally. Both operate at high voltage similar to the traction battery, not at 12V.

MG1 is required to start the gas engine. If it fails, or the traction battery can't power it, then the gas engine cannot be started.

MG2 alone can move the car if there is sufficient charge on the traction battery. But the range is short until the battery is depleted.

The gas engine alone cannot move the car without both MGs working. But because it cannot even be started in this condition, it applies only in the case of the MGs failing after the gas engine is already running. It could continue running, but not do anything useful except produce heat.

"The inverter" has several elements, so I don't know just what your scenario was.

 

MG1 and MG2 both reside in the transaxle (aka transmission). For all practical purposes, it's one thing.

 

That's an easy one.
You are NOT connecting to the right place.