Warning signs the 12V battery is on life support

Is there anything special that needs to be done differently when replacing a battery in a Prius?

 

Very good points there. I could be speculating, but I've noticed one thing. When the 12 volt battery is fully charged or at least at some normal voltage, the engine shuts off a lot faster, even in a cold weather. The battery could be immaterial once in READY mode, but it needs to be charged, so the engine will have to run more. This does not happen during the summer because the 12 volt battery doesn't drop the charge as fast. Once again I could be speculating.

You're correct about the B-mode. Today it was -4 f in the morning, and the whole 40 minutes driving experience was like in B-mode. I could not get the traction battery fully charged and I also couldn't figure out what the heck the engine was doing. The mpg was terrible, around 30mpg I believe.

 

Ah, but 'noticing' something and establishing it are different things ... to cover the ground between "it feels like I've noticed this" and "yes, it's established that's what's happening", the 'noticing' needs to get a lot more specific. For example:

How many times have you noticed this? How many were in the "12 volt battery is fully charged or at least at some normal voltage" category, and how many in the other category? What observations were you relying on to sort these instances into those two categories? Have you been checking the 12 volt state of charge habitually when you get in the car? By what method? In the category that isn't "fully charged or at least at some normal voltage", how far from full charge has it been, on average?

How different are the engine-times-to-shut-off between those two categories? That is, within each category, what's the average time to engine shutoff, and how much do individual cases vary around that average, and then how does that difference between categories compare to the variability within each category?

What other surrounding conditions that could affect engine time to shutoff are you also monitoring? Do you have records of the ambient temperature, coolant temperature, traction battery temperature and state of charge, that you can match up with the same cases where you've 'noticed' the connection between engine shutoff and aux battery charge? Have you done the same comparison of average shutoff times grouped by those variables, to see if they're correlated equally or even more strongly than the effect you 'notice' with the aux battery?

I know it sounds like a lot of work. But I can't help that. What makes the difference between just talking about 'noticing' things, and establishing that those things are what happened ... just is a lot of work.

You're not speculating there, you're stating things that are clearly true on basic principles, but unquantified. It's hard to have perspective before throwing some numbers in.

For starters, there's no direct connection between running the engine and charging the 12 volt. The 12 volt battery is being charged whenever the system is in READY, and the rate it's being charged at depends on its state of charge, its temperature, and the DC/DC converter output voltage, but not on anything the engine is or isn't doing at that moment.

There's an indirect connection, of course. The engine will, over time, end up running a certain amount longer or a certain amount harder, when it runs, to make up whatever energy the hybrid system has been putting into the 12 volt battery to charge it. Here's where some numbers fit in. The nominal, specified charge rate for the 12 volt battery is about 50 to 60 watts, about the same as one headlight. The various electrics and electronics in the car add up to a few hundred to several hundred watts, depending on what's on. Meanwhile, the hybrid system, in normal operation, is routinely shuttling around amounts of power, in one direction or another between the engine, MGs, and traction battery, ranging from thousands to (low) tens of thousands of watts. The charging of the 12 volt is one skinny straw sucking out of that river. How sure are you that it is the one thing you're 'noticing', when you're trying to guess why the car is doing something it does?

There's another line of speculation someone usually brings in at this point, which is that a bad-enough 12 volt battery (that is, not just low on charge, but failed) could be drawing "significantly" more than the nominal 50 or 60 watts. This is another one of those things that isn't wrong, just a statement of something that clearly can be true, but hasn't been quantified—which would be super easy to do. I've been on PriusChat about 9 years, and I can't tell you how many times I've read posts saying that definitely has to be the reason car X is doing Y. But I can tell you the number of times I've seen a post that said "yep, I have a car here right now where I think that's what's happening, and I put an ammeter on it and here's what I read", because that number is easy to remember, zero posts so far. And ammeters aren't that hard to lay hands on....

If that were to happen, it would lead naturally to the next step, where we'd try to quantify, out of N cars doing weird thing Y, in how many cases out of N is that the explanation, and not something else? But so far, that's still on hold for somebody who's convinced of the explanation in principle, to measure and establish an instance in practice.

-Chap

 

I can confirm from my own scangauge that regen is limited (seems that the friction brakes come on at 50 rather than 80-90 amps) when the traction battery temp 2 is between 35-70 degrees F. At 70 it switches to normal mode. I would suspect below 35 at some temp (haven't actually seen it lower yet) its limited further much like how it is above 120 F where regen is basically off.

 

no, but you may want to add 12v to the jump point so you don't lose trip, radio and window settings.

 

Another sign (at least with the smart entry system): hatch remains locked even though the other doors open. The buttons in the door did open the hatch if I managed to start the car, so I guess it really was a low power issue.
My 8 year 12v battery died whilst on a holiday in Portugal. Happened in the end so we'll be home a day late.
I did manage to start the car a few times with little juice: start first in acc mode. Wait until nav and radio are fully booted and start the car. This helped me to start and enable me to drive to a Toyota dealer.
I guess it is not completely dead, but just can't deliver the voltage to start both the car and nav simultaneously.

Guess I could also have bought a booster, jump starter battery, to help me drive home, but we'd rather have a fully working car before driving 2385 Km (1481 Miles) home