Possible 12 volt battery going bad

Read this link about testing your 12V battery and see if this helps.

http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

 

So how long did your car sit without driving? It may test ok but it's on the beginning edge of needing replacement. Also if you know it is going to sit for a week you may need to let it idle for 20min or so to fully charge up before another extended sit period. It's what I do on airport parking 1 week gone trips. Make sure I leave it in the green and no problems.

 

The battery is now 5 years old ... REPLACE IT! And yes, the symptoms you describe can be caused by a bad 12v battery.

 

From other reports you are hitting it just about right point for a normal dead battery. I wouldn't say its NOT a battery going bad (a guess, do the test). I've only left mine 6 days max (05 also). Others report dead battery after 2 weeks. Turning off the sks also aids in extending the battery sitting life.
You can pull the dome fuse under the hood. That leaves the car dead. Thats how the cars are shipped. It will reset everything in the car and leave it so that you need to use the key to unlock the drivers door.
If it looks like something you are doing regularly you may want to do the power outlet mod so it is live and purchase a solar panel charger for the power outlet to leave in the window.
I;'ve not replaced mine in my 05 since it doesn't seem to be a problem if I watch the sit time. It could be time though to start shopping and reading on how to replace yours. After reading, check out dealer prices. I've been able to dicker with mine when I pay cash. If you are a DYI person, it doesn't look difficult.

 

If the 12V battery is discharged, it could benefit from 8 hours or more of the Prius being READY. 20 min. is not going to do much to improve the charge level.

I suggest that the OP should replace the 12V battery now.

 

So it takes 8 hours or more to charge the battery?
Leaving it in ready will charge enough then shut off the engine until the level reaches low enough to turn on the gas engine again. Yes?
So what does 8 hours do? Just curious.

While I'm all for maintenance, hank1968 shopping for a battery is prudent. The battery can be a sudden problem. I personally haven't experienced it as others have. I do also keep mine on a BatteryMinder which has probably extended the life.
Lots of posts on Peps changing battery and dealer experiences.

 

I am talking about the 12V battery. If you are depending upon the Prius charging that battery from a near-dead level, that really requires an overnight charge (assuming that the battery is in good enough condition to hold a charge.)

When you refer to the engine running, then turning off, then running again, this relates to the traction battery SOC dropping low enough so that the gasoline engine will spin MG1, thus providing power to charge that battery. The engine usually will turn on when the SOC drops to two red bars and turn off when the SOC increases to three blue bars, so not much of an SOC change is needed.

 

Yes that is what I understand. Is it that it takes 8 hrs because the gasoline engine will turn on and off, on charging the 12v battery?

 

When the Prius is READY, the 12V bus is maintained at ~13.8VDC. That charges the 12V battery. 13.8V is not a very high charging voltage, hence the need for an extended charging period. (Some vehicles with alternators will run at 14.5V or more.)

The traction battery provides power via the DC/DC converter to keep the 12V bus at 13.8V. The gasoline engine runs as needed to keep the traction battery charged up at 3 blue bars.

 

Appreciate the explaination.

So as the 12v battery goes bad aside from possibly stranding the car because the electronics can't start, is it more of a drain on the DC/DC converter 12v buss and keeping the gasoline motor going more that can affect the mpg? OR why does the 12v battery affect the mpg?

 

There are lots of ways for a battery to fail, it might evaporate the water, it might vibrate into parts, the plates may sulfate and no longer have the right chemistry, etc. One failure mode is that one (or more) cells may short circuit, directly connecting + and -, making it a 10 volt battery. No amount of charging will ever get it to 12 as there are only 5 working cells, but the computer tries constantly, using a LOT of power. So MPG is down in this failure mode. And it is low voltage, so it may not be able to start the car.

 

The Prius just doesn't charge the 12V battery as rapidly as an alternator equipped car does. Leaving the car in READY mode is no way to charge the 12V anyway, it's just an example of how long it would take.
If you really want to try and salvage this battery you'd have to leave it on a "battery minder" type charger overnight or more. But the best suggestion is to bite the bullet and get a new battery at this point.

- D

 

The OP said his main battery was in the purple when returning from trip. I have left mine for 14 days to return to essentially the same main battery bar reading, maybe one bar down from green, depending how far into green the bar was when stopped I suppose. I don't understand why the purple bars and dead 12v. I assume the bars weren't purple when car was left. What happens when a Prius is left in the ig-on, or even ready mode, for extended times, unattended? I know the engine runs on and off in ready as I have used a/c while parked in hot weather. Not that this is what happened here.

 

just purchased an 05 manufactured 4/05 with 82K runs great no smart key option...reading this thread I checked with a volt meter at the battery car off read 12.4v can I assume this is the second battery since its 10/12 now? Also looks very clean not like a battery that been collecting dust for 5 years. Also btw getting 54mpg over first 100 miles mix city/hwy I rented a prius before over a weekend so a little familar with driving one.

 

Usually you can find a date code on the battery. The original equipment battery date code is six digits, DDMMYY format IIRC.

If the battery doesn't have a sticker that shows Toyota Motor Sales and the other US distributors (for Hawaii, Puerto Rico, etc.) then it is not original equipment. If you take and post a photo we can help you to identify it.

 

If the 12v system goes dead, the Battery ECU loses it's current SOC tally and has to reinitialize. It attempts to calculate the SOC by reading the voltage which is not very accurate, and I think it intentionally estimates low.

The traction battery will eventually discharge when off, as that is an artifact of the battery design. If your battery is at over 50% SOC and you park it for a few months, the SOC will be down, but it will recover when you start using the car. (your 12v battery will likely be dead at this point though) If your SOC is 40% and you park the car for a year, you may be looking at a new replacing both batteries!

 

That makes sense then that he had purple bars. After I posted before I realized of course I knew ig-on will discharge the 12v, and ready will discharge the main battery. Three things that may have been discussed before, but I have questions, are: the 12v. charge by the main battery, is it a tapering charge or continuous float? and I wonder with a full tank of gas how long the Prius would last when in ready mode in Park, unattended? Not how long it would last before being stolen. I have left the car in P and gone into a house for even half an hour, don't know if I could travel out of state with the car in P and the fob in my pocket?