Need Help Identifying What Happened

They cannot test it with most modern (load or "battery") testers as they will not accurately test a battery that small. The Prius battery is more of a garden battery than a car battery, that should help them understand.

With the car OFF, so the inverter cannot supply power to the 12v system, if you leave the dome light on for a minute (to remove the surface charge) then close all the doors; the 12v must measure 12.5v or more with a digital volt meter to be any good, at 12.65v, it would be in good shape.

 

Unfortunately the battery testers most shops use will indicate that your battery is fine, when it is not. I am also guessing your 12 v battery was probably not in great condition before it went into the shop. I only get about 3 years out of the 12v battery. They work fine when it is warm but sometimes its well below freezing where I live and you need a strong battery.

Your fuse box cover looks broken the same way ours is. Its not the cover thats broken but the tabs on the box. It would be quite a pain to change the box. The tabs on our 2005 have been broken since we bought the car used. The fuse box cover, like many other plastic parts on these cars are pretty flimsy and easy to break. So I would not be too hard on a shop that does not regularly work on these cars.

The cover should sit just fine on the box without them. Ours has never fallen off and it makes it easier to get the cover off when you need to jumpstart the car. I am guessing your cover will not sit on the box because the little red tab that covers the positive post in the box is still flipped up. The cover will not seat properly unless this it flipped down.

 

Actually, that is backwards. Most shop testers place too large a load on these small batteries and falsely fail them. They are NOT starting batteries!

No Prius uses the 12v for starting, so that comment makes no sense. All you need the 12v for is enough capacity to power up the computers, the hybrid system actually does the starting of the engine.

 

One problem that Prius has in cold temperature with 12V battery is its location. 12V battery sits in cold corner of the trunk. When the lead acid (including AGM) batteries are cold they charge very slowly. And in gen 2 Prius there’s almost nothing heating up the 12V battery. So if you drive short trips cold temperatures and park outside 12V battery is slowly depleting all the time. Especially as SKS system is constantly draining the 12V battery.

 

@valde3, what do you recommend someone do for a gen 2 Prius 12V battery in those circumstances: driving short distances in cold temps and parking outside?

 

First make sure that you actually have a problem:
How short are you’re trips? How long does it sit between them? What’s the coldest average temperature of week/month? Can you actually measure the battery draining then?

You could turn the SKS system off when parking for longer times. You could just use charger to charge the 12V battery.

But the best fix would be to install 12V battery heater. They work by using the charging system to heat up the battery while the battery is charging. Akkulämmitin Arctic Battery Heater Many of those have been installed on Toyota hybrids.

 

I believe the OP is in California where the temperatures are more moderate than Finland, though.

 

I don't know what is wrong with you. You and others jump on my posts almost every time, and thats why I seldom post. I thought this was supposed to a forum where we tried to help each other out?

I am fully aware that the 12v battery does not start the engine. I live in a cold climate. The Prius battery when new has barely enough reserve capacity when the temperature is well below zero (celcius). At 3 years old a dome light left on for a couple of hours will deplete the battery enough that the car needs a jump start. If its left at the airport for a few days it needs a jump start. The keyless system will sometimes deplete it overnight in the driveway.

As for the shop testers. The last time I replaced the 12volt battery I had it tested at Canadian Tire (a common parts store in Canada) and they tried to talk me out of getting the new battery because it tested fine. I knew it was not fine because I had to keep jumpstarting the car. And replacing the battery solved the problem, so it was not a parasitic drain. So from my "personal experience" they can test fine when they are not.

 

I always connect a battery-maintainer (Sears <$30) overnight when car is in garage, although I guess you could run an extension cord since it's low current! - hope this helps - Wil

 

Thanks, I might try the battery heater. I am in Chicago suburbs, so pretty cold in winter. 2008 gets driven for about 12 minutes then sits for a few hours outside some days then drives a few minutes, sits, drives a few minutes, sits a few hours, etc. The 2007 takes more short trips as well - 20" then sitting for few hours, etc. I am not knowledgeable or handy with cars, but I want to take as good care of them as I can.

 

I was really just replying to conversation started by jessiejosco and continued by 05PreeUs and Chodronish

I don’t really have that problem. And it’s basically the same climate here. If I have driven enough that the 12V battery is full I can leave it for six days with SKS on.

Have you measured the charging voltage? Should be 14.4V. No constant drain on the battery? No bad connections in 12V system? Battery posts, ground cable to body, and in ground gable are the most likely problem areas.

 

Actually.... charge voltage is a function of battery temperature, so the colder the system the higher the charge voltage will be. Most modern cars use PCM/ECM controlled voltage regulators to accomplish this, but wit ha Prius you have the inverter doing the job.

The inverter can supply nearly 100% of the 12v system loads, so the 12v should not be discharged unless the inverter is off.

 

But it does not raise the voltage higher than 14.4V as higher voltage would cause all kinds of problems including bulbs burning out faster. Some cars do raise there charging voltage to 14.9V but in cold temperature you could actually raise it a lot more to get battery to charge faster.

Of course the battery does not drain while driving. But the problem is that it drains all the time when Prius is “off” and locked as it’s constantly looking for a key and lots of control units still get power. And then when driving cold 12V battery is charged very slowly.

 

Sounds like a happy end to an unpleasant story. Glad to hear that. As for the plastic pieces under the hood, they break and it doesn't really matter. They could have been already broken for all you know. Mine looks the same as yours and has been like than since I bought the car in July. The red piece in your trunk is from your 12V battery positive terminal. That does not need to be on there for full functionality. In many cars it's missing altogether. Again, for all you know yours may have been off the battery before those guys touched it. It just snaps back on, no big deal. I just had the 12V battery replaced by Pep Boys. These are some tiny batteries as cars go. The guys that worked on my car seemed to be familiar with Prii and did a pretty good job replacing the battery. They even managed to retain things that would be lost while disconnecting the battery like trip odometer readings and fuel consumption numbers.

Hopefully it will all work out in the end.

 

A 0.25A drain is about all most cars are designed to tolerate, I'd bet Toyota designed the Prius for even lower given the tiny reserve these batteries have.

 

Sounds like they are trying to operate a reputable shop, definitely in the minority that would take care of this.

 

Yes you would have a similar climate. I would not say it's a problem and I am pretty sure there is nothing wrong with the charging system on either our 06 or 08 gen 2s. As you said the battery does not charge as quickly in the winter and we often do short trips.
I could use a trickle charger or a battery heater, but for the cost of a battery every 3 years or so it is worth the peace of mind. It's just not very pleasant to come home to the airport to find a snow covered Prius with a dead battery. I do have one of those little battery boosters in the glove box just in case, and it works great to jump start a dead Prius.

The point I was trying to make in my original post is that the 12v battery probably also looses reserve capacity as it ages in warm climates, but you would not notice it unless you happened to leave a door open for a long time.