Prius 12V Battery is a Wimp

I have to agree this small battery is a design flaw. Mine went dead 36 hours after being parked in the garage. Maybe I left the dome light on, or the door ajar; dont know specifically what drained the battery. But I know for sure this is a big problem because without the 12V battery you cannot shift the car into neutral and pull it out of the garage to jump start it or haul the car to the dealer to recharge the battery. The savvy tow truck operator knew enough to place a jack under the front to lift the wheels off the driveway and managed to scoot the car out to the street.

I just read through all of these comments, and it is clear that a bunch of folks have no desire to think about the design, so they jump onto the grammer. The unfortunate thing is that a battery is a "consumable" item, meaning that WE KNOW that it will fail, just like tires wear out. Some sooner than others. My limited experience working on an older toyota that belonged to my son while he was in college convinced me that they were designed with the intent to only be serviced by a dealer. I was so impressed with the "quality" of the design that I would never consider even thinking about owning one.
A squestion comes to mind about the basic design concept, which is"why have the secondary battery at all?" Run everything from the one battery, with a small switching regulator to power those items that need to run a bit with the unit "off", and then take a hint from the way cars used to be, and when they are off, THEY ARE OFF. We really don't need all of those silly gimmics that the marketing people demand be added.

Yeah, the little 12V battery is a wimp. It is designed to allow it to be, as it does not need to crank the engine. I have never replaced the battery in my 2002 Prius even though I could notice it getting weak for years, but it was still holding a charge enough to power up the computers and boot the whole Hybrid system with it's own high voltage traction battery, so even after more than 6 years the OEM battery is still going. Just never leave a light on or other 12V drain or leave the car unused for a month and there is no problem.
BTW, if you can't jump the car, that means that you did not make proper contact. I have had my Prius jumped when I did leave the lights on for too long, it started after a few seconds of jump starting. Sold the car last year. Still regretting it.

You can power a house with a Prius. Google something called "Field Day Prius Power" and "PriUPS". Those ham radio guys love to tinker and hack, eh?

Does anyone know the total energy of the main battery system? Energy is not measured in Watts. Watt-hours or kilowatt-hours would work for me. There is no reason to get too excited about a battery failure- cars have had battery failures for decades. We should get excited about having lots of batteries power a car, though, because they are not environmentally friendly to make or to dispose of, and they are very expensive to replace when they fail. Not IF they fail, WHEN they fail.

Some posters seemed to miss the tell tale "So catastraphic(sic) was the failure that even with a charger attached I could not get more than 2.5V from it." A shorted battery can HOLD the voltage so low that putting a charger on it, jumping the "front terminals" or other gyrations will have NO POSITIVE EFFECT in getting the vehicle moving. That seems to be the problem that we're discussing... Having just gone through the same problem on a very conventional GMC Jimmy maybe I caught it a bit earlier than the folks dissing the user's experience... (High tech Schumacher Battery Charger/Conditioners, BTW need SOME voltage to pick up and charge a battery. Had to put an old fashioned brute force charger on it to get the initial voltage boost.)

Some vehicle, like the Mercedes and BMW, have power management system that limits the current to a certain level after the ignition is switched off and the vehicle is locked for a period of time.
The problem of difficult accessibility of the battery is not new, and you are not the first one complaining that (so do I!). I think the vehicle manufacturer have think about it during design stage but you know nowadays we want smaller vehicle but bigger passenger compartment it can be a headache for designers to house the battery in a convenient place. It is really the packaging problem.

Take a look at one alternative to supplement the tiny battery. Search for "wa8lmf prius"

Regarding David's comments about Grammar:
Good communication is important in both written and design documents. If either contains flaws, the end product risks having problems.
PS: Please reexamine your use of the word 'commenter's [sic]'.

My wife and I just retired our first Prius. Over 180,000 miles of trouble free driving. That battery your fussing about never let us down, in fact we never had to change it. I think Toyota hit the target with the design.

Amazing, all the respondents have their own ideas about the battery technology, it's function, how to get at it and more.
If this is the state of engineering today, we are in a sad state of affairs.
I won't lecture you on being factual and accurate with the statements you make as an engineer.
Oh, there, I lectured you anyhow.

I can not believe all of the commenter’s that missed the point of the battery problem and focused on the grammar. That could be the issue with most of the bad designs we encounter these days.

I believe the battery is NiMH, not lead-acid. They have superior charge/discharge characteristics, haven't researched their capacity vs size, but I believe it is better than a lead-acid. Some of their batteries, according to Toyota, are still going after 8 yrs, 100's of thousands of miles (taxi's). I don't own one -- yet...but thinking, heck it gets better mileage than my motorcycle!

'cmon Ucster, I'm not a native English speaker, however I realized that Karen meant "a battery that powers a comparable sized car" instead of "a comparable sized battery that powers a car"... got it?

ucster. Please read this as:
"this puny battery is about half the size of a comparable sized car, battery"

This last sentence needs work: "There is a kind of irony in the fact that a car featuring 1310W of total energy in its main battery system, can be shut stopped short by a wimpy little 12V battery!"

We learned something here...Toyota needs to better communicate the differences in the use of accessories with this new technology.

"this puny battery is about half the size of a comparable sized car battery"
If it is "comparable sized" how can it be "half the size"?

Help me.

Yes the battery is a bit of a pain to get to. I just replaced the original battery in my 2005 with an optima Yellow Top as a preventative measure. 4-5 years is pushing it for any battery, particularly here in Phoenix. Procedure took about an hour, and that included changing over the terminals to fit the new battery. Used the excellent kit sold here:
www.elearnaid.com/12vo1topraub.html
As mentioned above, if your battery is dead you can supply 12V power (jump pack, charger, or another car) to the jump terminals under the hood. This will allow you to open the rear hatch normally, as well as start the car or recharge the 12V battery. If you can't get at the front, then you do have to crawl through the car to open the manual hatch release from the inside. A bit of a pain, but not a big deal.

Most of the information here is completely wrong. The Prius 12V battery is small, because it doesn't need to provide the current to start the engine like a "normal" car battery. Whether the engine is running or not is irrelevant, since the Prius does not have an engine driven alternator. Anytime the car is "ON", the high voltage system keeps the 12V battery charged and runs all the accessories via a voltage converter. Sitting with the engine off this will eventually run down the HV battery, which the computer does monitor, and will automatically start up the engine to recharge. Where you will get into trouble is if you leave lights or accessories on when the car is off, or in ACC mode, just like any other car. Since the Prius battery is about half the size of a "regular" battery, it will run out about twice as fast if you do that. Like any lead battery, running it flat a few times will greatly shorten its life. Unlike a regular car, when you want to run accessories all you have to do is leave the Prius ON (with the engine auto-shutoff), and you will never have to worry about running your battery flat. As you say, you've got ~1.3kWh of battery in the trunk so you might as well use it! Not only will this run your radio or reading light roughly for ever, you can look up the stories about the folks in last years hurricanes that were able to power their entire house of their Prius for days on end w/o running down the battery. The Prius is a revolutionary vehicle, and that means there are some changes to get used to. IMHO thats a small price to pay for the 52mpg I've averaged over the last 2 years.

I've just had a similar experience with my 2004 Prius. While packing for a trip, I had an electric cooler plugged in for about 20 minutes. When it came time to leave, I couldn't start the car. I tried jumping the Prius from another vehicle (including 5 minutes at 2000 rpm) without success. After being towed to a dealer, they jump started it about 2 hours later. Why they could jump it and I couldn't, I don't know. The battery now accepts a full charge. Now I'm going to be more careful about using the 12V power.

Lead is not good for the environment. Prius 12V battery has half the lead than a normal car.
This 12V battery is only used to boot up the computer to start the car.
If you want to listen to the radio for extended hours or use the power window frequently, you can use the HV battery. Press the power button twice without holding the brake pedal.

While the 12V design is not perfect, this is mostly PEBKAC. Author could have used the front terminal (under the hood) to energize the 12V system, which allows one to use the electronic lock to open the hatch. This is, of course, described in the owner's manual. The bit about the service department is a useless anecdote (why would a service dept. know about successful DIY replacements?). I don't know what the author is getting at in paragraph 5. The 12V battery will not drain if you leave the car on (assuming the battery can hold a charge and there is gas in the vehicle).