Low voltage battery...

welcome! if you can find a plug, you could shut the hatch over the wire and hope no one unplugs it.
if not, you only need to drive it a few times a week. you may want to buy a ten dollar digital multi meter, and keep your eye on the voltage at the jump point under the hood.

the battery indicator on the display is for the hybrid battery, not the 12 volt. you don't have to be concerned about that.

i'm not sure what 'maybe the battery is conked out, but the toyota folks didn't say anything about it' means?

 

Personally I am not one of those persons who would run down and buy a $200 battery the first time the battery runs down. They can take several discharges and still last five years or more. It is possible the hatch was not closed completely or the map lights were left on. I still have the original 12 volt battery in my 08 Prius and it has been discharged several times. That's seven years old and counting

Or you could have a battery tender output permanently connected to the power points in the fuse box under the hood and leave the end somewhere in the grill. One of the best is the Battery Tender Junior, available everywhere for around $25. It comes with two detachable ends (see the pics in this ebay ad). Of course you would have to plug it in periodically.
New Battery Tender 12V Maintainer Trickle Charger 12 Volt Junior 021 0123 | eBay

 

It sounds like your current battery might be bad due to insufficient use/charging. Once you replace it, you are going to have to figure out how to keep it from discharging to very low levels since you don't drive it enough.

Perhaps one of these, or something similar can be used to keep the 12v auxiliary battery charged up:

The charge pack could be left connected to the 12v auxiliary battery to keep it charged, then periodically you take the charge pack up to your place and recharge it. Then repeat.

 

Dealers have a nice 12v tester that spits out a test report that dealers have to include when they replace 12v batteries under warranty. I would ask for a copy of that to make sure it was tested properly.

More important to 12v battery health than driving 200 miles per week is how you drive x miles per week. Lots of short trips are draining to the 12v because after powering up vehicle Electronic Controll Unit (ECU - think car computer), priming the brakes, and switching the relay to connect the big traction battery to the electric system, the big battery deliverers a little more than 14 volts to the 12v battery to replenish it. I'm guessing it needs 5 - 10 minutes to replace the electrons in the 12v battery and drop the voltage to more of a maintenance charge. If you are doing lots of short trips in a day and think you are not getting enough run time to charge up the 12v you could just turn the Prius on and let it sit for 10 minutes. A little wasteful and unnecessary for normal (even light) operations, which makes me think that your 12v may be weak and ready for or close to needing replacing.

 

The time spent stuck in traffic is not a problem for the 12V battery whatsoever. The whole time the ignition is on and 'Ready', the HV-to-12V inverter is carrying the entire 12V load and charging that battery. That battery is used only to start the car, and to carry the accessory load whenever the ignition is not 'Ready', such as any time you are listening to the radio while in ACC mode, or powering the headlights during the delay period after the car is turned off, or powering the interior lights.

As already pointed out, the dashboard battery indicator is completely unrelated to the 12V system. It is for the HV battery only.

 

Your 12v battery falls within the basic 36 month/36,000 mile warranty. If you drive 5000 miles a year, then it's not your lack of mileage or type of driving. Your battery is dying and soon will be dead. Our 2000 Honda Odyssey after 5 months would not start and we thought we left the map light on. It died three more times within the next month. The dealer replaced it and that one lasted 9 years.

It only has been driven about a 1000 miles during the last 11 months, usually a 20 mile trip every 2 weeks. Battery is 4 years old and it always starts. Have your battery replaced under warranty.

 

To clarify, it does not "start the car" in the sense of cranking the engine, but it does power everything else that has to happen before the system is "READY."

 

A certain number of 12v batts are just bad and fail early. The Prius batt is barely more than a motorcycle battery to begin with.

In any case, if it has failed once it will certainly fail again soon and leave you stranded. $200 seems like a reasonable cost to avoid that scenario.

 

I continue to be one of opposing views here assuming the battery has been discharged only once and is working fine now. First Toyota apparently tested it (did they?) and aren't replacing it under warranty. You still have at least a year of warranty left, so IF you are concerned, get a portable jump box, learn how to use it (wrong polarity often burns up expensive parts) OR just go back to Toyota, ensure they test it, and then go on with your business. If you said "I left it at the airport for three weeks and it was dead" I would believe it self discharged. Still not a death sentence for the battery.

 

I was thinking along similar lines. Maybe consolidate your trips, try to drive the car on alternate days, or every third day, and combine chores/trips, as much as possible.

Being in a semi-insecure, underground parking garage with no 110 volt plug-in convenient does make a charger tough. I'd start by getting a decent multi-meter, just monitoring the 12 volt's voltage, which is a so-so indicator of it's health.

 

There's something not quite adding up, between the two items made bold. At rest voltage should be around 12.6 volt, 12.5'ish if it's getting on, 12.7 or higher if it's completely in the pink.

I think in a year or so, if you acquire a few gadgets and read up, it'll all be good. One thing: turn off the hatch and cabin lights, never sit listening to the radio, never leave the doors hanging open for any longer than absolutely necessary.

 

i would pull the battery and put it on a smart charger, or see if you can find a mechanic who would do it, if you don't want to invest in a charger. it's possible the battery is toast.

keep an eye on the hybrid battery indicator, if you notice it rapidly fluctuating all the way up and down, it could be a sign of the battery going south, although, you will get an array of lights on the dashboard if this happens. are your mpg's okay?

 

The above is a smart charger and does not require the battery to be pulled to use it but you could if pull it if you have to...

Removing and reinstalling the 12v battery is not as easy as on conventional cars.

 

There's no real difficulty taking out the 12 volt, maybe a bit more digging to uncover it, that's all. While you can charge the battery while it's in the car, in Kondwha's case that may not be practical: he's in a parking garage, no nearby outlet, and not that secure either.

My favourite is CTEK 3300 for smart charger, in the amperage range recommended by Toyo, but there's a lot of choices for sure. The 3300 is older too now I think.

Our car usage is not that different than his, we're lucky if we put on 10,000 km's per year. Our 12 volt is now about 4.5 years, I may replace pre-emptively this fall, but it's doing good. Even with a recent screw-up: the charger manage to pull it back.

I happened to check it's voltage this am, is at 12.7 something. Actually surprisingly good. Again, I'm not complacent, it may not have decent reserve, having had "incident" or two.

 

If you are not driving the car enough the Smart key will drain the 12VDC battery. This is especially true if you leave the 2nd fob in the car because.then the car and fob are "talking" a lot.

One option to reduce drain in the 12VDC battery is to turn the Smart key OFF and use the mechanical key to lock the car. Once you are in the car you can either turn on the Smart key or plug the fob into the slot (near the Shift Lever).

Turning the Smart key OFF/ON is easy as there is a button below the steering column. However, it is a pain to remember to do this.

Personally, I just enjoy driving my Prius (2005) so much I usually just take a short trip of 25~50 miles every weekend.

SM-G900H ?