Original 12v battery?

Another option: with the money you would spend on a replacement aux battery, give your daughter a little lithium jump pack to keep in the glove box.

That will save her from strandings for any reason, whether it's the age of the aux battery or just leaving a map or mirror light on. The jump pack doesn't care about the back story, it just gets her back on the road.

It also provides the flexibility to avoid spending money on the aux battery until it needs to be replaced. If it gets to where she has used the jump pack a couple times without having left anything obvious turned on, she can then decide it's time, drive to a shop, and pick up a new battery.

 

What does the battery look like; can you post a pic? Preferably with hold-down bracket off, presuming it's obscuring some of the text. I believe there's a date code on the Yuasa's. @Raytheeagle has the code down pat IIRC.

If it turns out to be original, I'd definitely replace it. Maybe get her a jump pack too. And do a trail run with the jump pack, a how-to on how to hook it up.

Oh, and a charger, lol.

 

Snap a shot of the top of the battery and we can decipher.

 

Well, YOU have had it for ~3 years, so you know the battery is at least 3 years old.
Under the circumstances, I vote for a replacement........just because.

 

Here's a way to do a preliminary check without a multimeter or load tester.

Turn car to ACC mode (not "ready")
Hold INFO button on dash
Turn headlights on/off (go all the way back/forward) 3 times. (or until display shows diagnostics screen).
Choose Signal Check.
You can voltage and compare. With lights off and doors closed/AC/Radio all off, should be at least over 12V.
With lights on, best if it stays above 12V, but could go a little below and still be considered "useful".

This is all assuming you have the center screen. If only radio, you'll need to get a multimeter (HarborFreight is good enough) and measure from the battery in the trunk. There, you should be at least 12.5V with nothing on (12.7V brand new) and should stay in the 12's with lights and other accessories on. Don't put in "READY" mode, otherwise, it'll be charged by the HV battery and invalidate this test.

Also, don't test immediately after coming out of READY mode. Let it sit idle for 10-15 minutes so the battery level can stabilize or leave lights on for a couple minutes after parking before testing.

For two months in the summer, she should be good if it passes the above checks. If leaving again in the future, especially for Northern/colder climates/months, maybe better to get a new QUALITY battery for her, just based on age. Or at least a Li-Ion battery pack 12V jumper (compact and can use as a 5V phone charger as well). The Prius doesn't need significant amount of 12V power to start up.

FYI, just checked battery prices and, my gawd, batteries got expensive. Was ~$120 a few years ago and now it's $200ish (minus coupons/sales if you can find them). You may also want to consider an Optima yellow top battery (specific model for Prius) for around $230ish if "regular brand" batteries are almost as expensive.

 

In the Prii that I have owned, the voltage displayed on the MFD 'signal check' screen has been consistently around 0.6 to 0.7 volts lower than what you might measure at the battery, or even at pin 16 of the diagnostic connector right nearby.

The difference may vary some from car to car, but I have not found it to vary much in a given car, once you've measured it once (for the same conditions, either ACC or ON).

That's not an awful thing necessarily, just something to keep in mind, and do a little mental arithmetic when seeing the number. In particular, to demand a reading of 12.0 or more on the signal check screen would be to expect a very-ample-to-near-miraculous state of charge in the battery.

 

So, what do we define as a quality battery, now a days?

 

That's interesting as I've replaced 12V batteries in both my (2007 Touring) and my brother's (2004 base) Gen 2 Prius before and in both cases, when they were bad, they were always reading below 12V and with a load (lights/fans) it would be around 11V. They weren't totally dead, but couldn't hold a storage charge longer than a few days before it wouldn't have enough to "start" the Prius. On the brand new battery replacements, I put on a tender for a few hours until full before installing and got 12.7V on the dash and IIRC, around 12.3ish under load. I also got my hands on a used yellow top for the Gen 3 (2010) Prius I had (my second Prius) and that registered 12.5V w/o load and 12V under load. IIRC, the original battery was reading around 12.1V w/o load, but the yellow top was cheap (guy crashed his Prius and salvaged his yellow top) so I went ahead and swapped it.

I know it's been a while since I've been on here so things may be different, but first thing I found in search reflects similar voltage levels as what I described, along with the readings from others who responded to the thread with their readings with aged batteries. (In order I found in the thread in the first few pages) 1 Year old: 12.4V; 80k old: 12.2V, failed battery: "barely registering 12V"...
Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

Also, this youtube video shows similar results, with a 5 month old battery and sitting 10 hours before testing, that read 12.7V via signal check and 12.97V on a multimeter at the fuse box jump point. 12.97 seems awfully high, so maybe his multimeter needs calibration?

I did have everything off (no radio, doors closed, dash lights at low, no lights, etc.) and never stepped on the brakes when getting in the car so it would be as high a possible. I do recall using a standard (Harbor Freight) multi-meter (yeah, not perfectly calibrated), but don't remember being off as much as you are. The youtube video shows ~.3V difference, which seems more reasonable. There may be some variances with people leaving the door/trunk open (dome/courtesy lights stay on), which would put it under light load and show lower values, but clearly, people with 'aged' batteries are able to get above 12V on the signal check screen so there may be something off when you're testing?

@Yea Right , It's been a while since I researched batteries, as evident by my shock in battery prices, so I really can't say. I'm probably going to look for a used yellow top again as I've had good experience with them, even before my Prius owning days. I do recall Optima got sold a few years back so quality may be an issue as well, but that's research for another night. Already WAY past when I should've logged off the Internet.

 

I've had the original 12v since May 2013. Still kickin.

 

The battery's condition is most important, really the ONLY thing that matters. Most big-box automotive retailers that sell batteries will have a pro-level electronic load tester (new gen tester, not old-school load tester), that can determine its current Cold Cranking Amps, and give you a verdict on its viability.

Typically they will test for free. Dealerships also will do the test, but the jerks will charge $75 or more. And THEN sell/install new for a kings rsnsom.

Or you can DIY test with something like Solar BA9 pro-sumer level tester.

Tip: if CCA is lower than spec, it's time to replace.

 

140412w

i'm guessing 04/14/12?

if so, i would replace it.

 

Excellent decoding work.

Probably not your first rodeo based on your time around these parts though .

 

Check with a volt meter?

 

i was only off by 7 weeks