At what point do you replace the 12v battery?

The first indication of trouble will be a failure to start the system. I'm not a person who waits for that to happen, YMMV.

 

See post # 2.
First time starting difficulty is the clue.
Also if listening to the stereo for 20 minutes causes a no start is a clue.

Below is not Prius specific but based on my experiences:
Conservatively batteries last 3 years...so change every 3 three years and one is usually "golden" and never stranded.
Typical life is 4 to 5 years.
6 years...you are on borrowed time IMO.

 

Get a half-decent digital multimeter, and periodically check voltage. If you see numbers 12.6 or higher you've got a very healthy battery. 12.4~12.6 is "middle aged". Lower than 12.4, it's time to start shopping.

To really check health, look into some sort of digital load tester, like (prosumer level) Solar BA5.

To recover from partial drains, an intelligent charger in the 4 amp range is good.

All of the above will set you back close to the cost of a new battery, but keep you from being in the dark, and will last foreverer. Good for helping family and friends too.

 

2012 w/ 42k. car is garaged and doesn't see much heat or abuse. 12v is still in good shape, but i'm more in the camp of; if it ain't broke, don't fix it.

i have found on my past two prius that if you don't run the battery down, it's good for 7 or 8 years.
of course, that includes time spent at the dealership before purchase, where you may not know if it was run down.
i do check the standing voltage every 6 months or so, as mendel mentions above. takes 2 minutes under the hood.

 

My 2010 12v battery lasted 64 months. Of course it failed on a -5F morning in January. Luckily I didn't really have to get anywhere important. AAA got me started and and Toyota replaced the battery.

 

A jump pack is another good item to have. Once you get all this crap you rarely need it, but get to play hero, lol.

 

deleted (double post)

 

Trip meters (if you're tracking mpg it's good to rely on the odometer, it's dependable), and maybe auto-up on the windows will need to relearn. You can get an OBD cable "memory saver" that plugs into jump pack.

 

Gathered Prius Vee'erz - FWIW FYI: 2012 Prius v Five put into service December 2011, 47,000 miles on the odometer, operated in Central Florida climate, drive it "Hybrid $mart," Accessory Battery remains robustly healthy.

 

My experience has been these batteries are fine for six years+ as long as you don't leave anything on, like a vanity light. I recommend a 1.5 Amp battery minder 1510 to charge it along with an OBDII adapter so you don't have to access the battery or pop the hood to charge. This also makes it easy to check the voltage with a voltmeter, and as long as it is above 11.8v you are not in immediate risk of being stranded. If you charge it regularly it should be good for a very long time. The battery minder has a pulse desulphation feature and operates below 15v so it should be safe for the prius while extending battery life (it brought voltage up on my four year old battery from 12 to 13.2 volts gradually). Be aware that the OBDII port should not be subject to more than 3amps of current. The Battery Tender brand of OBDII adapter works with the Battery minder connector. There is an inline fuse that can get loose and then blow out but fuses are cheap (happened to me once, noticed it was loose and blown after I noticed it didn't work anymore). Anyways the batteryminder costs about a third of the cost of a new battery and an auto ranging voltmeter is $20 on amazon, manual range voltmeters can be less than $10. Even if you eventually replace the battery, you should make sure the new one is fully charged before you install it.

The battery just boots the computer so cold cranking amps isn't important, voltage is. Keeping voltage above 12.4v prevents most sulfation. Also the OBDII voltage is live while the car is off, the 12v charge port turns off.

 

I have a 2012 with 109k miles on the original battery. I don't want to spend foolishly either. Although, I admit, a dead battery is no issue for me to replace and I am extremely lazy. How is the 12v used differently in a Prius?

Anyway, my biggest issue is regaining the backup beep. I don't go to the dealer anymore so I'm going to have to buy an OBDII reader as well. I bought a portable jump pack so I can continue using the original 12v battery as long as I can. I'll probably cap it at 6 years as I don't know of anyone making it to 7.

 

In the prius the 12v boots the ecu. The ecu then tells the hybrid system to spin the engine and power up the inverter so you have 13.8+ volts from the inverter as soon as the ecu boots. Only once the hybrid system has the engine up to 1000 rpm does it add gas and spark and by that time the 12v battery is just chillin. This is why bad batteries give no hard start warning signs in a prius. When the voltage on the 12v gets less than 11v you can get weird glitches from the computer not getting enough at bootup or suddenly fail to boot.
I never disabled the beep but The obd reader i got that works with torque over bluetooth is ScanTool 427201 OBDLink LX Bluetooth: Professional OBD-II Scan Tool for Android & Windows

 

First indicator on our 2008 was the "red triangle exclamation" light came on. Actually drove it for almost a week while trying to figure out why this light was on. Toyota dealer said it would go off after a couple days. Whaaaaat????? Obviously they failed on this one!! Luckily, it wouldn't start while parked in the cozy garage. Lucked out here! I replaced it with a Optima Yellow Top 12V battery. Maybe overkill but should outlast the car.


Tapping on my iPhone

 

just tested mine after sitting overnight, 12.87 volts.

 

That's a pretty good voltage. I am getting the 12.22V after overnight on my 2012. I also don't do long trips to fully charge the battery. I keep it on maintainer occasionally to fully charge the battery, but after days it goes back to its regular 12.2 resting voltage. I am not sure if I would get a higher resting voltage by driving more.

I don't really want to buy a new battery yet. I still think it can last some time. Just weeks ago was -12F and it started just fine. Thinking about getting a battery pack.

 

you probably have a few more years, if you don't drain it. one of the benefits of pip is a top up every time you charge.

 

Our car sometimes sits 3 days, maybe 4. If it's late in the day, after a day without driving, I've taken now to just hooking up the 12 volt charger. I let it run to completion of it's cycle, then leave it connected until we do need the car.

 

So a fully charged battery read 13.30V. Immediately started driving in 35F weather and within minutes I was on EV. A 12 min drive gave me 51mpg.

With a weak battery in the same weather I get 43mpg driving the same distance. The EV would probably kick in after 10min or so.

So yea, the weak battery does effect the mpg and its really obvious.