At what voltage should the OEM 12 volt battery be replaced?

Would replacing it be a financial hardship? If not just replace it and be done with it.

 

If you read the reviews for that expensive OTC digital tester, one person said he tested a battery, got a good report from the digital tester, then put a load test on the battery and it fell flat on its face.

The technology to accurately predict a battery's life may not exist. Look at Boeing...

One modern single engine, 2 bus, 2 battery, aircraft calls for replacing one of the batteries every 2 years, staggering the replacement.

But what's the worst a Prius will do? Not start one day without warning?
When that happens buy an Optima. Or buy one now for not-so-cheap insurance.
Or test your battery with the 'headlight load test' once a month. But even then you could have a failure without warning. Dang Lead Acid old tech batteries.....

(Keep in mind there are many, many other systems that will leave you stranded in a Prius. Life is uncertain...)

At least it won't make your cockpit go black some dark and stormy night....

 

I've had my car for just over four years. I plan on replacing the battery with an Optima Yellow Top this fall, just for peace of mind. Better safe than sorry.

 

If the battery reads 12.1 volts @ 68F after the 30 sec headlight on test with the power switch off, and doesn't have any reason to be down in charge, that is you have driven it for at least 30 minutes in in the last 24 hours (which would have charged the battery to the correct level), and then let it rest for at least an hour and nothing was left on to diminish battery charge, then with 12.1 V readings, I would be concerned that the battery is near the end of its life.

The Toyota test specifies recharging perhaps because in a dealer repair environment doors may have been open leaving dome lights which caused the battery to drain excessively.

 

@rdgrimes: thanks for this valuable information. I got the idea how the heavy use kills the battery but still I am not able to understand how the lack of use kills the battery? And also is there any way to fix the flattened battery?

electronic assembly

 

Well you're talking chemistry now, which is over my head. Suffice it to say that a lead-acid battery likes to be used, and undergo steady cycles of discharge-recharge without extremes. Temperature affects things too, as is always the case with chemistry. On a hot summer day a battery can just die without warning, and given the absence of any health indicators like cold cranking it can be a nasty surprise.

I've had them seem perfectly normal one day and dead the next. Or normal one day then sit idle for a couple weeks and be completely dead.

Once damaged, there's no going back and you need to replace it. I've been known to replace them on spec because they're 3-4 years old, simply because a dead battery is no fun. So my advice is: when in doubt - replace it.

 

I just had my 12V fail a month ago without warning. The car has 40,000 miles on it and is a little over 4 1/2 years old. No warning signs such as poor MPG or trouble starting. It was a hot day (94 degrees) and after driving the car from work to a shopping center, it died when I tried to leave. When I pressed the start button nothing happened except a high pitched whine coming from under the hood. The headlights were very dim. I smelled a slight sulfur odor in the car. I call the AAA dude and to my surprise he was very knowledgeable about my car. He tested the battery which read 12.57V, Measured CCA - 111, Rated CCA - 325 (battery replacement highly recommended). He then attached his jumper / tester to the jump terminals in the fuse box and the car started right up. The real surprise was on the way home, the slight sulfur smell turned into a major stink. When I parked the car in my garage and pressed the start button to shutdown, the high pitched whine returned and the stink was overwhelming! The whine noise seemed to be coming from the brake pump area. I decided to remove the battery and place it in the far corner of my back yard (down wind of the house). Upon disconnecting the battery, the noise stopped. I ordered a new Optima Yellow Top from eLearnaid and when I connected the terminals, the dash instruments lit up and the engine started! I wasn't expecting this and I rushed in to shut it down. I brought the car in the next day to the dealer to do a recall and the tech inspected the electrical system. He said everything looked good and no codes were thrown. I don't consider the OEM battery to be cheap. From my experience even the highest quality batteries can fail after 3-4 years. Rock-on.

 

I see a gulf of contradiction between the first and second sentence. Anyway:

If the battery's consistently under 12.5 volt, I'd go shopping.

 

Mine has been right at 12.5V with no load since about a month after I got it. (It would measure lower with the trunk open and/or interior lights on.) Four years in, and apart from one event where it discharged down to <1V (2 months after I got it), it's still going strong.

I think it's more of a personal preference. If my battery dies, it'll probably be when it's parked at home unused for a few days, and it won't cause me too much trouble to have to jump it. If jumping it would be a big problem for you, then maybe you want to replace it sooner. But even 12.5V seems a bit high for that, when Toyota says that 12.5V is between half charged and fully charged.