will the 12v battery keep the car from starting

Yes possibly

 

11.27V is a completely dead 12v. ~11.8V is around 80% dead. Time for replacement.

 

This thread has already touched on three questions that are not the same:

1. Is the battery dead (damaged, worn out, no fix besides replacement)?
2. Is the battery discharged (run down near the end of its stored charge, needs to be recharged)?
3. Is this the cause of the problem the OP is posting about?

The OP was asking question 3. The answer to question 3 is "almost certainly no" (if you look in the repair manual, you will see frequent notes to make sure you've got at least 11 volts before taking diagnosis further, and you've got more than that here).

The answer to question 2 is "definitely yes", so it would be a fine idea to give the battery a long, full charge. The answer to question 1 is "maybe yes, maybe no, it can be tested after recharging".

 

Hmmm, well then I will have to disagree with the manual because 11 volts is definitely not enough to start the car. Are you sure you read this in the manual?

My car didn't start when my battery was dead and my battery was showing over 12 volts. However just the voltage doesn't mean anything. Voltage under load tells you how the battery is doing. I did measure the voltage under load while the headlights were on and the voltage dipped below 10 volts. I knew the issue was the battery. I got a new one and the problem solved.

In this case, I feel very strongly that the OP's issue is a dead battery. Of course, I cannot be 100%, I am not there but 12 volt battery is where he should start at...

 

Pretty sure, yep.



My experience has been that some of the car's ECUs will begin logging low-voltage trouble codes in the neighborhood of 9.5 volts (and those thresholds are also printed in the manual, in the "DTC detection condition" boxes for the corresponding trouble codes), and a bit further below that some difficulty starting the car may set in.

However, now that I know "11 volts is definitely not enough to start the car", I'm going to have to figure out where I really was on the days I thought I was driving the car after starting it at 11 volts or so.

 

I see it. I still stand behind my statement. 11v standing voltage on 12volt rated Lead-Acid battery means that the battery is entirely discharged. Under any load, that voltage may dip way below 9-10volt level instantly. It is gonna cause all sort of problems on Prius. Vehicle may not start or the portion of the dash (MFD) may go dark.

Well now if we are talking about a battery say that has a nominal voltage around 10-11volts, then it is likely to start the car because at 11volt level, that battery is partially or maybe fully charged. And it would likely keep its voltage under load. But this definitely not the case for a 12volt lead-acid battery showing 11 volts. I don't know what they were thinking when typing those lines.

 

Key word being "may". It's definitely possible that a particular 12 volt lead-acid battery, reading 11 volts standing, depending on its age and condition and the surrounding temperature and so on, takes the big dip you are picturing here. On the other hand, if that always happened, people wouldn't be starting cars from 12 volt batteries standing at 11 nearly as often as people somehow manage, and always have managed, to do (even in cars with heavier nominal starting loads than a Prius).

There were one or two instances with my Gen 1 where it was necessary for me to unplug the brake pump relays, start the car, then plug the relays back in. But those were times when something had been left on and drawn the battery down far deader than 11 volts.

It's probably enough that they know what they were thinking, which my own experience IRL with the actual car suggests that they do.

It's probably something like this: they give 11 volts as a rule of thumb for deciding whether you begin your diagnosis by fussing with the battery, or just by proceeding to other normal diagnostic steps like getting the trouble codes. If it's below 11, you fuss with the battery first.

If it's above 11, you just go ahead and gather diagnostic info in the usual way. If, in your particular case, the 11 volt battery fell below 9.5 when loaded, you'll find out anyway, from the trouble codes that say so. If it didn't, and you have no such codes, you've saved some unnecessary fuss, and you're already en route to solving the real problem using the info you now have.