12V battery dead?

Yes, if you want to continue the drama described in your OP.

If you prefer the car to actually start when you depress the POWER button, then replace the 12V battery now.

Or, charge the 12V battery overnight using a charger with a 4 A setting. Driving the car for 40 minutes plus 90 minutes is insufficient to fully-charge a depleted battery.

 

I get the impression that your Prius is trying to tell you something! …like "PLEASE REPLACE MY AUXILIARY (12V) BATTERY!!! …PLEASE!!!! …PLEASE!!! …pretty PLEAseeesss… ……(…and as W. Shakespeare might say, "The rest is silence!!!")

 

No battery in that scenario would have bounced back completely. You could try charging: use a smart charger, let it fully charge (if it will), not exceeding the max amperage for charging recommendation in the Owner's Manual. Then drive it for a day or two, before doing any testing or making a decision.

You can do a rudimentary check of voltage with a digital multimeter. Better would be a load test. There's now prosumer level digital load testers, Solar BA5 for one. Or have some pros do the test.

Or just do the charging session, and see how it goes. Still, a 3.5 years old battery with several months downtime is not going to last much longer.

 

optima's are supposed to be able to handle full discharge better than oem, so i would put a trickle on it, and then keep an eye on it until comfortable one way or the other.
next time you are leaving for a few months, disconnect the neg bat. cable.

 

When you decide to replace it, Toyota currently has a Prius Battery with a 84 month (prorated) warranty.

 

You need to fully charge the battery, let it rest and then check it. You can charge it with a battery charger or by leaving the car in READY mode. In either case the battery should be charged for at least 12 hours. After charging, wait at least 8 hours and then check the voltage. The resting time will allow any surface charge to dissipate. Checking the voltage immediately after charging it will give you a high false voltage.

Letting it sit discharged for an extended period of time is bad for any 12v battery. The Optima yellow top is supposed to withstand this treatment better than a conventional 12v battery, but you probably have lost some capacity. I have a 4 1/2 year old Optima in a low mileage 2007, and I now get 12.8 volts when I check it after allowing it to sit overnight. I have discharged my battery once, but it only sat overnight before I recharged it.

If it were my car, I'd replace the battery if it won't hold 12.5 volts or so after a full charge cycle.

 

red hot with a hissing sound? that battery would have been out of my car before you could say, 'where's your mama'. and an immediate phone call to optima.

 

So now the drama is going to include a battery that catches on fire?

Get rid of the battery, now!

 

Get rid of that battery right now! It looks like you posted that tidbit while I was composing my previous reply in post #8.

 

I would buy a new battery DESIGNED for your Prius before driving it again. Neither Red hot nor hissing should ever describe a car battery.

 

i hope the o/p is still with us.