Is 11.5 volts good?

Yes, your battery is ~ 6 years old if original and it would be reasonable to replace it as a preventive measure.

 

At that voltage its so low you may see a red exclamation point in addition to all the bulb check lights on the dash that come on when you first start the car.

 

Your low voltage is either due to needing a new battery (very likely) or due to many short trips leading to a low State-Of-Charge (SOC). My OE 12v lasted over 8 years and 200k miles of service before I replaced it with an Optima Yellow Top. The OE battery still holds a reasonable charge a year later and I use it to start my 16hp twin cylinder B&S engine (made in 1979) that drives my generator. I attribute the long life of the OE battery to never subjecting it to extreme discharges and many long trips giving the slow Prius charging system time to keep it fully charged.

Letting a lead acid battery sit at a low state of charge will cause sulfate crystals to grow on the lead plates. The resulting reduction in active plate surface area lowers battery capacity and is often the cause of short battery life.

JeffD

 

Hi Rich, it would really help to know if that voltage was measured in "Acc Mode" (one press of start button without foot on brake) or "Ig-on mode" (two presses od start button ...).

If it was measured in "Acc mode" without any other electrical loads on (lights fans etc off) then yeah that's really low, like the point where you may start to see anomalous behavior. If it was in "Ig-on" mode then not quite as bad (probably won't see anomalous behavior yet), but still pretty low and about time to put in a new battery anyway.

 

You might hook up the 12V battery to a battery charger overnight, using a maximum charging current of 4A. Then after letting the battery rest 8 hours or so, repeat your voltage test when IG-ON. If you see improvement then you are good; if not then you might consider replacing the battery at a convenient time.

Your fuel economy will depend upon ambient temp as well as the kind of driving you do. Very short trips, very high speeds, the need to climb and descend substantial terrain, and frequent braking all are not helpful.

 

The above comments are helpful so I'll add one that is less so. It's called a 12volt for a reason Not an 11.5v or a 16v, but 12v. You've done well for it to keep going this long.

 

Hi Rich. The issue seems to be that the voltage can drop fairly significantly, just a for a half a second or so, as the system boots up when you're entering ready mode. So a weak battery that's only giving 11.5 at "ig-on" might transiently drop to under 10 volts during boot up. This seems to be what causes the problems.

This is one of the reasons that it's good to know both the voltage in "acc mode" and also at "ig-on". Observing the amount by which the voltage falls gives useful information about the battery condition (resistance/impedance).

BTW. My 12V battery is pretty old. I get 12.1/12.2 at "acc mode" and 11.8/11.9 at "ig-on" (after the car has been sitting for at least 6 hrs, all other electrical loads off). Notice that my voltages are also a little low, but the 0.3 volt drop (from acc to ig-on) isn't too bad. So despite the fairly weak voltages, at least its impedance isn't all that terrible. It hasn't caused me any problems yet, but I suspect I'll need to replace it in the next year or so.

 

What kind of of problem will you have if my 12v battery is at 11v

 

Best way to measure voltage would be to use a multimeter, with the car off. If it's reading under 12 volts thus it's time for intervention. If it's say 4 years or older, probably simplest to just replace.

 

The Prius is a 4 wheeled computer that uses the 12v battery for power. The same thing happens to a Prius that happens to your cell phone if your cell phone battery is at 1%. It either gets really flakey or just doesn't want to work.

Some symptoms of a very weak 12v battery:
The SKS system will not allow you to open a locked door.
No lights come on when you open the driver door.
Pressing the power button has no response or unusual response.
The dash lights blink on and off (especially the gear position indication)
Trunk will not open.
Exterior and/or interior lights won't function

 

A low 12v battery voltage can produce several intermittent problems. One that often occurs is a parking pawl alert as it takes significant current to withdraw the parking pawl (locks the front wheels in "Park") when you are parked on a hill.

JeffD

 

Eventually it will fail to start.
Not knowing WHY it reads 11 V, it might even stop running while going down the road.
Get it fixed.

 

I have had intermittent starts with the cheap 12V battery degrading after 2 years now at 11.6

DO NOT BUY AAA batteries!!!!!!!!!!!!!!!!!

 

argh! i just had them install one in my truck

 

Bisco ... Just better be ready to replace it in 2 years exactly


I am not going to buy that AAA battery crap again.


---wire

 

i am trying to sell it, hopefully sooner than 2 years

i never knew there was such a thing as a poorly made battery

 

The cost of the AAA battery was $241 the Toyota OEM battery was $260 tax included. Better get the good one compared to the crap AAA battery.

---wire