Something is draining the battery

If the 12 volt battery is original it likely needs to be replaced. The loading on a Prius 12 volt battery is different than other cars. Many times if it has been discharged too low even once, it never recovers and needs replacement.
The 12 volt battery just boots up the computers. In a traditional car, the battery is needed to start the engine too.

 

Is it the original battery though? All bets are off if it is: replaced it first, ensure the new battery is fully charged before installing.

That aside:

How are you having it checked? An electronic load test? Or?
How are you charging it, or having it charged? A full charging session with a smart charger? Or?
Any added-on accessories that might be using electricity when the car's off?


You could check for phantom loads: disconnect the neg lead, and splice in (in series) a multimeter set to ampere scale. Commence with ampere, and then switch to millampere, for safety. Run leads out through the hatch and gently close it on them. Let the car sit for 20 minutes, then see what kind of milliamps you have. Around 20 with just occasional spikes is what I've read, which seems typical.

 

Start with an electronic load test I think. Most battery retailers will provide this service for free. Or for DIY, google Solar BA5

 

since you have dash warning lights, you have trouble codes set. you need to go to a dealer, or someone with a prius aware scanner to get the trouble codes.

 

If the car is off there should be no extra drain even if there is a problem elsewhere in the car. The lights you see may be from the computers being confused because your battery is low.

Batteries can short internally from various causes and that won't show on a test. I just had one do that to me. They go dead in a day or less and a test won't show the short. It can be tested for but not by any of the standard battery testing methods.

If this is your 2009 Prius the most likely problem is the battery is too old/has gone bad. They only last 3-5 years for most people.

 

If it is the original 12V battery, then it is 8 to 9 years old. That is seriously old even in my battery friendly climate zone. Your LA area summer heat is less friendly.

Weird stuff happening? MPGs dropping? Test The Battery

 

NO. But when batteries get old and weak, the battery itself will "self discharge" when just sitting there.
AND the standard battery tests don't always give reliable results.

If you MUST get a test failure to convince you, take it in to the battery shop to have it tested near to the time it has failed and do the test WITHOUT first changing it up. They can also test your charging voltage at the same time.

But I agree with the others that the battery itself......or a bad connection on the main cables......is very likely the source of your problem.

 

I'm having this same issue. Sometimes my car turns on other times it doesn't. It 100% is not the 12v battery but that doesn't mean it's not a bad connection somewhere. If you find out what's going on with yours please tell me and if I find out I'll tell you. I think we are having the same issue

 

What exactly leads you to that conclusion ?

MANY other folks have said that in the past........only for it to turn out that it IS the 12 V battery after all.
Even brand new ones sometimes fail.

 

I'd do this also if you haven't. Also inspect both battery terminals and reseat.

 

could be your brake switch.