Horrible gas mileage :(

It could be related. I think you learned your lesson to leave the car in READY mode. Some of the poor gas mileage from a bad battery is likely due to the car wasting energy to charge up the 12 volt. The rest seems to be from one (or more?) of the ECUs getting into a bad state. At least, that seems to be the hypothesis.

Do you have a relatively slow charger (I think 4 amps is the limit) to charge up the battery to see if it can be revived? If not, you're going to need to replace it.

 

Yes. Leaving it on in READY is fine. Leaving it in IG-ON or ACC is bad.

If you're going to go overnight security again and it's say 8 hours, you may as well leave it on in READY for 8 hours to see if that charges up your flat battery well enough.

Even one of our resident repair experts has suggested READY for 8 hours (No sure where my battery is? 2006 Prius | PriusChat). I'm definitely no repair expert... not even an amateur.

 

^^^
Don't know about where for a good deal but Specializing in: dictionaries including the Collins Cobuild, Oxford English Dictionary (OED), Webster's & Random House is in So Cal. Battery at Optima DS46B24R Optima battery direct fit replacement for Prius 2004 + with Instructions.

Another So Cal guy drove to them to pick up his Yellow Top. But... you're still going to need to keep the car in READY if you're going to run stereo, accessories or whatever for more than say 10-20 mins.

There's a night security guard for a supermarket/mall area near me w/a Gen 3 Prius. I've seen him BLAST his stereo real loud when he's sitting in his Prius. It didn't look like he was in READY mode. It seems like he's doing it for an extended period of time (blasting when I go in the store, still blasting when I come out). If I see him do it again, I'll have a chat w/him about them. Maybe he's already learned the hard way that he's killed his 12 volt...

 

Advance Auto online. 15& off + $50 gift card. PU at store. Make sure its charged when pu as not big mover and may have sat on shelf. Call ahead & request charge.

 

a u referring to this? Buy Autocraft 12 Volt Battery 31S-750 at Advance Auto Parts

 

If you don't buy an Optima Yellow Top or an OEM Toyota replacement, be careful about buying other batteries which might be regular flooded lead-acid batteries and/or have no vent tube connection.

Take a look at the posts at Toyota Dealers are Killing the Golden Goose | Page 5 | PriusChat and some of the ones around it along with Replacement of 12 volt auxiliary battery | Page 2 | PriusChat.

 

Autozone has a 20% off coupon today and they carry the optima yellow top for the Prius.


Autozone- today only 20% OFF orders of $100 or more - automotive, coupon

 

I would bet that there is a pretty good chance to catch him off guard, his hearing is probably shot, and if he is in the car, even better, he will never hear you, or the bad guys!

 

Hasn't this been a common problem with people who have recently modified their stereos? Battery failure, poor MPG and even car not starting?

 

In READY, IG-ON or ACC?

 

Deep discharging a lead-acid battery (the 12-volt in Prius) causes corrosion on the plates, consequently reducing the surface-area available for electron transfer. In other words, there's no fix available. The damage is done. Replacement is required once you start experiencing vehicle operation issues... which sounds like you're at that point now.