Been reading that if aux battery is weak your milage decreases because Prius is spending energy keeping aux charged that could be used to propel the car. Found the "how to check your aux" by various button pushes, and indeed its time to change it out (6+ years now) and hopefully the milage comes back up. Now my real question which I can't seem to get a definitive answer reading threads to is when to change the BIG battery. Prius was brand new 2006 off the lot, so far no major problems and normal oil and bulb replacements etc. and minimal dings to tinfoil thin body shell. Coming up on 7 year warranty or 100K miles expiration in 2013 sometime. Should I be proactive and bite the $3K+ bullet and change it out then, or just drive Prius until one morning the dreaded red triangle of death lights up (or whatever indicates the BIG one has gone to that great battery in the sky). Thanks
If you have 100,000 miles you have a ton of life in the traction battery. No reason to think about changing it out to be proactive.
It's likely your 12 volt needed changing awhile ago. I changed mine on my 06 >1 year ago. Tinfoil thin body shell? I'd disagree. The Honda Fit seems to have thin metal. As for proactive? I wouldn't bother until you get codes that point to it. There are cheaper alternatives as well. See Failed traction (HV) battery, what to do? - Prius Wiki.
(Part one) Some of the many ways a Lead Acid battery can fail would involve continual charging in a Prius. Other failures will retain normal voltage but lose wattage to last any time. Low voltage is one way to fail, but not the only way to fail. (Part two) Unlike the 12 volt battery, the computers watch the HV battery like a hawk, I would wait until they say something is wrong, then get a used/rebuilt battery.
My wife and I have a 2004 & 2005 Prius with 120,000+ miles and the Aux or small battery are regularly checked and they remain in great shape which continues to amaze me! Even the breaks are green in the Toyota servicing report. So far our biggest expense is three sets of tires.
If you're on the original, that sounds doubtful, esp. in TX. I wouldn't be surprised if it reads below 12 volts (or possibly well below) via http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html if left overnight. If so, it needs to be replaced. And no, ignore the >12 volt reading when in READY mode. The DC-DC converter is charging the 12 volt at that point.
