12v Battery Wierdness

I converted to the Optima Yellow-Top AGM battery (elearnaid kit) when my 2004's original battery died in late 2008. I feel pretty lucky, I got 4 1/2 years out of the original battery. Today, 3 1/2 years later, I got back from a 3 day business trip and opened the door to the Prius and nada, nothing, zip. Not even the dimmest inkling of any power at all. So I throw the booster pack on the jump terminals under the hood and go back and open the door. When I open the door I hear a relay click and everything leaps back to life. I press the power button once and everything is like normal...clock is set, stereo is resuming on the CD where I left off, etc... Before kicking her on so I can back in and easily attach the BatteryMinder I notice the booster pack isn't sitting where I want it in case the engine starts before I can pull it off of the terminals. I get out to readjust and while doing so I pop off the negative battery terminal. Much to my surprise the stereo is still running and the lights are on in the cabin. What the?? Without reattaching the booster pack I hop back in, press the power button again and do the sequence to get into the diagnostic screen. Ok, battery is at 10.9v, certainly drained down, but enough to run the computer, screen, stereo and the A/C fan is running slowly. I closed the door kicked off the A/C and the voltage bounced back p to 11.3v. What the heck, so I attempted to start...tada, it did.

I think I didn't get the driver's door completely shut on Tuesday morning, so I think I shot myself in the foot and I know I may be in for a new battery considering the age and the 2nd time I've flattened it, but what is going on here? Did the battery get so low that the Prius kicked open a relay to prevent the battery from being completely flattened? I've nearly flattened the battery by leaving a map light on for a week at the airport. I still got a very weak dome light when I got into the car, but it wouldn't start without a jump.

 

Re: 12v Battery Weirdness

I'm not too worried about the Optima. I do have a BatteryMinder wired in so I put it on 2A overnight and will see how the voltage settles after 12 hours at rest. I'll report back on the results.

What concerns me is the weirdness where the Prius appeared to be completely dead...not even the dimmest dome light, but after having a booster pack on the terminals for seconds I suddenly had power back. This somewhat acts like a traditional vehicle when the battery cables get severely corroded and while you get accessory power OK, when you try to turn it over you just get a click and lose all power for 10-20 seconds, but then the stereo and dome light all come back on. I'm going to get into the battery compartment to check for corrosion and/or lose connections, but there are no other signals of this.

 

Just to report back on this anomaly. The Prius appears to be just fine and operating normally. I got into the battery compartment and found nothing amiss...no corrosion, no loose connections, or anything else out of the ordinary. After the full charge via the battery minder we're back to perfectly normal operation. I'll report back if there are any other occurrences.