Electrical (Battery) Mystery

Hi! New to the forum. I have a 2009 Prius. It's got about 120K miles on it, still with original hybrid battery. Bought it used from its first owner back in about 2016 when it had about 88K miles. Been pretty trouble-free really. I know a little bit about cars, but next to nothing about hybrids. But I've got a mystery on my hands. So let me take you through it.

First, I'll start by saying that about 8-10 months ago, we went out to the car and the 12v battery was dead. I hoped for a cheap fix, so I got a Duralast 8A A/C charger. Let it charge overnight. Didn't take. So then I went out and got myself a new 12V battery. Installed it. Fine!

We were living about 1K miles away at the time. Since then, we moved. Drove the car the 1K miles. Fine. It's our daily driver, but we don't drive a ton. It's been fine. Fine, fine, fine. Worst I can say about it is this: when we were hauling the family and cat and lots of stuff in the Prius across the Mojave Desert, the A/C stopped blowing cold. It occasionally did that throughout this brutal Denver summer, but 95% A/C worked.

OK BUT...
On Monday morning, I went to drive my kids to school. There'd been a light frost overnight. Clicked the fob to unlock the doors. Nothing. Took the key out of the fob, manually unlocked. Tried to start the car. DEAD AS A DOORNAIL.

Put the kids into my '79 Alfa and drove them to school in that. Got home and investigated. The kids had left the dome light on from the prior Thurs or Fri. Between that and the plunge in temp, I figured, okay, no mystery. Hooked up the Duralast charger and let it charge all day. NOTHIN'. Not even a sign of life.

Called Autozone. Said hey I bought a battery from you guys less than a year ago at a different location yadda yadda, and the guy looked up my account and sure enough the battery was under warranty. Removed the 12v from the Prius, drove it over to the Autozone (in the Alfa) and was given a brand new battery no questions asked. Came home, popped in the new battery, and the car came back to life. Drove it around for 20 minutes or so. Right as rain.

Tuesday car was fine. Wednesday (YESTERDAY) car was fine until...at about 6:30pm (would have only been three hours since my wife had last driven it), my wife says "hey that's our car alarm!" Now, we had JUST noticed the sound. Fairly sure we'd have noticed if it had been beeping and beeping for a long time.

The car was parked right out front. I grab the keys and go out to the car. I notice that the car is not so much beeping as weakly bleating like a tired sheep. I hit the unlock button to get it to stop and it sort of doesn't take the first few tries and then it finally does. There's no sign of any forced entry or anything. I get in the car and put the key in the slot and foot on the brake.

What happens next is that the dash lights up like a Christmas tree. Including the BIG RED TRIANGLE OF DEATH. I try starting it, but no, it won't go. Then the lights start dimming and flickering until, finally, they all go out except for the Check Engine light. I'm thinking, shit, that's not good. (Also: no dome lights were left on.)

Hook up the Duralast charger and try charging it for a couple hours just to see if I can get any sign of life, but there's none. I can't charge it overnight because we live in the city and someone will just walk off with the my charger and I really don't feel like having my catalytic converter stolen or the car stripped. So I unhook the charger and lock it up for the night.

Wake up this AM and get the kids up for school. Figure I'll just try to charge it from 7am - 7p to see if I can get the battery back to life. I drive the kids to school in the Alfa (which is pretty cranky in the morning, but go figure---it works) and I get back at 8:30am. I am surprised to see that the charger now says CHARGING COMPLETE/MAINTENANCE CHARGE.

Hell, that was fast! I gleefully unhook it and go to fire it up but....nothing. Just clicks coming from the driver side of the engine compartment. And that seems to be when I depress the brake.

Frustrated and confused, I pull my Alfa right up in front of it, hook up the jumper cables and figure I'll just try to jump it. Was NOT expecting it to work since the charger said my 12V was juiced up. But almost immediately, the car comes back to life. Give it a few minutes with me revving my Alfa, and then disconnect it. Prius stays running.

So then I take the Prius out for a nice long drive (45 minutes). Park with engine running and do the headlight off-on-off-on-off-on thing while holding INFO and get to the screen where it tells you the 12V status. It's running at around 14.4V!

Drive home. Park it. Open the trunk take out the luggage compartment and the battery panel. I tighten the bolts on the neg terminal and ground and on the positive terminal. Then I grab my multimeter and with the trunk open, take a reading. 12.45 volts. Mess around a few minutes then go up front and pop the trunk and take a reading from the jump terminal. 12.38 volts.

So...low. Not fully discharged. But not 14.4V.

So now I have it hooked back up to the Duralast BEFORE it's dead. I'm figuring I'll give it 5-12 hours to see if I can get it fully charged. But, I don't feel like I've addressed the problem. And I am not sure I know what the problem is:

-Is it a bad battery that won't hold charge? Would have thought so, except that this new battery replaced one that was less than a year old. Was that also bad? OR was that just a coincidence considering my shrimp kids really DID leave the dome light on for 3 or 4 days?

-Do I have a parasitic leak/draw somewhere? Do I have to hook up my multimeter in series and start pulling fuses?

-Or is there something else?

I'll update this post as it develops, but I'd love any predictions/advice/etc.

Thanks!

 

Update: car started fine this morning. That, in no way, indicates anything is fixed, of course. But currently, car is still operating. Again, I welcome any theories/suggestions.

 

Sorry, help me understand this one. Do you mean, the car will need to be quite literally turned on and just left on for 15 hours? Or is having "the car in READY" something different?

Right now, the car is completely OFF but it's in my garage where I have the charger hooked up to jump terminals. The car started fine today after 30 hours of sitting. But I just figured maybe a good 12 - 15 hours of charging might help bring it up to full charge if, indeed, it still partially discharged.

But what you are saying sounds different. If you indeed mean that it should be left on for 15 hours, then I'm wondering if I can do that in my garage. We live in a city where things "walk off" if they're not locked up. So I would feel comfortable leaving my garage bay open with the car running (as it faces the alley), or with the garage door open at the rear of the garage, as people walking along the side of my house would be able to see the garage was open, hop the fence and then, well, you guessed it.

On the other hand, I'd be concerned about running my car for 15 hours in a garage with all the doors closed. That seems...hazardous.

 

When I removed the battery, I used a 10mm socket to loosen the terminal clamps. Didn't force anything with a flathead. But you're the third person to suggest that it sounds like a loose cable or loose ground. So I did tighten them back up. The positive one wasn't loose,but I was able to rotate it (and those components). So I cranked them 'til it was stiff. Again, the negative SEEMED secure, but I tightened the ground and clamp as much as possible.

 

Quick update: I had my 8A Duralast charger on the thing from 7pm last night 'til 9:30am this morning. Had checked on it at around 11pm last night, and it was still in "charging" mode. This morning, the status light was a brilliant, gently and slowly pulsating emerald green, showing that the battery was now fully charged and the charger was in maintenance mode.

If I'm incident free for the next week or two, I think I'll chalk it up to loose clamps/grounds. If not, then it's either got to be a bad converter or a parasitic leak.

On that note, I wanted to ask a question. I've seen it asked in this and other forums about a thousand times, but I'm not sure there's been much consensus: When the car is off, I've noticed that the security/immobilizer light is flashing on the dash. To be clear, this light shows an orange/red car with a key inside it.

Now, I absolutely know 100% that this light was not constantly flashing when we bought it used back like 4 years ago. But it's been doing it for a decent amount of time now. It seems like this is normal. It's a deterrent to would-be thieves ("Move on, buddy, this thing's got an alarm.") So I'm not concerned that it's a warning of something being amiss. But my question is: how much of a draw is this light on the battery?

(At this point, of course, I'm like Scrooge when it comes to my amps and volts.)