Prius with 193K miles and repeatedly dying 12V: repair or donate?

Trying to figure out whether to fix or scrap our 2005 Prius with 193,000 miles and a mysterious 12V battery problem. What would you do in my situation?

We've had a recurring problem with the 12V accessory battery: I have replaced it myself three times. The last failure was the most mysterious, because after replacing the 12V battery for the third time, I conducted nearly daily voltage tests using the Prius' "secret" vehicle signal check. Voltage coming off of the 12V was normal, if not high the entire time in all of the Prius's various starting and ready modes. I wired up a solar panel to trickle-charge the 12V battery when we weren't around, and we plugged the 12V in at night to a trickle charger to keep the battery topped up.

Nonetheless the day came a few months after the latest replacement when we had our usual 12V failure symptoms: the time on the clock started resetting, the car didn't turn on on the first press, the Prius computer rebooted, and eventually the car wouldn't start without a jump start. This has been super-frustrating, and googling around, I haven't been impressed by people's reported experiences trying to have the dealer try to resolve such problems. My worry is that a dealer repair person will change the accessory 12V battery without fixing the underlying problem. I would suspect a parasitic draw somewhere, but voltage tests when the car was overnighted without any kind of charging did not suggest a serious voltage draw on the 12V battery.

Other details about the car: our keyless entry hasn't worked for several years (dead batteries in the key fob?) and we open the car manually using the key each time. We stick the fob in the slot each time we want to start it. MPG is comparatively low vs. other Priuses, perhaps 44-45 MPG, and lower when the 12V battery starts to die (37-40). I have had the car serviced (oil, etc.) like clockwork every three months; recently, I've noticed the engine is starting to burn oil a bit (level is reported low on each change).

Most of the driving has been done in the flats and the heat of California's Central Valley, and we've put something like 120,000 miles on it just in the last five years. Besides the fact that the 12V battery keeps dying, the car has been very reliable.

We just inherited a free Prius with just 60,000 miles on it, and I have a 1993 Camry with 150k miles on the chassis and maybe 60k miles on the replacement engine I had dropped into it a few years ago. The Camry just passed California's smog test and seems to be running well. We don't want or need three cars.

Our choice is to try to fix the Prius with 193k miles on it and donate the Camry (which gets about 20 miles to the gallon, the horror...), or keep the Camry and donate the Prius. My instinct is to not drop thousands of dollars into the high-mileage Prius, especially if these issues might be pointing to a dying main battery.

I'm in the Sacramento area and we are proximate to San Francisco and the Bay Area if that helps the repair picture.

 

Thanks. The 12V lasts for months in some cases before dying. This has led me to speculate that what might be happening is that we are occasionally inadvertently doing something that may be killing the 12V battery in one fell swoop. These seems particularly likely given my repeated voltage tests that indicated normal volage. The replacement 12V batteries we've been buying are said to be the best (Optima and Exide.)

The fobs don't work, to my knowledge. My wife tried to replace the batteries but keyless does not function AFAIK. She lost one of the fobs awhile back -- she left it on top of her car -- and one of my crazier 12V theories was that perhaps it fell into the engine compartment or other place in the car and was permanently waking the keyless entry, leading to premature 12V battery death. But she lost it more than a year ago, and I am skeptical this is what is going on. As I mentioned in my original message, we enter the car using the manual key and insert the fob into the slot before powering the car on.

I attempted to use a multimeter to do a parasitic draw test on the 12V battery, but I found it is very difficult to do this in a prius for a few reasons. I didn't track anything down. I don't pretend my analysis was particularly good or complete.

Car will go to charity if we decide to give it away, alas.

Yes, this has been driving me crazy, and it seems so sad to scrap a car that we've maintained well, and still seems like it has a lot of life in it. My instinct has been that this is something I should be able to fix. But I haven't tracked down the problem.

Answering your questions, I repeatedly followed the 12V battery check procedures on the tutorial at PriusDIY. After putting in a new battery the voltages were on the highest end of what was recommended. In "ready mode" (test #3 as described at the PriusDIY tutorial) the charging voltage was repeatedly, if I recall correctly ~14.1 or 14.2, which is in the "normal" range as described by the PriusDIY tutorial.

You asked about the ground cable. You're talking about the cable that leads off the neg terminal of the 12V battery and presumably screws somewhere into the frame? Maybe I should look at that, especially since I've been the unschooled guy replacing the 12V battery repeatedly. If it's not tight, that will kill the battery? Should I clean both ends of it? Is there a way to test if the 12V battery is getting the juice it requires to recharge? Should I be giving up and bringing this into a real Prius repair-person?

 

I tried that after the last battery died, though I am very tall and it is possible my knee has inadvertently toggled the SKS button in the interim. Do any of you know offhand which position disables the SKS system (push it IN to turn it off, or push it OUT to turn off SKS?) I can check to see if it was set to the wrong position.

It would be nice if there was a way to pull fuses to disable the cabin lights, trunk open lights, SKS system, and anything else that isn't essential to safely run the car -- we could eliminate a lot of possibilities.

 

Thanks. First I bought an Optima 8171-767-FFP (DS46B24R) YellowTop Prius Battery and then for the next two switch-outs I bought two of the Exide Edge FP-AGM51JIS Flat Plate AGM Sealed Automotive Battery. In an attempt to keep the battery charged, I bought a NOCO Genius G3500 6V/12V 3.5A UltraSafe Smart Battery Charger and used the AGM setting. Voltages were reading normal, and then one day the Prius clock started resetting, the computer started rebooting, and eventually the car would only start intermittently.

 

Verdict: keyless entry system was disabled. I don't think this was the culprit.

 

hmmmmmmmmm. you may be getting at something, here. I permanently attached terminal "loops" to the 12V battery so that I could easily hook up the NOCO Genius G3500 6V/12V 3.5A UltraSafe Smart Battery Charger and the NOCO Battery Life BLSOLAR5 5 Watt Solar Battery Charger and Maintainer in an effort to keep the battery alive. There is a quick-connect system along with a NOCO Genius GC004 10' Extension Cable (check links on amazon if this isn't clear; I can't yet embed links in messages myself.)

Well, the last few times I've attempted to plug in the charger to trickle-charge the 12V battery via the system I'm describing above (after the 12V battery "died"), the charging has either not started, or it reports 25% and then stops. I thought maybe something had gone wrong with my charger, or the 12V battery was dead. But maybe the issue is with the connections to the battery, or with the "ground cable" that was mentioned earlier in the thread.

Note I did not have this charging/ quick connect system for the first two 12V batteries I replaced, and those batteries failed as well.

The last time I replaced the 12V battery was in late April of this year. It shouldn't have failed so quickly. The replacement I put in place was a Exide Edge FP-AGM51JIS Flat Plate AGM Sealed Automotive Battery

 

I'll try that, thank you. If all else fails: can somebody suggest a 12V battery disconnect switch that works well with the ergonomics and placement of the Prius' 12V battery? Ideally there would be some kind of a switch attached to a lead wire, such that I won't have to disassemble the entire rear compartment of my Prius every single time I want to disconnect the 12V battery.

I like the Prius, but boy, the inaccessible, underpowered 12V accessory battery is just about the stupidest car design decision I've ever seen. I can't fathom why Toyota designed the Prius this way, except to make maintenance shops perennially happy.

 

thanks. and if I want to test most effectively for possible parasitic draw, what's the best and easiest way to do that? Should I shut myself inside the Prius, wait five minutes for everything electronic to quiet down, and crawl to the back of the car where I've pre-disassembled the space so that I can get at the 12V battery terminals? or can I do those checks from the under-hood terminal? I have an auto-ranging multimeter, and a clamp-on multimeter.

Is there an authoritative way to test for the amount of current that's actually going into the 12V battery via the Prius' version of an alternator? My prius reports normal levels, but ideally you could test this at the battery connection itself.

I'm wondering how easy or how hard it is for an authorized Toyota shop to do these diagnostics. The harder they are to test, the less confidence I have that the workers will actually do the necessary work. I don't want someone to just guess at the problem: I can do that a lot cheaper myself.