Many Toyota Prius owners (or indeed, just about any hybrid owners) fear that at some point they’ll have to replace their big bad battery. Well, this blog isn’t about that.
No, this blog entry is about replacing the little battery. That dinky 12V battery that you wonder why it’s still even in your fancy high-tech hybrid.
The Prius 12V battery is a strange animal. It’s used for powering your accessories, pretty much just like on a regular car. Except it also … isn’t. It’s confusing and complicated. But the main point is, what it’s used for doesn’t really much matter. The important thing that it does is start your car.
The 12V battery powers on your Prius onboard computer. Just long enough so that it can switch itself over to the high voltage hybrid battery instead. More or less, that’s it. You just need it on long enough to “turn on” your Prius. After that, the 12V accessory battery is pretty much useless. One could even ask why it’s there at all. Why not just use the high voltage hybrid battery for everything? My guess would be some archaic law somewhere “for safety”.
Anyway, so, as with any car, that 12V battery will eventually die.
In my case there was absolutely no “dummy” indicator that said “low voltage”. (Nor was there a voltage indicator like on many old cars.) It’s a serious failing in the Prius in my opinion that it tries to minimize your service information for you. No engine temperature, oil pressure, tire air pressure, or even battery voltage indication on your dash (which with the “modern” design of the Prius, one could expect) or on your computer screen, called the Multi Function Display or MFD, at the simple touch of a button. Nope. No indicator anywhere, no dummy light went off.
The first indication that I got that there was any problem with my 12V battery was … you guessed it … when my Prius suddenly one day wouldn’t start. It was working fine one day, dead the next. Not even so much as a groan to indicate there might possibly be a problem.
And even then, honestly, I didn’t think it was actually a problem with the battery itself. I assumed that it was an accessory forced to be plugged in by a certain insurance policy that had just drained the battery even when the Prius was turned off. (I’m still not convinced this wasn’t partially involved in the situation.)
But at least then, a simple jump-start and the Prius was off and running once more.
For a while anyway.
But since I walk to work, and my wife and I tend to use her car for errands, as my Prius is a “business car”, it wouldn’t take long before my little Toyota Prius would be dead again.
I was trying to drive it every so often to keep it charged, but it never seemed to last very long. Since I almost didn’t need the car anymore though, it was hardly a problem. Especially as by that time I’d just kept the portable jump starter in the trunk.
And through all of this time, not once did the Prius ever complain about the battery level or status in any way … except for when I tried to start the car and couldn’t. No duh. There’s a problem. Really? At that point do you really need a light for that? The point is, there was never a single indication of a problem before I’d try to start the car and couldn’t. It’d either work, or it wouldn’t. Any given moment, I’d never know until I tried.
And so, as I didn’t need my Prius often , and jump starting it was so easy, I just things degrade.
Well, when it got to the point when the 12V battery literally couldn’t even hold a charge overnight, I figured I really should replace it.
Now I could go with the stock OEM battery. It’s an ’06 Prius, and it started failing in 2011, which put it at four to four-and-a-half years before it started acting up. In a low-usage vehicle. So let me see, replace the piece of crap that had started failing me in four years of ownership with the same part? Yeah, um, no thanks.
Let me take a moment to stress this, whereas Volvo made it a habit of building their cars to withstand anything, to over-engineer any safety or reliability issue imaginable, Toyota seems to have done the exact opposite with the Prius. Where a corner can be cut, Toyota cuts that corner and then some on the Prius. Its parts are cheap. Its stock OEM parts are crap. And if you have the opportunity (or need) to replace a part, please, by all means, replace it with something not stock. Trust me on this. Even the air filter works better with a K&N air filter than the stock Toyota air filter. (It fixes some hesitation issues when you put your foot down that you might attribute to an uncertain hybrid synergy drive or even to the notoriously bad traction control, but is actually just literally your engine not getting enough air to breathe.)
So when it came time to replacing the Toyota Prius 12V accessory battery, did I go to the dealership and screw myself with the same bad part? Heck no! I searched online. I ended up with the Optima DS46B24R battery at BatteryMart.com. It would seem that (I guess because of the weird shape?) all Toyota Prius 12V batteries are ridiculously expensive, in the $200 – $300 range. A little research shows some dealerships even charge over $300 after labor is involved to replace this battery. When for a little less than $200 I can replace it with a better one myself? Seemed like a no-brainer to me. So that’s what I did.
I almost wish I hadn’t.
Okay, so I should have done more research. But I figured, it’s just a freaking car battery. How hard can it be to replace? I knew it was in the trunk instead of under the hood, but other than that, it’s just a car battery … right?
So in any normal car to replace the battery goes like this: Open hood. Undo negative terminal. Undo positive terminal. Remove bad battery. Place in good battery. Connect positive terminal. Connect negative terminal. Close hood. Enjoy. It’s quick and easy. The worst thing that you have to worry about afterward is if your stereo will still work. (Anti-theft features sometimes really suck.)
So the Toyota Prius has the battery in the trunk instead of under the hood. That’s all the difference … right?
First, yes, it’s in the trunk … but not just in the trunk. It’s in a hidey-hole in the framework of the trunk. So you open the trunk. You pull off the carpet. You open and remove the false bottom of your trunk to reveal the hidden storage bin. You then haul out the hidden extra storage bin … hoping that you haven’t really stored too much in it because it could get darned heavy. Now that you see your spare tire, you’re almost to it … to the reveal that is. So to your left is a little corner floor panel where your brake lights are. You try to unclip the plastic part so that you can pull that floor panel out. After fighting it for a moment, there she is, the 12V accessory battery … mash-up.
Because just having the 12V battery there would have been too easy.
There’s some kind of brake controller box thingy in the way though. And there’s a vent from your high voltage battery to the side of the car also in the way. And there’s this really big red thing clipped to the top of the battery itself … which is hiding down a well in the body of the side of your Prius.
To call that 12V battery “inaccessible” is a mild understatement. No simple replacement here.
So now the fun part begins. And here is where I really wished that I’d researched this more before starting.
There are two online videos that I got when I Googled this. Neither actually show replacing the battery. Both say the same thing however. You can remove the vent. And allegedly, you can swing the brake box thing out of the way after removing most of its bolts. The most questionable part to their videos, and really makes me wish that someone had actually shown this process in either of them, comes when they say that you can just remove the negative and positive terminals, leaving the big positive terminal cluster-frack intact, undo the tie-down, and swap the battery with ease.
Why didn’t you show that part? Oh, right, because you can’t do it that way! But later, you’ll wish that you could.
So, if your Prius is anything like mine (and I’d imagine it would have to be) you’ll find that there’s pretty much no way in hell that you’re getting any kind of wrench down near those terminals to loosen them up. That even if you could, you’re still going to find it amazingly difficult to then pry the terminals free once you actually do that. All without banging your head, shocking yourself, or goodness knows what other mishap awaits anyone who tries to work in that confined space with a (mostly) live battery and all of that junk surrounding it.
Me? I had to move the brake box thing completely out of the way by disconnecting it entirely. Including from the positive terminal. I had to disconnect the negative wire from the car wall. I had to open up and unclip all of the positive terminal harness and wiring so that I could actually get to the positive terminal. I had to remove the bar bolted into place to hold the battery down. And don’t forget that little vent tube connected to the side of the battery, near the top, positive side. Then I had to pull the battery out of the well, so that I could finally loosen and pry off the actual +/- terminals, which were stuck on like glue. I swear I was afraid of tearing apart that battery just to get the leads unhooked. Someone put them on super-tight, which I thought was a battery no-no as you can break the soft metal of the terminals, right? Anyway, it took a lot of work just to get to the point where the 12V battery was officially removed from my Prius.
And then the whole thing had to be done in reverse. So reconnect the negative terminal to the negative post, reconnect the positive wiring harness to the positive battery post, drop
little Timmy the battery back down the well, hook up the little vent tube, lock the battery down, reattach the teeny tiny little positive wire to the positive wiring harness, reconnect the ground wire to the car frame, bolt down the brake box next to the battery again, reattach its wire to the positive wiring harness, put the big red cap back on the positive wiring harness, twist and turn the rectangular high voltage battery vent tube back into place, bolt it back into the little plastic clip in the side panel. Drop the bolt while bolting the vent back in … crap!
Now the bolt is in the battery well, behind the battery no less, where I surely can’t reach it.
Undo everything again. Redo everything again. This time DON’T DROP THE LAST BOLT! Put the battery floor panel back into place. Put the hidden storage compartment back into place. Put the storage floor panel back into place. Put the carpet back down. Start up the car.
And … nothing.
Something, somewhere, has gone horribly wrong. Maybe. Or maybe it’s just that the new 12V battery was dead? Well then, I can just charge it up, right? Sure. No problem. I jump start the Prius, everything seems fine. I even did the “flip my headlights three times while pressing display” trick to activate the MFD diagnostic mode. I check the 12V line, and sure enough, we’re almost at 14V, which is recharging. Congrats. No error messages anywhere. I now have a working battery. I just need to let it sit for a while to recharge.
So I do. Then I power off my Prius.
You know when you turn off your Prius and your 12V battery is dead because it just doesn’t do everything that it should. Everything just … dies.
So, again, no warning light whatsoever. Apparently my 12V battery isn’t even hooked up right. Or is DOA. Or something. And the Prius never bothers to tell you.
Frustrated, I packed up my tools, flipped off my car, and called it a day.
Later that night, I took another crack at it. One thing was niggling at me. Reconnecting up the cluster-frack that is the positive terminal harness, there are two main wiring connectors that slide into place on it. One is really long and it feels like a solid grab. I think it even clicks into place.
The other however, has one teeny wire, is short, and does not click. At all. As I played with it some more I realized that there is a slight feel like it’s being grabbed by something, if I push it in really hard. But that’s it. I did a visual inspection and there is a connection in there, so it’s on right. But it isn’t a very confident connection, that’s for sure.
So I clicked it in again with a shrug. Seemed legit.
So then I started unscrewing the connection of the negative terminal to the car frame. I figured if I could make a spark from the ground wire, I’d know battery was hooked up right. As I’m unbolting it, zappy zap! Huzzah! Not only that, but because the key was still in the ignition (I’d hoped that jump-starting it would let me unlock the trunk, since it’s an electronic “handle”, and while I’d disconnected the jump starter afterward, I’d forgotten to yank out my key) the Prius booted right up into accessory mode.
Whoah! It’s a magic start button from the trunk!
So that worked then. The battery was charged enough to start the car. Things were alive. All was good.
You see, my choice of a non-stock better battery is slightly larger than Toyota’s OEM 12V battery. It fits in there just fine. But it’s a teensy tiny bit taller, just enough so that you can assemble everything back up, but the floor panel doesn’t quite fall into place like it should. It’s a little loose. But once you put the storage compartment back in place, that holds it down just fine.
And it all worked for literally one day.
The next day, it was … shaky. Like an unsure connection somewhere shaky. It wouldn’t start. So I checked the wires again. Same deal. That one stupid little wire connection in the positive harness must have slipped out a bit during my drive. So I reseated it again.
And the next day, again.
Now, I finally put my thinking cap on. Obviously reconnecting it every time I parked my car after a drive seemed like a pretty crappy solution. Not to mention does that mean the batter isn’t actually charging while the car is running because it breaks loose? I contemplated glue, but I wanted it to be able to be undone if need be. So I used the world’s best fix-it tool: duct tape. Yes, I literally taped that one little wire’s connector onto the harness so that it’d no longer come loose. With purple duct tape no less, so that any Toyota service person could see their car’s design flaw in epic detail.
And since then, things have worked beautifully.
Which raises a huge concern, and makes me think I should seriously consider some kind of glue. Because what if the tape ever gives? Tape gets old. And seriously, how do Toyota shops fix this issue? Can this really happen to every single Toyota owner? What do they do, replace the positive wiring harness each time they touch it?
So, long story short, if you have to replace your Toyota Prius 12V accessory battery, be prepared for a painful experience. You can buy your own battery and do it cheaper than taking it into the shop … but for all of the hassle that it is, you might actually prefer the shop anyway, if you don’t mind the really shoddy OEM battery.
If you do go the “do it yourself” route though, be prepared to laugh at the two videos on the internet, because chances are, you’re going to have to take everything apart to get the job done. If you have to pull apart the positive wiring harness, be sure to tape that one little connector down so that it can’t get loose. And because it’s such a pain in the asterisk if you lose a bolt down that battery well while you’re trying to close-up, make sure to grab some magnetic tools so that the bolt won’t fall even if it slips out of the socket wrench. And be prepared to curse Toyota a lot for the way so much is jammed in and around that 12V battery, making what should have been a simple job really complex. Plan to lose an hour. At least. And that’s if you’re prepared and know what you’re doing.
Speaking of cursing Toyota, so how will you know when your Toyota Prius 12V battery is in need of replacing? You won’t. Your car will sure never tell you before it fails. Nope. One day it’ll just be dead. Your car won’t start. That’s how you’ll know. And technically even then, it could be other problems. But if you’ve owned your car for a couple of years, chances are it’s the battery. They’re not meant to last as it is, and Toyota really didn’t build it to last at all, even if it is barely used. Just like plenty of other parts on the Prius, like your brake rotors, for example. (No, not the pads. The rotors literally rust out! In just three or four years! But your brake pads will be perfectly fine!)
And that 12V battery is an expensive little sucker with a really odd shape compared to most car batteries.
But the weirdest thing about it, I have to say, is how teeny tiny the wires are connecting to that 12V battery. Even the ground wire looks woefully small. I’m sure for as little as it is actually used, those wires are in fact sufficient to cover the need. But I have never in my life seen wires connected to a car battery that are so ridiculously small!
Why does the Prius even have this battery again? Obviously, it’s not used for much.
One final parting note, as you try to start up your Toyota Prius with a dead (or dying) battery, you may get all sorts of weird results. One thing that seemed to happen frequently would be that the parking brake would click/grind/jitter/something itself into a weird state. Then your MFD would give you an error about it. Put it into drive and then park again once you’ve jump-started your Prius and it’s fine. But it’s scary the first time it happens. You’re like, “Oh nos! It’s gone all broken!” No, just a dumb design that tries to change the state of the parking brake … while you’re still in park no less … when you don’t have enough juice in your battery to complete the operation. It’s probably a “start-up test” that should have waited a second or two longer until the car had actually switched over to its high voltage battery upon starting up. I don’t know. But it’s freaky.
Also freaky, and possibly related or not, is that when you go to jump start your Toyota Prius, there’s an awful lot of clicking noises that go on under the hood. It’s awfully weird.
And one possibly really annoying “feature” of the Toyota Prius to consider, is it’s dumb-asterisk jump-start terminal under the hood. After you remove the fuse cover you flip the red positive terminal cover open. To find a tiny bolt sticking up … surrounded by stuff. I really had a dickens of a time figuring out how to get my big bad jumpstart cable (and it’s smaller than most jump start cables!) to actually get in at any angle sufficient to grab that little bolt hiding in there and hold a connection. Sheesh! Did no one at Toyota actually grab a jumper cable and try to connect it? WTF?!
Basically, this whole post identifies everything I hate about my Toyota Prius. It’s a theoretically nice car, but ruined by a lot of bad design decisions and poor quality in many places. My next car probably won’t be a Prius, and likely won’t even be a Toyota, because near as I can tell with each new update to the Prius, they just aren’t fixing any of this.
The 12V battery is a joke. The jump-start 12V post under the hood is almost unusable. The brake rotors rust out before the brake pads go bad. Both pads and rotors are insufficient to stop the Prius in an emergency. The TRAC traction control system is a hazard to everyone on the road and kicks in all too easily. The “regenerative braking” system drops out if TRAC kicks in. TRAC drops your power to be insufficient to climb a hill if it kicks in up a hill. Heck, you can run over a pebble while making a turn and TRAC will kick in, causing your Prius to practically stall, scaring the crap out of you and the driver behind you. The stock tires might as well be low-energy tires for all the grip they don’t have. Even the air filter chokes the engine so that it can’t operate properly. The speedometer is consistently 1MPH slow, like it’s an integerization bug. There’s no way to simply check your oil pressure, your 12V charge, your engine temp, etc. without installing your own expensive tool. And what warning lights the Prius does have don’t actually warn you until after the problem has already occurred, by which time you already know.
And now they’ve restyled the interior to take away pretty much everything I liked about the Prius interior. No more “space ship” layout. Even though one you get used to it, you appreciate it. And no more handy little storage bins because of it. Meh.
So anyway, that’s the 12V battery on a Toyota Prius. At some point it’ll fail. You won’t know when. You might not even be sure that’s why your car won’t start anymore. You can fix it yourself, but Toyota has made the easiest part of a car to replace now one incredibly tough part to service yourself. But in the end, you just might want to go through the pain anyway, because the stock OEM parts on your Prius are crap, so the more you can replace with quality parts that will last, the better off you and your Prius will be.