Prime 12v battery dead

Connect an external battery charger to the 12v battery and fully charge it up. Then remove the charger and let the car sit for 3 hours. Then, use a volt meter and measure the battery. What voltage do you get?

 

How did they check the battery, because I seriously doubt the drive would've recharged the battery enough for a load test to pass. Did they give you any numbers or any indications of what they actually did? I'd stop by an auto parts store for a free battery test. If it fails, take pictures of the Fail and test equipment they used and confront the dealership - ask to speak to the service manager. They should have their own test equipment and tell them you want to watch them test it again.

Good Luck....

 

yes, a lot of people here have had the problem. search, and ye shall find.

 

It took me about a week to realize that my battery drain is variable and unpredictable. The car can be unstartable in as little as 2 hours, or it can be fine for a week. I've gone full circle with 3 different dealers; even having the battery replaced, but no solutions at all. My workaround was to install a battery maintainer, which I plug in while parked at home.

 

What's your use of the car like, say miles per week? Are there protracted downtimes, or you're using the car fairly regularly? Ever left a light on, or door ajar? Do you listen to the radio in accessory mode (car not fully on, aka "Ready" in Toyota parlance)?

 

Sounds like my dealer, concocting all sorts of ways to blame me for the problem.

 

Correct, the 12-V system on the Prius (at least in Gen 4 and Gen 5) is entirely independent of the engine.

Chances are that you are not driving the car regularly. On top of that, the 12-V BMS on Gen 4/Gen 5 is buggy. The 12-V BMS does not fully charge the 12-V battery. It likes to keep it around a 75% SOC, but since it is buggy, this could drift down much lower.

Today is Cyber Monday. Get a Noco Genius and use it every now and then so that you fully charge the battery at least every few months and don’t run into this problem again. A battery that has a full drain has degraded and ideally should be replaced; so, get the Uplus AGM 140R/H4/LN1 battery as well. There is also a chance that your car came from the dealer with a degraded battery anyway if it sat undriven in the dealer lot for too long.

The UPlus AGM 140R/H4/LN1 battery on Amazon is currently on a Cyber Monday sale for $116.99. You can also get $10 for your core if you take it to Autozone or such. Go get it while you can. It fits on both Gen 4 and Gen 5, with side-vent ports and a plug for the unvented port.

The Noco Genius battery maintainers (1 A, 5 A, 10 A, etc.) are also on sale.

UPlus AGM BCI 140R/DIN H4/EN LN1 battery on Amazon

 

Und

Indirectly, the power to charge the 12V battery does come from the engine. MG1 primarily charges the hybrid battery, which in turn is converted to power to charge the 12V battery.

 

Is there an easy way to set up a 12V trickle charger from under the hood or do I have to access the battery directly in the truck?

It's -5oC out and our 12V battery is dead. I want to hook up a trickle charge for the future

 

A trickle charger, no, a battery maintainer like Noco Genius, yes.

I think Gen 5 has a positive 12-V post under the hood, yes. You can route the AC cable through the gap over the headlamp.

 

It's eminently possible to connect a charger under the hood, either temporarily with the charger's alligator clips, at the jump-start point in the under-hood fuse box, or you can permanently wire a quick-connect to the point where 12 volts feeds into that fuse box. The negative connection can be to any convenient, bare-metal point on the car body, say under a bolt head.

There are chargers that are external to the car, and some that mount under the hood, and an cord with plug is typically run out through the front grill (or you can get fancy, install a capped, plug-in port in the front fairing (aka bumper ). Aforementioned Noco brand has both styles of chargers.

I'm using a CTEK charger, near-steady usage for over a decade now, but they were pricey to begin with, and price has escalated as well.

 

I think I had one once that came with a magnet for under hood attachment.

 

A trickle charger is a bad idea; they will destroy your battery eventually.
A battery maintainer tests your battery periodically, and gives it a top-up charge as needed.

 

Please explain the difference between a battery "maintainer" and a "trickle charger". To me they are the exact same thing and the difference is opaque and sounds like sales BS gimmicks. As far as I'm concerned they both check the battery condition and periodically apply current/voltage to top up the battery.

I don't intend to leave the devices constantly connected, only to top up the 12V batteries as a preventative maintenance program say once a month like I do on my motorcycle batteries when the bikes are parked for the winter.

 

In the old days a trickle charger was not looking at the voltage and current draw of the battery with any logic; it was just a cheap low current transformer with a couple of rectifiers. It might put out a poorly regulated 13.5v at a half an amp all the time.

When you have a bunch of old guys in a forum, most never mechanics or electrical techs, you get some confusing language along with outdated knowledge from their heyday. It gets worse when they buy a Prius but can't sit still long enough to digest technical training like the Weber State hybrid videos.


The first is available at your nearest estate sale

 

A tempest in a tea pot, @notspam3 simply using a generic term?

 

If it is like the 2025 Camry, there is a jump point under the hood.

 

^This is a workable solution from @Paul Gregory. I ‘think’ the positive post circled calls for an M10 sized eyelet, others can correct me if I’m wrong. For the negative eyelet, a good metal-to-metal contact point on the engine or body as @Mendel Leisk notes. The plastic fuse/relay cover does need to be notched for the connection to exit, a small price to pay.

Being new to Prii/hybrids/Toyota, I went through this same learning curve and the members here set me straight.

Presently, I hardly drive my new plain old HEV-only LE FWD at all so 12v discharge over time is a concern. Prepping it for my daughter in Socal (ultimate end-user), only a couple hundred miles on it after ~1.5 months of ownership, has sat for 1-2 weeks a couple of times w/o being driven. Have put it on a small (750mA) maintainer twice overnight, first time around 24 hrs to bring back to “green”, second time was last night, less than 12 hrs to “green”. So overall no issues.

Of course, 2 ways to go about this – either the above ‘front’ solution, or the ‘rear’ hatch battery location (12v plastic battery cover needs notching too for this method). Whatever works best for you.

My daughter’s an apartment-dweller so has issues with access to a 120v/15A power supply (at the apt. or at work) in either case. I considered doing a ‘rear’ solution (in concert with a suitably large enough portable power station kept in the hatch area overnight) but didn’t really like that (nor did she). And realistically she’ll be a daily drive user so only on rare occasions will have it sitting for over a week or more. In that event I’ve advised her if she’s looking at a couple of weeks or more of idle time, to be safe she’ll just have to find a way via a friend or whatever to keep the 12v topped up.

Prii owners can take solace in the fact they’re not alone – belong to a number of Ford/Hyundai forums, they’ve got the same (and worse) issues. Common complaint is, it’s a new car I shouldn’t have to do this (use a maintainer). I get the thinking, but the reality is different. I’m just thankful Toyota’s BMS doesn’t seem as troublesome as Fords (their F150 and Maverick forums are full of dead 12v battery/’deep sleep mode’ woes).