Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime?

It's mostly a Prime issue, but can also happen to regular Priuses.

I just had mine replaced last month under warranty. With the HEV, I think the battery is generally good, but there's very little safety margin. So if you damage the battery by draining it by leaving it in ACC just once or twice and then you have a really cold snap(say below 0F), that can end up being enough to take it down. At least that's what happened to me.

I drained it once this summer while working on the car. Then we had a really cold snap(approaching -20F) in Dec and that forced me to break out the jump pack. From there it was a downward spiral. My dealership is 30mi away, so I let the battery die several times to make sure it would fail the test the first time. I didn't deliberately drain it, I just let it die on its own without trying to stop it with a maintainer. The Toyota replacement battery is quite a bit bigger than the factory battery, so I think everything will be now be fine for at least 5-7 years(maybe longer).

I can't speak for every dealership, but it took about 2hrs for them to finish everything. They have a quick test(~15min) and a full test(~60min). They'll do the quick test first to see if there's a reason to do the full test. If the quick test is borderline or bad, then they'll do the full test because Toyota requires a failure on a full test before they'll pay for a warranty replacement.

 

Well, as I keep saying, the problem was not that you damaged the battery by draining it. The problem was that the car's BMS won't fully recharge a drained battery. In such cases where the battery drains, you need to connect a battery maintainer like Noco Genius for at least 24 hours (preferably 48 hours) to fully recharge the battery. That would have probably solved your problem. It is also a good idea to periodically (every few months if the car is driven regularly—or more often if the car is not driven regularly) manually charge with a battery maintainer just in case.

 

Agree to disagree. And leave it at that.

 

You can disagree and be also wrong at the same time. And you are wrong. I know this because I have extensively experimentally studied the 12-V BMS. I know all the antics of it. You haven't; so, you are in the dark on what is really going on.

 

I had a full battery drain last summer after leaving the car undriven for over a month. The battery voltage was 1.3 V as far as I remember. The battery was also almost four years old.

I did get a jump start from AAA, and the car ran fine. However, the car's BMS never charged the battery to much over 50% SOC. I eventually replaced it with an AGM battery. Later, I recharged the OEM battery with Noco Genius (5 A), and the battery voltage reached well over 12.70 V. I did not install it back to test its capacity, but certainly, Noco Genius put a lot more charge in it than the car's BMS.

Get the BM2 Bluetooth battery monitor. You will learn a lot about how the BMS behaves and see it when the battery SOC is low. The BMS is very miserly and uses a very complicated charging algorithm and often does not charge the battery no matter how long you drive the car.

And, last but not least, get a battery maintainer like Noco Genius and use it occasionally. Do not trust the BMS alone to recharge your battery even if you drive your car regularly for most of the time. You don't drive it for a week or use the accessory mode for an hour, and the complicated BMS algorithm will get confused and fail.

BM2 Bluetooth battery monitor on Amazon

 

Did they put a wrong-sized battery? They should have used the Toyota TrueStart DIN H4, which is the identical size to EN LN1/BCI 140R. Also, its RC is apparently 74 min, which is 45 Ah—not 50 Ah as some other batteries of the same size (like my AGM).

If you want everything to be fine for seven-plus years, I certainly suggest hooking up a battery maintainer like Noco Genius occasionally rather than trusting the 12-V BMS alone to do the recharging. Installing a Bluetooth battery monitor will also help.

 

This TSB confirms why I think so many Toyota 12-V batteries are failing.

It basically says that the dealers are required to fully recharge the 12-V battery with a battery maintainer before they deliver the car to the customer. And they say, the battery warranty is void if they don't do that. However, we know that they don't, and many new Priuses come with a 12-V battery with a very low SOC, and, without connecting a battery maintainer, the car's miserly BMS with its buggy algorithm is not capable of fully recharging it no matter how long you drive the car.

Note that the TSB also states that the car should not be left undriven for longer than a week unless you disconnect the negative battery cable.

Your only remedy: Get a battery maintainer like Noco Genius and fully recharge the 12-V battery (for 24–48 hours) on Day One when you buy the car. Then, use it occasionally to fully recharge the battery and definitely if the car wasn't driven for longer than a week or some other drain event occurred.

Toyota T-SB-0024-23 rev. 2—Battery maintenance during PDS

 

A few weeks ago, there was a thread on reddit where a Gen 5 prime owner parked the car long-term at a lot, so no access to a plug. Instead, they setup one of these solar battery maintainers.



Clipped this from amazon, and there are numerous brands to choose from. 10W and 5W also.

Anyone ever tried of these? You have the option of plugging it in via the cigarette adapter, but the reddit post said they used an ODB2 adapter like this:



Is there any advantage to charging via ODB2 port vs the cigarette adapter?

 

The 12V outlet/cigarette adapter in most cars these days is powered off when the car shuts down. Thus won't work for this.

 

The Lojack security device added on by dealers uses the hot wire in the obd port to power it. I just took the fuse to the Lojack out. Someday I’ll remove the obd adapter they used to get the 12 v power. It isn’t a very thick wire.
So putting power into the obd port works? I wouldn’t do it as I don’t mess with things like that. Others can do as they please. I would use the clamps, or get ring connectors. Or just use one of my battery chargers. I have one on all night again.
Not a bad idea on the solar, but do I really need more stuff to have around? It is possible to charge the 12 v in ready mode when parked, of course not when the 12 v is already dead and car can’t go into ready mode.
So have more stuff and carry a 12v jump pack to get in ready mode. That seems less stuff than a solar array.

 

As others said, you can't use the cigarette lighter, but perhaps there is a plug in the firewall that you can remove to route a cable from the battery into the passenger cabin.

You don't need to use a battery maintainer regularly. You only need it if the car is not driven for more than four days or if it sees only short trips. If it is not driven for more than four days and you don't have a battery maintainer, you should disconnect the negative terminal of the battery.