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

Actual reported failures seem to be almost all PHEV/Prime, which given the lower numbers suggests a strongly PHEV-specific fault.

In which case it seems likely most are due to the potential current-draw-while-charging issue there's a TSB for. (Only in the US, afaik - it could well be specific to the US charging standard).

There have been some reports of the comms module going nuts and sucking out power - that's presumably not PHEV-specific.

Obviously there are always going to be some reported flat batteries on any car, and there are people here obsessing over the general battery management, which is the same on PHEV/HEV - it's true that it doesn't keep the battery as full as it could, but that's never going to be the direct cause of failure.

(Non-US PHEV here - no problems).

 

Everything you said is completely false.

On top of that, as I stated in my previous post, the Gen 5 Prius Prime PHEV aggressively recharges the 12-V battery during an active plug-in session.

 

I agree with Gokhan | PriusChat . I recently traded in my Gen4 Prime for a Gen5 non Prime and according to my BM2 monitor the voltage/drain on the Gen5 12volt is similar if not identical to what I was seeing on the Gen4. And it's not limited to just Prius. My '21 Venza suffers from 12v battery drain when not used although since we use that car more often it's less of a problem. I have a NOCO Genius2 and use it as necessary.

 

Oh. Ok. Interesting. Seemed most comments coincided with being plugged in too long. Perhaps that’s a more common similar but different complication resulting in ‘won’t start / 12V battery issue’ than for non-plug-in vehicles.

Since my original post, I have found and read a few other 12V discussions here. It seems the current hypothesis boils down to the algorithm of when the revised 12V charging happens - only kicking in after reaching 70%. Some drivers will reach that faster than others based on a variety of factors. Yeah?

And then if at 70%, you might/might not have sufficient power to start. Edit: more likely the further below 70% you go, which could happen over periods of non-use (all models) or extended plug-in time for Primes. And reoccurrences of too little charge may affect battery life. Am I still on track to the core concerns about the 12V battery?

 

I just did charge via Noco Genius- necessary or not, and plan on doing so once a year.

 

A 70% SOC is more than plenty to start the hybrid system (about a 100-A current draw or less?). I don’t know the exact number, but it should be pretty low like 20% or less.

 

Thanks. I suspect you may be onto some of the only definitive information on this subject, validated by Toyota and the only consumer focused Technical Service Bulletin about the 12V battery, and specific to the Prime/Plugin Model. ((Read: frequent enough to publish with a fix that, interestingly, addresses the ECU sleep function. Unclear if that’s one-in-the-same with the oft referenced BMS (presumably Battery Management System) in this thread.))

T-SB-0054-24 Toyota Engine Technical Service Bulletin

“Summary: Some 2023 - 2024 model year Prius Prime vehicles may exhibit a condition where the 12V auxiliary battery charge may deplete due to the plug-in charge control ECU not entering sleep mode when the Plug-in Hybrid Electric Vehicle (PHEV) charging cable has an extended connection without charging (for more than two hours). The plug-in charge control ECU has been modified to reduce the possibility of this condition.”

There are the Dealer focused TSB’s that describe dealer obligation to manage batteries for vehicles stored for long periods of time and a pre-delivery dealer policy defining who is liable for replacement if not followed (Toyota corporate vs dealer). As expected, it appears to cover just about every model Toyota offer.

Lots of other input and discussion, which is interesting but after a lot of attentive reading appear as hypothesis, speculation, perception or true one-off’s (like the member that hooked up a radio to the battery and left it there). And of course several ideas to reduce the potential issues that may or may not be likely under normal or typical conditions.

YMMV as they say.

 

I see. I read the 70% BMS recharge threshold in your post in the Technical sub-form ( When there will be 12-V-battery issues, when not, and what to and not to do | PriusChat ) and took that as fact/proven. I see now it isn’t. Thanks for clarifying here. It might be nice if you update that thread with the same acknowledgment.

Here are the points you’ve written there that I’m referring to:

“Unfortunately, the BMS is very miserly in charging the battery, which is the main cause of the 12-V issues. This is by design by Toyota. While older cars always charge the 12-V battery, Toyota has chosen to program the BMS to minimally charge the battery in order to improve the fuel economy. This is explained in the Toyota new-car features manual. What the BMS does is like a thermostat—it aims to keep the battery SOC in the 70–100% range, and it won’t charge it much until it falls to 70%, and then it will charge it to 100%, and then wait until it falls to 70% again, and the cycle will repeat. What I mean by it won’t charge is that it will keep it at a float voltage (of about 12.89 V) or slightly discharge it while driving the car, and the battery voltage and SOC will then drop due to parasitic drain and self-discharge while the car is not driven.”

“ In fact, the BMS algorithm is somewhat buggy and may not be happy with only ten-minute trips, and you might want to regularly hook up a battery maintainer if you are only doing short trips as well. By the way, a “trip” does not necessarily mean that you need to drive the car. You can turn the car on to the “READY” mode (the hybrid system is on) and sit in it for 30 minutes of longer, and the BMS will charge the battery if it thinks that the SOC is below 70%. That would be an alternative to hooking up a battery maintainer for those who don’t drive the car regularly and don’t have a garage.

If the battery comes with an SOC greater than about 70% from the dealer and if you drive the car regularly, you do not need to hook up a battery maintainer.”

 

It is a moot issue.

As clearly stated both the Gen 4 and Gen 5 the owner’s manual, you should never leave the car plugged in for longer than overnight, as it increases the parasitic drain with or without the firmware update. (I guess leaving it plugged in for the purposes of extreme-cold-temperature battery-heating function in Gen 5 is an exception.)

As I also stated, the Gen 5 charges the 12-V battery during an active plug-in session, which more than compensates for the parasitic drain that happens after the active session ends as long as you don’t leave the car plugged in for days.

 

I don’t know why the point isn’t getting across and you are still confused. “70–100%” is the BMS target SOC range, not the minimum SOC the car would start at.

What I also stated in that thread was that if the SOC gets too low (such as below 50%), the BMS may not be able to pull it back up to the target range, and you may need manual recharging with a battery maintainer. Otherwise, there is a chance that the SOC could keep getting lower and lower to the point that you have a dead battery. In any case, you don’t want the SOC to fall below 70% due to reserve-capacity as well as sulfation-damage issues.

 

No, it is ok. I do understand. Took a minute, but I get it. What is sorely missing is probably. In turn, possibility comes across as if it were inevitable if not imminent for just about everyone.

Yes, all batteries capabilities diminish and eventually become unusable over time. Factors such as temperature, application, energy demand vs rating all matter to various degrees. But that is just stating what is normal and obvious. Not the point of this ongoing discussion it seems.

My takeaways:

If you take delivery of a vehicle, any vehicle, that has been in inventory for more than 30 days, the probability of a premature dead battery is increased, more so with more days on lot. That probability decreases on the same vehicle if the dealer properly maintains inactive vehicle batteries all along and the probability also decreases if this is done a day or two prior to delivery. Least probability when both happen… but never 0% probable that a new vehicle has a defective or otherwise bad battery.

Same goes for post-sale / owners and extended periods of no usage (i.e. more than a month with an otherwise normal battery not approaching or past its rated life).

If you have a Prime/plug-in model that has not had the TSB update for the ECU Sleep issue, your probability of a premature dead battery is higher.

Lastly, Prime/Plug-in models often driven in EV mode only or mostly (not that uncommon given the 40+ mile capability) increases probability of a dead 12V battery. One should consider adopting the same practices as the ‘on lot aging’ / storage state battery maintenance if this fits your typical usage as a Prime/Plug-in owner, even for vehicles that have the TSB ECU Sleep fix applied.

Again, took me a bit to separate the signal from the noise, but that’s pretty much it.

 

It is part of the officially documented Toyota predelivery service that the dealers are required to measure the 12-V-battery SOC and fully recharge the battery using a manual charger if it is too low, but of course, probably most don’t do it.

There is absolutely no difference between HEV driving and BEV driving in a Prius Prime PHEV as far as the 12-V charging system works. In either case, the DC–DC converter controlled by the BMS does the charging, and the system does not even care a tiny bit if the engine is off or on. (And in Gen 5, there is the added bonus that the 12-V battery is aggressively recharged during when the traction battery is plugged in and being recharged.)

 

When we showed up to test drive a brand-new 2010 Prius, as we’re standing beside it the engine started up. I was a little surprised; salesman explained “we warmed it up for you”. Ok, we go for a test drive, loved it, decided to buy.

At some point in the paperwork the odometer reading was needed, so we went out, salesman tried to start it, nothing happened. They finally got a service guy out with a jump pack.

I stipulated new, no-cost 12 volt battery as a condition of sale.

a few months after purchase I noticed the car’s build-date: about 15 months prior to purchase.

This was Open Road Toyota in Port Moody, always like to give them a plug.

 

Ok, my bad. I appear to have misunderstood the Prime 12V charge conditions while operating the vehicle; I wouldn’t/didn’t think the system running EV only would prioritize 12V charge state. I also didn’t assume the vehicle would always be plugged in daily/nightly after every use.

Luckily for us all, regardless of having a Prime or not, it appears the need to do anything specifically to manage 12V battery condition, besides the TSB on the Prime model addressing the ECU sleep issue while plugged in, is non-existent. Save a vehicle ‘in storage’ (not in use) for extended periods pre-delivery or post sale, of course.

 

Here is the official Toyota documentation on this.

Excerpt: “As a matter of policy, Toyota does not provide battery warranty coverage for discharged and/or failed batteries due to lack of maintenance. It is the dealer’s responsibility to maintain the specified SOC of the vehicle’s battery while in stock and ensure proper SOC at delivery.”

Toyota T-SB-0025-23: Maintenance for HEV/PHEV/BEV and auxiliary batteries

 

Here is the official Toyota documentation on Gen 5 Prius HEV/PHEV battery problems for your entertainment (probably the same manual for Gen 4). I doubt it has more useful advice than I provided though:

Official Toyota repair manual: Troubleshooting and identifying the cause of discharged Prius HEV/PHEV 12-V battery

 

My 12V battery will go for several days without discharging, and then suddenly drain down in less than a day. The parasitic drain seems to be very variable and unpredictable. I am guessing it has something to do with over-the-air updates. I've had no trouble since I started using a battery maintainer. The battery is also maintained when the EV charger is connected.

Please don't suggest a new battery. I've been down that road already.