How Long Can Prius Prime Sit With Good 12v Battery

It’s confusing, but I think they’re referring to the hybrid battery’s longevity.

I’d be concerned if I saw less than 12.5 volts. That said, an electronic load test is more revealing: a battery can have low voltage but still be in good shape, testing Cold Cranking Amps. Or vice versa.

 

See attached document. There may be a newer TSB, since this is from 2021 revision 2.
It states that the first 60 days, before you need to fire-up the engine or move the car for at least 30 minutes - then every 30 days there after. On page two, it states 6 weeks between startups for Hybrid system; so I interpret that as 6 weeks max; without a trickle charger or plugged-in. You know that you should remove the AC charge cable when the traction battery is fully charged - otherwise it'll drain the 12VDC battery. Storage specifications states to store car at 80% traction battery charge or lower, so as not to damage the pack.

Hope this helps.....

FWIW, I've only gone 5 weeks without using my car. While the battery was only at 11.5VDC, it started fine - but I did put it on a 6A charger for 2hrs it get it back where it needed to be.

 

i don't think these questions can be answered accurately

 

The document @BiomedO1 is interesting, I'm going to save it for reference. Toyota's aspirations vs dealership realities is comical though:

Our '10 was built in August 2009, and we bought it in November 2010. It was at the back of a sales/storage shed, hemmed in by a lot of other cars, very dusty. A deal bird lay on the ground beside it. I doubt it had gone anywhere much, since build date. 12 volt battery was stone-dead, which we luckily discovered prior to signing, in spite of the dealership's attempts to mask it. Temp spare tire pressures when I checked later, was around 40 psi. Basically paid around $1500 CDN PDI, for a car wash...

From that document, this is interesting:

 

With respect Mendel, there are vanishingly few people spending the big bucks for a Prius 23 to let it sit idle for several months or even several days. Probably the best advice on using the car after several months storage is to try to start it, and if it does not work charge the 12V battery and try again, and so forth..

 

True. Our circumstances have changed, over the 14+ years of ownership. Early on it saw about 15k kms yearly, my wife driving it for work. She retired, then I did, then COVID, then realities of fixed income, getting older, less enthusiastic about automotive excursions, though we still get out and about.

so yeah, precious little expenditures for automotive, but still there when we need it. Helps us knowing how to keep the 12 volt healthy, with no (somewhat absurd) obligation to drive every day or two. There may be others in that scenario.

 

One thing I noticed in the TSB referenced earlier is that the quoted timeframes of 60 / 30 days between 12V recharges is based on a **disconnected** 12V battery. That tells us nothing about what a connected battery would do.

So I snooped around and found the following TSBs. They all have the same title "Maintenance for HV and Auxiliary Batteries ", but are for different model years.
2018 - 2019: https://static.nhtsa.gov/odi/tsbs/2018/MC-10141569-9999.pdf
2020: https://static.nhtsa.gov/odi/tsbs/2020/MC-10171448-9999.pdf
2021: https://static.nhtsa.gov/odi/tsbs/2020/MC-10177586-9999.pdf
2022: https://static.nhtsa.gov/odi/tsbs/2022/MC-10214322-9999.pdf
2023: https://static.nhtsa.gov/odi/tsbs/2022/MC-10227923-9999.pdf

This covers most of the gen 4 years and nudges into the gen 5 timeframe.

They specify that the 12V battery must be disconnected for storage periods of more than 30 days. I guess that implies that the car should start fine if stored for less than 30 days. Of course, that's for a brand new, fully charged battery.

One thing I question is that they say to charge the HV battery, put the car in Ready mode. That's it. Nothing more. But if you do that it will just run down the HV battery until the ICE starts. Shouldn't it say to put the car in Ready mode and then go to Charge mode?

Ed

 

You're correct if you're just recharging the 12V battery. But the document is poorly worded and throws in the idea of recharging the HV battery without specifying the Charge mode step. They state:

"When the remaining capacity of the HV battery is low, the gasoline engine automatically starts and the
HV battery is charged."

Yes and no. The ICE will start, but it won't charge the HV battery unless you use Charge mode. This is for the Gen 4 cars. Are the Gen 5 cars different?

Ed

 

The traction battery ECU will ask the ICE ECU to activate the ICE and beginning charging the traction battery. No different than your traction battery getting low on your commute. On our Primes, we have the option to plug-in and charge the traction battery - on non-plug-ins the traction battery ECU handles everything; so you don't have to force the car into charge mode. That happens behind the scenes as the different ECUs talk to each other.

FWIW; turning on the car for 30 minutes will at least activate the DC-DC converter and give the Aux. battery a bit of a charge.

Hope this helps......

 

It will charge the HV battery up to the top HV mode buffer. The the engine shuts off.

 

I've owned my 2020 Prime since it was new. I have never seen the SOC rise unless the car is plugged in or I manually select Charge mode.

For example, when I go on the highway, I always select Hybrid mode rather than EV. The traction battery's SOC at the end of the trip is the same as it was at the beginning, even if the original SOC was 50% and even if the drive was 2 or 3 hours long. During the drive, the car only charges the traction battery enough to maintain that SOC, give or take a couple of per cent. The only way to increase the SOC (up to the maximum of 80%) is to manually select Charge mode, which I have done a few times.

What am I missing??

Ed

 

Guilty as charged. But I get suspicous when a supposedly official document has obvious errors. It makes me wonder if there are other errors that aren't so obvious. But, let's get back on topic.

All we can take away from that document is that if you charge the 12V battery for 30 minutes every 30 days, the car will start. That doesn't mean that the battery will be on the edge of failure after 30 days or that 30 minutes of charging will fully charge a depleted 12V battery. Also note that although the document refers to the Prius, it doesn't mention the Prius Prime, but it does cover the RAV4 Prime. Perhaps there is or will be a newer document that includes the Prime.

Bottom line is that there are too many variables to give you any kind of definitive answer. I'm sure you know that the Prime's 12V performance is questionable at best. The Gen4 models weren't good and it appears that the Gen5 models may be worse. I advise you to install a battery monitor to watch over things so that you can decide how long it's safe to let YOUR car sit. I use the BM2 monitor. It measures the voltage every 2 min. and saves the data for 30 days. You download the data via Bluetooth to your phone whenever you want and view a graph of battery voltage vs. time. It only draws about 3ma so it has a negligible effect on the battery life.

Ed

 

That the dash display on the PHEV is not fine enough to so the SOC fluctuations going on during hybrid operation. It is quite easy to see on a 2005 dash display. Stuck in traffic with the AC running, you can watch the display drop. When it hits the lower buffer point, the engine comes on, and stays on until the SOC reaches the upper buffer point.

Let your prime battery fully drain down to that point, which is lower SOC than bottom EV range point, it will do the same. It won't charge into the EV range portion of the SOC, cause that is generally wasteful, but it will let the SOC used for hybrid operation fluctuate this way. Does the same when put into HV mode while there is still EV range available, with the goal of keeping the SOC near where it was before going into HV.

 

If I didn’t misread, this was with neg cable disconnected, so no phantom load.

 

Measured CCA is lower than spec. That alone says replacement time for me. Then the at-rest voltage: way too low.

recent measurements done on mine, an optima yellow top installed September of 2015, attached. To be fair, I keep it on a smart charger (CTEK 4.3) near-constantly between drives. The battery’s spec’d CCA is 450:

 

The definition of a good vs bad battery is the battery's' ability to do 'work'. That's measured in CCA 'Cold Cranking Amps'. The Prime is a bit different because the battery isn't used to turn/start the engine - just turn-on the computers and close the main traction battery relays. I believe you should be able to 'start' the car all the way down to around 10.75 volts. If the battery is still good, the discharge rate should start slowing down to a crawl around there.
I would give it a few more days to see if it stabilizes. If it keeps dropping, it's bad. Again, the gen4 will start all the way down to at least 10.75 VDC, your gen5 should be similar.

Let us know how it goes.....

Oh your in Florida, it's not that cold over there, that battery should be doing much better........
FWIW: every-time you walk by the car with the FOB in your pocket, the car runs a pre-ignition check; so just being around your car with the FOB will draw more power from the battery. Both sets of my keys are in a Faraday bag when not in use.
I realized this about a year ago, when I was dropping something into my trash can. My interior lamps lit-up. Didn't touch the car - I backed away and everything went back to sleep mode. seemed to be about a 3-5 foot perimeter.