2026 Prius Plug in won't start

12v battery is dead. You'll need a jump to get started.

 

If you've got a multimeter, check the voltage. Anything less than 12.0 volts, it's seriously depleted. 12.0 to 12.5 is so-so. 12.6 or higher you're in good shape.

If you don't have a multimeter, a good-enough one is around $20~30, any hardware or automotive store will have them, or Amazon.

For a better assessment, one of the new generation of electronic load-testers is your best bet.

Barring DIY, try a jump-start. Be very careful with polarity, red to positive, black to ground. Beforehand, phone a few automotive places, verify they have compatible battery in stock, and can run a load test on your current battery.

Or maybe someone like AAA can come to you.

 

Please read manual....

Toyota Manuals and Warranties | Toyota Owners

12V battery ONLY receives a small charge while it's actively charging the traction battery. Your traction battery was fully charged on day ONE; so the other 13 days was discharging your 12V battery - due to more readiness testing while plugged-in. That's why the manual states to unplug the car when the traction battery is fully charged. Fully charging the traction pack and not using it for two or more weeks is another discussion - again read the related sections of the OM, because your degrading your traction battery pack faster than engineering projections.

You've probably killed that 6 year old battery 12V battery. Try recharging it and get it tested at any auto parts store for free. Since the battery was drained, you may get some phantom error codes (CEL), just erase them and see if they come back. AFTER you take care of the 12V battery issue.

Good Luck......

Sorry; that's a 2026??? It's similar to a gen4; just a larger traction pack. Call the nearest dealership's roadside assistance number. They will fix it for free and replace the battery, if needed. I wouldn't mention leaving it plugged-in for 2 weeks solid; since that places the system fault at your feet.... Operator error.

Toyota Manuals and Warranties | Toyota Owners

 

This is to be expected with my car. Leaving it parked for more than 4 days usually results in a dead battery, or rather a battery at too low a charge to start the car.

When the battery voltage falls below about 11.5, the car enters an error state, and will not start up. The only solution I found is to use a booster pack. The rest of the time, I use a battery maintainer if power is available.

This seems to me like a serious design flaw; it seems absurd that you need to get a boost or make a dealer service call every time the battery goes flat, which is quite often, especially considering that there are kilowatts of available power in the hybrid battery. It seems simple enough to me that it could be designed to transfer some power to the DC-DC converter, when needed (as long as there is sufficient charge in the hybrid battery). But Toyota has been no help at all with this problem, so I get by with my workarounds.

Post edit: Plugging in the charge cable will maintain the 12V battery in a limited fashion, but I found it to be unreliable over longer periods.

 

I use one of these in my live socket:

 

Paul, if you highlight some text, then embed the link in it, using this function:



It works better. Right now I'm just seeing blank space.

 

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Yes, if you leave the Gen 4 or Gen 5 Prius PHEV plugged in for two weeks, it will drain the 12-V battery. You should only keep it plugged in during active charging and unplug it within a few hours once the charging is completed.

Now, at this point, you must buy and hook up a Noco Genius and manually charge the battery for about 60 hours; otherwise, it will be seriously degraded.

Last but not least, you should not keep the traction battery at over 50% SOC if the car will not be driven immediately, as this will cause serious degradation of the lithium-ion cells. Ideal long-term storage SOC is around 25%.

 

If I had a PHEV, I'd use a home automation product (I am a Hubitat'er) and a smart switch that can monitor watts, and write a rule that when it gets down to whatever low level 'charge complete might be', shut off the switch. You could even charge it up to something less than 100% if you could roughly correlate it to watts.

 

Again, you don't want to leave the Gen 4 or Gen 5 Prius PHEV plugged in for longer than overnight, as it induces a significant parasitic draw on the 12-V battery once active charging is completed.

The Gen 5 PHEV has an adjustable SOC limit on the traction battery. The Gen 4 PHEV doesn't have it.

 

My iPhone is set to charge to 80%. Every once in a while it charges to 100%, for calibration purposes.

I ran across calibration with my Bluetti LiFePo power stations. I charge them 100% ('cause in a power outage-you need it), and drain them and recharge periodically. LiFePo apparently is not as sensitive. This is interesting:

BLUETTI Pioneer Na Sodium-Ion Portable Power Station

 

If you're going to drive a car with an eensy-weensie start battery then you REALLY need to invest in a 'jump pack' and learn how to use it properly.
No.
I'm not picking on Priuses.
MOST cars these days have undersized batteries and MOST of them will fail with zero warning.
Priuses just seem to be worse than most.
Jump packs are 'crayon eater' easy to use and most of the modern units have polarity protection built into them - meaning they won't hurt your car if you mess things up.
Even the best models are under $100 these days - but you WILL WANT TO read the instructions on the unit that you pick otherwise it might be more useful for you to just keep the $100 bill in your cargo bay.
Really.
Leaving the house when you want to (or a gas station in the middle of the night!)....can be kind of important.

As stupid as it sounds, you're probably better off driving without a spare tire these days than you would be without a jump pack - especially if you have a patch kit and a pump.

Good Luck!

 

They just need a larger 12v to handle the drawdown, it’s not that complicated

 

No lead–acid battery is large enough to be left undriven for extended periods. The 45-Ah battery on the Gen 4/Gen 5 is sufficient. Aftermarket versions are 50-Ah.

 

It is very unlikely that you will have a dead battery on the road. It usually happens at home. Although, it happened to me once in Vegas because of the 120 ℉ heat, and once in a parking lot because I forgot to turn off the headlights. In the first case, it came back to life after it cooled down (and I made a trip to a Vegas Walmart to get a replacement the next day), and in the second case, a friend gave a jump start.

And a AAA membership helps with tire and battery issues.

 

Neither is unplugging the car when the traction battery is fully charged. I've left my car on the driveway for 2-3 weeks at a time on multiple occasions, w/o issues. I don't top-off the traction battery, unless I'm going to drive it the next day.

Bottom line; if you follow OEM recommendations and don't hang a bunch of electronic garbage on the car's systems - it should perform as expected. That's been my experience...

PS: Fobs live in a Fariday bag when not in use. Prevents people from amplifying the fob signal to steal the car and also prevents the car from 'seeing' the fob - no unintended readiness testing.

 

This one:
GENIUS1
Or:
NOCO - 2-Amp Smart Battery Charger - GENIUS2