Hybrid system Malfunction (Visit Your Dealership)

I have had my 2021 Prius Prime Limited for more than three and a half years now, and I have never had any 12-V-battery issues. Last summer, I left it undriven for over six weeks in hot weather with an active full Toyota Connected Services subscription, and there was no issue.

Regular long trips help.

 

A Prius battery does not have to supply much current to start the car.

As a test a few years back I replaced the 46b24r battery with an old UB1250 alarm battery rated at 5ah. It is a 4x4x3” package available for $15 new at Home Depot.

It ran the brake booster and brought the system to Ready. Then the dc to dc took over in seconds.

Obviously I did not begin with the headlights on. I also did not try to jump a discharged 46b24r which would need to be raised in voltage for the ecm to function.

The 3 pound alarm battery’s absolute maximum current is 50 amps for 5 seconds. So just about anything from 5 amp hours or better will Ready the car in a pinch.

 

Well, that makes no sense. Since the Prius has no regular alternator, it relies on a DC-DC converter to take power from the hybrid system to charge the 12V battery. If you think the system needs to draw100 amps from the 12V battery on startup, you are obviously befuddled in some way.

 

It makes no sense to you.

 

Gen 4 and Gen 5 are entirely different beasts than the previous generations. They are very hard on the 12-V system.

It not 100 A, I would guess the starting current to be at least about 50 A.

If you could start Gen 4 and Gen 5 with only a few amperes, hardly anyone would have the infamous Gen 4/Gen 5 start-up problem, but many people have reported here that the car wouldn't start when the 12-V battery got weak.

The idle power drain of my Gen 4 Prius Prime is very high. From how fast the traction battery loses charge, it seems to be as high as about 100 A at 12.6 V.

@Mr.Vanvandenburg's battery monitor is showing a brief 0.3-V drop at start-up. (The green curve is for his Gen 4 Prius Prime.) That is probably consistent with a 50–100-A (~ 1C–2C discharge rate for a 45-Ah battery) start-up current. A brief 5 A (~ C/10 discharge rate) would hardly cause any voltage drop at the battery terminal. Now, that is for a healthy battery. If you have a weak battery, especially in cold weather, that 0.3-V voltage drop could end up to be several volts, and the car could not start.

Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime? | Page 6 | PriusChat

Last but not least, if both the Ah capacity and CCA rating weren't important, the owner's manual wouldn't warn about both of them. It is not a misprint or oversight by Toyota. They know that the car has no starter. For a healthy battery, the CCA does not really matter. But a battery with poor build quality and hence a low CCA rating will increase your chances of needing a jump start for your Gen 4/Gen 5 with a notoriously hungry 12-V system if there is battery degradation, especially in cold weather.

 

That's correct, it makes no sense that the 12 V battery delivers 100 amps or even 50 amps on startup. It only has to activate the computer system to engage the systems including the DC to DC converter that serves to maintain the 12 V charging system. You obviously got it backwards, saying that the DC to DC converter required lots of amps from the 12V battery on startup. I have jump-started dead Prius batteries many times, and the connection draws far less current than you are asserting. What you said was completely wrong again, and you should acknowledge your mistake instead of doubling down on your error.

 

I agree that the newer Priuses have a starting problem. Below a certain threshold voltage, the car will not even try to go into READY mode. The Gen 3 would start if there was any amount of charge in the 12 V battery. Not so with the Gen 5. It seems to have a much higher threshold voltage and shows only an error when you try to start it. There may be enough power to turn on the systems, but it will not go into READY mode due to the low voltage error state.

I have remedied this by installing a 2 amp battery maintainer. It keeps the battery at a steady 12.5 volts. I haven't checked the voltage when it fails to start, but I'd guess it might be below 11.5 V. Such would have been plenty to start my Gen 3.

 

it is the loaded voltage that is important, not static

 

You made the naive statement that since the Prius didn't have a starter, it drew a very small current from the 12-V battery at start-up. That statement was false. Toyota does not agree with you. They emphasize the CCA in the Gen 4/Gen 5 owner's manual, and they mention the starting performance (SOF) under battery sensor in the new-car features manual. Open up your owner's manual and read it, and then argue with Toyota instead of with me. You also didn't know that the battery voltage depended on the load current, and for a battery in poor SOC, poor health, or poor function, the voltage with a load would be a lot less.

The entire 12-V system needs to be powered by the 12-V battery at the start-up, as the DC–DC converter won't kick in until then. I don't know exactly how much that current is, but it is definitely not a few amperes. It is tens of amperes, probably closer to 50 A than 10 A. That is why your car wouldn't start—not because the open-circuit voltage was too low but because the battery couldn't provided enough current. In other words, connecting eight AA batteries in series instead of the lead–acid battery wouldn't have started your car.

 

There is no need to guess. It is on page 591 of your owner's manual.

 

He didn't know that the voltage of a battery was a function of the load current, and for a degraded lead–acid battery, the voltage decreased steeply with the increasing load current.

That is why the new-car features manual describes that the starting performance (SOF) is one of the things the battery sensor measures. The sensor also measures the state of charge (SOC) and state of health (SOH). That is also why the owner's manual also emphasizes the CCA rating, as in cold weather and/or with a battery with a low SOH and/or low SOC, the SOF will get worse.

It turns out that the SOF, which is referred to as the starting performance by Toyota in the new-car features manual, is an acronym for state of function.

What is battery SOF and how to estimate it?—MokoEnergy—Your new energy-solution provider

When you buy a battery with a higher Ah or reserve capacity and a higher CCA rating as the owner's manual emphasizes, you are simply increasing your chances that the SOC, SOH, and SOF will remain high enough to start the car in more scenarios and the battery will last longer as well.

 

I can tell when someone is just guessing.

 

Like I already said, I've jump-started plenty of Priuses, and there was never a 100 amp load or even 50 amps.