2024 Prius Prime dead battery

First of all, since your 12-V-battery SOC neared zero, it resulted in sulfation and permanent capacity loss. This is irreversible. Keep that in mind.

The problem is that the Gen 4/Gen 5 Prius/Prius Prime battery-management system likes to keep the SOC around 70–90% and won’t charge the battery much no matter how long you drive the car except once in every 20 driving hours. This could sometimes result in the SOC getting too low. Therefore, the only way to eliminate 12-V-battery problems on Gen 4/Gen 5 Priuses/Prius Primes is to periodically use a battery maintainer (such as Noco Genius) to bring the SOC to 100%.

AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system | PriusChat

 

Search the site using google to discover the many gen 5 12 volt battery issues.

the 12v only boots the ecu’s and closes relays. It does not start the engine.
That task is performed by the hybrid battery, so no need to run in charge mode

 

That is correct. There will be a significant but very brief current draw (perhaps 50 A or more and for about a second) from the 12-V battery during the starting to fire up the electronics, but there is no need to put the car in the HV mode or charge mode, as it is the DC–DC converter that charges the 12-V battery, and the DC–DC converter is agnostic to the selection of the HV/EV/charge mode, as it is powered by the traction battery, not the engine.

 

Do you think there is a way to change the programming of the 12 volt charging? It should always try to charge to 100%, like old cars do. It wouldn’t cause extra traction battery use in the simple way to look at it. What is the savings to keep the 12 v 70-90? It still is replacing all the lost charge from the 12 v either way. No more or less with either.
Could it be the Toyota system is so primitive it doesn’t stop charging like a 1964 Plymouth alternator/regulator would? I may try and see if the dc converter ever turns off charging when car is in park. I always see charging, never “charged”.
I remember the P2 never stopped charging? Don’t remember exactly. Then the Camry hybrid added a temp sensor which must work to stop charging. Now we have this battery sensor thing.

 

the pip kept the 12v fully charged. prime engineers made a conscience decision not to, you'd have to ask them why.
change the programming; anything is possible, no one here has the technical knowledge to pull it off.
and so many things are proprietary with toyota, it would be a difficult task for the best software guru.

 

So you agree that the only way to eliminate battery problems in a Gen 5 is to use a battery maintainer to bring the SOC to 100% ?

 

With my Uplus AGM battery in My Gen 4 Prius Prime, I’ve been observing the next-day resting voltage, and so far, it has not fallen below 12.71 V, which is 85%. So, there is no need to hook up to the battery maintainer at the moment. However, I will definitely do so if I don’t drive the car for more than a few days.

 

I don’t think there is a simple way.

Interestingly, the Gen 5 new-car features manual mentions that when the 12-V-system load is high, the DC–DC converter produces the maximum voltage [14.24 V] to avoid a system instability. For example, I see this when I use the electric power steering brought to the stops. The voltage will rise to 14.24 V when the steering wheel is brought to the stops, and it will stay there for ten or twenty seconds or longer even after the steering wheel is brought back to the center.

I am still waiting to see when the next once-in-every-20-driving-hours fully charging event will take place—perhaps this week.