Just so I understand 12v charging

yes, and this is now definitive, as per the 'new car features' booklet, and weber state dismantling.

however, it may be a defective design, leading to dead batteries in some cases. just a guess though

 

IF......the "normal" charging system is working correctly, there should be no reason to also charge the 12 V during "plug in" charging.

 

and yet, toyota has added extra equipment to do so, at considerable expense, no doubt

 

There is no "normal" alternator. All charge to the 12v battery comes from the traction battery. That is charged from the regenerative functions, or the engine, or by plugging in. When the system detects that the 12v battery is at the minimum allowed voltage, it just does its thing. Stops at the max allowed voltage. It's invisible to us.

 

When the car is off, the traction pack may not be connected to the 12volt. The safety protocol for hybrids is to physically disconnect the battery from the drive train when off since the first Prius. The starter battery may still be in circuit with the traction battery, or it may not.

 

either way, the hybrid battery does not keep the 12v charged when the car is not 'ready'.

 

Well that's confusing.
Since the original question was "when plugged in"......and I'm pretty sure that it would NOT be in the 'ready' mode when plugged in.

 

Yes. In the Prime manual (p. 126 of the one I found), note the last line.

■ Charging time may increase
In the following situations, charging time may become longer than normal:
●In very hot or very cold temperatures.
●The vehicle is consuming a lot of electricity, for example, when the head light switch is on etc.
●There is a power outage during charging.
●There is an interruption in the electrical supply.
●There is a drop in the voltage of external power source.
●The charge in the 12-volt battery is low, for example due to the vehicle being left unused for a long period of time.​

I have measured the voltage at the 12V test points in my PiP, and that voltage increases when the charger is running, but not when it isn't running.

 

very confusing, you should see the threads on it. it may be that that plugging in activates a relay, or there is a separate charging circuit.
i haven't watched the weber video, but this may be the most controversial feature of prime, or close to 'why did my engine start'?

 

you may be the first to find this, or i missed it. there are members here who have measured the 12v charging and not charging, and claim there is no difference.
if i under stand electroengineur correctly, prime will vary the 12v charge, where pip seems more fixed. and prime charges at a lower rate.

 

Anyone who really believes that, as you wrote it, just doesn't have any concept of how electricity works.

 

volt meter doesn't lie

 

The reason for it is that plug-in charging runs the HV battery fans and probably other things (HVAC fan during traction battery cooling, etc), which are 12V. If it didn't charge the 12V battery while plugged in, it could be drained from the HV battery fan running for 6 hours.

 

This makes sense.
The built in charger for the Prime and PIP may be a little different due to one having an AGM battery "I assume" and the other having a more standard lead acid battery.

 

If the voltage at the 12V battery is the same when the charger is not operating and when it is, the 12V battery is not being charged. Physics prevents otherwise. You need voltage to "push" the electrons into the battery.

 

That's true. Maybe it's just enough to compensate for the load of the battery fans and anything else that runs during charging. It's probably smart enough to check the initial voltage of the battery and see that it doesn't need to be charged. There is a current sensor on the 12V battery, so if they control the battery current to 0A, there would be no change in voltage of the battery, and the 12V charger would provide exactly enough current to power the 12V accessories that are running. There is a line in the manual that mentions charging might take longer if the 12V battery is low. Maybe the 12V charger will actually charge the 12V battery if it starts out low, but no one who has measured the voltage has seen that since we drive our cars regularly, and the HV system keeps the 12V battery topped up while driving.

I have not measured the battery voltage myself, but I would be surprised if it was always exactly the same during HV charging as the voltage with the car unplugged. It would be unusual for them to control the 12V battery current to 0, since it's easier just to float charge it at a constant voltage while running the accessories.

Or maybe 12V charging happens during the last 20 minutes of charging, after the HV battery is mostly charged but the car is still using some power (800W?) from the charger. They would still need to keep it topped up during HV battery charging though.

Without doing more measurements, all we can do is speculate about how it works.

 

however it 'works', it seems problematic, or perhaps just a bunch of bad batteries

 

How so?

Are you referring to a couple recent posts by people complaining about dead 12V batteries? It didn't sound like it's necessarily a problem with the 12V charging system. More likely they accidentally left the car in "Ignition On" or "Accessory" mode without realizing it, or somehow a door was left slightly open or lights were left on. It shouldn't be possible to drain the 12V battery that way, but maybe there is a bug in software that controls it somewhere. Since it's a small battery there is not much reserve capacity to run 12V loads for very long.

For example, on my 4runner the headlights automatically turn off after some time (30 seconds or a minute). But if I turn the lights on after turning the car off, they will stay on and drain the battery. I think I've heard there is a 30 minute timer that will shut off everything, but I haven't tested that. I don't know if the Prius works the same way, but it's possible since it is a Toyota.

The majority of Prime owners have had no problems with their 12V batteries, otherwise we would hear about it here.

 

this has actually been happening for the 2 years prime has been out. difficult to diagnose over the internet, but way too many dead batteries to be a coincidence. and those are only the ones reported here.