Please explain 12V system to me--how to keep battery charged while using accessories?

You cheat as bad as I do! Except I was in too big of a hurry to cheat this time.

 

Ya, that manual is ridiculous...

 

The diagram shows a 100 amp fuse for the DC to DC converter, so at 14.4 volts x 100 amps, you can hook up a 1440 watt inverter?

 

... assuming none of the car's own built-in equipment downstream of that fuse was drawing any current at the same time as your inverter.

... also assuming the converter's output holds solid at 14.4 volts even at full load. It may droop some.

 

It's not going to be 14.4 like a normal car - particularly if the ICE isn't running. As for 100 amp, that could be tenuous too - a fuse is always over the rating for the circuit - not an indication of output.

(2) How to check a Hybrid/EV 12V Charging System - YouTube

A bit like a speedometer - this is Dad's car in the mid-'60s - with a top speed of 85mph - if you were game. 110 was ... just a figure.

 

Whether the ICE is running is one thing that has scarcely any effect on the voltage out of the DC/DC converter (unlike in a conventional car, where the voltage usually goes up some with increased alternator RPM). The innards of the converter are more like those of a regulated computer power supply; in my Gen 3 it is generally 14.7 right on the dot, whether the engine is on or not, with scarcely a wiggle when it starts or stops.

The voltage can change at times, but for other reasons. It will be dialed back some from 14.7 when the 12 volt battery is thought to be hot or fully charged, and yes, it will droop some near the limit of the converter's capacity.

 

Maybe I'm missing something - but John Kelly on the video I quoted shows:

 

That's more a number you might see when the hybrid system is disabled (car is OFF, or in ACC or ON, not in READY).

If you ever see a voltage like that when the car is READY, your DC/DC converter has failed (or a fuse in that circuit).

 

OK - further on in the video (13:00), he shows it at 14v with it on ON. But he also shows that there is about 13 amps being drawn by the vehicle with "nothing" on.

 

Prof. Kelly is not prone to making mistakes of that magnitude, and I might not have time right now to find out what in the video might have given you those impressions, but at the 13:00 point the car is clearly in READY, not ON. The voltage reading of 14 is one dead giveaway, and the nonzero amp reading on the leftmost meter (showing the DC/DC converter output) is another. The DC/DC converter only operates in READY mode. It does not operate in ON, or ACC, or OFF. The car entered READY mode at around 12:06.

I'm not sure what significance you're attaching to that, or how we got from volts to amps. But with the car in READY mode as it is at that point, 13 amps is nothing surprising; there's plenty of gear in the car receiving power when in READY.

 

Post #43 was assuming that because there was a 100amp fuse that he would have 100amps supply available. But if the car, in READY mode is using 13 - that's cutting into the 100 isn't it?

 

Post #44 was clear that all of the car's built-in equipment has to be accounted for, and that can even be a lot more than 13. Watch the video for a couple more minutes and Prof. Kelly turns on some headlights and HVAC and raises the current draw to over 50 amps.

There are some subtleties to the wiring diagram; not all of the internal loads come from downstream of that 100 amp fuse. Some do, and some are on the converter side of it. The converter's actual capacity may be somewhat higher than 100 amps, but Toyota doesn't publish it. If you are planning to add a load drawing anywhere near what you think the total capacity is, your best practice is to monitor the converter's IDH signal (indicating when the converter thinks it is being overloaded), and/or watch for a significant droop in the voltage. (You can see in the video that quadrupling the current demand changes the voltage reading by barely a tenth of a volt. That's not a significant drop, but load the thing past capacity and you can see one.)

Whether the built-in loads cut into the "100 amps" you can draw through the fuse depends on whether they are downstream of it. The loads that are upstream don't contribute to the current the fuse is seeing, but they are still loads the converter has to supply, so you have to allow for them. The way the wiring diagram is divided up like that seems a bit arbitrary and depends on the model; there's no very quick way to sum it up.

 

Not because the ICE is off. Because the 12V battery has enough charge that the converter steps down its output. I see those changes on my "cigarette lighter" meter all the time while driving my Prime in EV mode. Starts out over 14V, then down to 12-1/5 or so. Then back up. And so on. Behaves the same in HV and EV.

 

I have a similar meter and I find that if it stays at 14v (charging) for any length of time, it's usually an indication that the 12v is low and needs a top up, so I stick the CTek on it for a few hours.

 

Later models of Toyota hybrids use a Hall effect sensor instead of a shunt resistor to measure auxiliary battery charge current, but the objective is the same. How best to attach any accessory 12v wiring depends on the use of this signal. Posts #31-34 above seem to assume that it is used in Coulomb counting, but this is notoriously difficult to implement effectively due to variability of lead-acid charging efficiency, so I doubt that Toyota uses it.

The alternative is that battery charge current is used as a signal to decrease the DC-DC converter output to a “trickle charge” voltage when the battery is nearly fully charged. I think this is the method used by Toyota. In my 2019 rav4 hybrid, DC-DC converter output voltage drops from 14.2v to 13.6v quite suddenly when 12v battery charge current drops to 1A. This method would be defeated if the sensor read substantial current loads into accessories (interpreted as a battery that is still only partially charged).

In this case, it seems best to attach accessories in a circuit to the negative battery post, before the battery charge current sensor. That way the sensor measures only the current flowing through the battery to the chassis, ie the true battery charge current. So the vehicle computer knows when to reduce the work of the DC-DC converter (improving fuel economy and converter life). I assume that battery voltage is used to restart charging, which is also fine when accessories are wired as indicated, because they will of course still deplete the battery voltage.

It would help a lot to know exactly how Toyota implements battery charge control. It would help even more if we (or Toyota service technicians on request from an owner) could ‘reprogram’ current and voltage control and output levels. This is not such a remarkable request. An owner can easily ‘reprogram’ a ‘smart’ 12v battery charger to provide charging profiles that suit different battery brands and chemistries. The cost of a ‘smart’ battery charger is a tiny fraction of the cost of a Toyota hybrid vehicle.

 

Could you share your source for this, please? In New Car Features for the 2019– RAV4 HV and 2021– RAV4 Prime models, Toyota writes, “To detect the current of the auxiliary battery, a shunt resistor installed inside the battery state sensor assembly is used.”

I’m not sure that’s true. The inverter with converter assembly has its own substantial connections to body ground. If an accessory were grounded to the negative terminal of the auxiliary battery, rather than to the body, the accessory’s return current to the DC-DC converter would flow through the battery state sensor assembly, causing an incorrect measurement. On the whole, I think it’s better to let the battery sensor measure accurately what’s happening at the auxiliary battery.

 

Shame on me: I just hadn't found where Toyota published it. At least for my Gen 3, it's right in the New Car Features manual, which normally should have been my very first guess.



I don't have the Gen 4 NCF manual handy, so I don't know if that figure is also published, or if it's the same. But that would certainly be a promising place to look.

 

For fourth-generation Prius and Prius Prime models, the maximum output current is only 100 A.

Toyota and DENSO made a major effort to reduce the size and weight of the power control unit (inverter with converter assembly), and I think the reduced output current was a result of these trade-offs. See Kitazawa, O., Kikuchi, T., Nakashima, M., Tomita, Y. et al., “Development of Power Control Unit for Compact-Class Vehicle,” SAE Int. J. Alt. Power. 5(2):2016.

 

Here is the sensor (and my accessory circuits to the battery post before it) on a 2019 rav4 hybrid. I believe that the pink and grey wires from the sensor transmit signals for current and temperature. I am not sure why Toyota remained with a shunt resistor for current measurement in the Prius, but I think either method would serve the purpose.


I can not see how an accessory circuit that is connected to positive and negative battery terminals will send any "return current to the DC-DC converter ... causing an incorrect measurement". Such a circuit has 2 wires, so it is complete without sending any current through the vehicle chassis.

IF I am correct in my deduction about the purpose of that sensor (ie IF the designed purpose is to sense battery-charging current), I think that is the correct way "to let the battery sensor measure accurately what’s happening at the auxiliary battery".

 

Thanks for sharing the photo. I’m not sure we can see enough of the internal construction of the sensor, however, to determine whether or not it uses a shunt resistor, as on Prius cars.

The Electrical Wiring Diagram for the RAV4 HV shows that the gray wire is +B for the sensor, and the pink wire is a LIN bus connection, via the No. 1 junction connector, to the hybrid vehicle control ECU.

Even if the connections are made at the battery terminals, the accessory circuit is still connected, electrically, in parallel with the battery, the DC-DC converter, and the car’s other twelve-volt electrical loads. When the DC-DC converter is providing the power (the normal condition when the car is in READY), there will be return current to it.

The hybrid vehicle control ECU uses the sensor to detect the current flowing in the battery. If anything other than the sensor is connected directly to the battery negative terminal, then the sensor could detect current that didn’t flow through the battery, or current could flow through the battery without being detected by the sensor, in either case causing an incorrect measurement.