Will the hybrid battery charge the 12V battery when car is turned off?

The answer is no, the hybrid battery will not charge the 12v battery when the car is off.

 

If you recharge the high voltage battery, the 12 volts battery will also be recharged at the same time.

 

That's assuming a PHEV; his bio, looks like a plain HV Prius.

You are correct on a PHEV. There's NO 12V battery recharge, just because it's plugged-in - ONLY while the traction battery is actively charging!

 

Lol, the hybrid battery barely is able to charge the 12volt when car is on...

 

I find that difficult to believe.

 

It might not apply to newer Prius. In a regular car with an alternator you get a jump start and they tell you to drive the car around for 20 minutes to recharge battery.

That won't work in a Prius because the computers are sending weak sauce low amp charge to the 12v battery so you'd have to keep the car running for 8 hours for equivalent recharge to 12v.

Because of this it's wise to put your 12volt on a charger once or twice in winter months if you're only doing short drives around town and not driving everyday...

 

I had a Gen 3, now a Gen 5. Never have I seen weak charging. It seems to charge very rapidly, with what I believe is a 100 amp DC-DC converter.

 

What often gets missed on all sides of the "how is the 12V battery charged?" question is that in most Prii (late model Primes being a possible exception) there is no separate "charging" circuit for the 12V battery. It sits on the car's 12V bus just like all the other 12V equipment sits on that same 12V bus, and the output of the DC/DC converter (when the car is READY) goes onto that big shared 12V bus. It powers everything that uses power from that bus, and when the bus voltage is above the 12V battery voltage, some current flows into the battery and charges it.

The gen 3 DC/DC converter has a published max rating of 120 amps output. I don't know the spec for later gens. That's just the max power the converter can supply to everything on the 12V bus that is drawing power, and does not mean the converter is somehow able to cause a specific amount of current to be drawn.

The control the car's computers have over the battery charging current has been greatly exaggerated. The computer is able to select different output voltages from the DC/DC converter, generally in the range 13.5 to 14.8 volts or so ... that selected output voltage goes onto the bus ... the 12V battery sits on that bus, and whatever charging current the battery sucks down at that voltage and its current state of charge is what it gets.

In gens 1, 2, and 3, the computer didn't even know how much current was going into the battery. Gen 4 added a current sensor there. Gen 3 has a temperature sensor mounted in the air an inch or two above the battery, probably as much to measure ambient temperature around the battery as the battery's temperature itself.

So the computer's ability to 'control' the charge rate into the 12V battery is very limited; it has not much info on what charge rate is even happening (better in gen 4 and later, lousy in gen 3 and earlier) and the only 'control' it has is by changing what exact voltage the DC/DC converter puts onto the entire 12V bus.

The battery has a label on it advising any human charging the battery to avoid rates above 5 amps or so. The car, of course, regularly exceeds 5 amps (make the car READY when the battery is quite discharged and the resulting charge current might be several dozen amps at first). But the charge rate rapidly comes down as the battery voltage comes up, so most of the time except right after starting with a low battery, the charge rate is in the ballpark of what the label says.

Just like the DC/DC converter feeds onto a big shared 12V bus, and has no special private connection to the 12V battery, the converter is also powered from a big shared high-voltage bus, not just by the traction battery. Most of the energy ever drawn from that bus is coming from the gas engine turning an MG or from gravity and momentum turning an MG. Even when the energy on that bus is coming from the traction battery at this moment, it's just energy that came from somewhere else earlier.

... which is why I generally don't say "the traction battery" keeps "the 12V battery" charged, but rather that the whole high-voltage system powers a DC/DC converter that powers the whole 12V system (and, in so doing, keeps a charge on the 12V battery).

 

I think it should be obvious that the 12V battery system is inadequately designed for the loads put upon it by the various electrical components. The number of complaints on this forum alone should make that obvious.

What's needed is a sensing circuit that protects the hybrid battery from over-discharge, when the DC-DC converter is activated to protect the 12 V battery from going dead. As it now stands, the 12 V battery can go dead in less than a day while parked, and the car will not start, only giving a message to the driver that the voltage is "too low to start." This requires either a battery boost or a service call to the dealer.

It should work like this: for example, the driver pushes the start button, but the 12V battery is too low to start the vehicle. Instead of an error message, the system should sense whether the hybrid battery has enough charge to activate the DC-DC converter, and "boost" the 12 V system to start the car. If the hybrid battery is too low, the error message can occur, as before, but this would be considerably more rare.

This would alleviate one the most annoying flaws in the design.

 

The high-voltage system is isolated, cut off by the system main relays, when the car isn't READY. Bringing the high-voltage system (including the DC/DC converter) online requires 12V power for the computers to run through their safety checks and power the system main relays to close them. It's not a whole lot of power needed from the 12V battery for that to happen, but if the battery is too dead to supply that much, that's kind of where the story ends. It's not like the computers are successfully waking up and saying "we could close the main relays now and make the car READY but we're going to give you an error message instead to annoy you". There just isn't the power to do it.

It doesn't help that there are some other large-ish loads like the brake pump that usually also try to run right at the same time you're getting in the car and trying to start it. If they had, say, a circuit that would delay the brake pump coming on below x volts until after the car was READY, that could help.

 

I understand the reasoning behind the way it's designed, but it's flawed reasoning.

 

Kind of ridiculous when you think about it, because there is at least 16 kWh of charge being unutilized, while you call for a boost or a tow.

 

You've described what you would like it to do instead, but you haven't described how you would accomplish it, subject to the same practical constraints Toyota's engineers faced (including the full safety isolation of the high-voltage system when not READY).

It would be fun to have a hand crank. I'm pretty sure it would be easy to crank up enough juice to power the computers and close the main relays. And it would have a great retro look.

 

You could adapt a $700 Antigravity Lithium 12v battery with a boost built in. A jump is on your keyfob. Some owners of garage queen supercars use them.

Personally with close to 500,000 miles of Toyota hybrid driving, I find the agm design quite robust lasting 8-10 years on average, compared to 3 or 4 years in previous conventional cars.



A $99 Noco GB40 is enough backup for me. I think I have used it once on my hybrids; several times on other conventional cars.

 

I could, but I shouldn't need to. That's the point.
I carry a booster pack for when I need it.

 

The hybrid battery is isolated, but it doesn't need to be. A sensing circuit could decide if the hybrid battery could "afford" to power the DC-DC converter to start the car when the start button is pressed, where it currently disables the car with the "battery too low" message.

Simple stuff, really.

 

Well, you could make that case to whoever drafted their safety requirements, and maybe you'd change their minds.

 

Are you sure about that?