Where does the headlight power come from?

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All the accessories and control electronics have to be powered by the 12V system. The one exception I can think of might be the heat pump but I'm not sure about that one. Anything else would have been crazy from several viewpoints - not able to use standard parts in standard applications (headlights being a very good example); and running hundreds of volts all over the car would be madness - people would certainly die.
 
I thought that the higher the voltage, the lower the amperage needed for a given task. And that amps were what killed you, not volts, was I wrong?
 
WoodlandHills said:
I thought that the higher the voltage, the lower the amperage needed for a given task. And that amps were what killed you, not volts, was I wrong?

Volts x Amps = Watts

They are all locked together. 12v can still kill, it only takes milliamps at high voltage to destabilise the heart. If you have an earth leakage circuit breaker or RCD in your switchboard, have a look at the rating, ours are 11 milliamps @ 240V

http://en.wikipedia.org/wiki/Residual-current_device
 
It's harder to push enough current through your body at 12vdc to kill you, but it is still possible.

Regardless, it would be foolish to source all of the electronics running off of 360vdc - there would be no commonality between models, and suppliers would charge a fortune for the small (relatively) quantity of high voltage bulbs, radios, window motors, etc., in any vehicle verses using standard 12vdc automotive equipment. Do you really want to buy a 360vdc windshield wiper motor, or window motor, or even a switch that can safely carry 360vdc?

Other than the custom LED bulbs that may be built-into housings, any bulbs that are separately replaceable are stock 12vdc automotive bulbs. AN example of this is the high beam bulb - a stock 55W H11 halogen bulb, available almost anywhere auto parts are sold. And, even those custom LED housings are fed from the 12vdc circuits, not the 360vdc main battery (well, the 360vdc battery does recharge the 12vdc one via a DC-DC power supply).
 
WoodlandHills said:
I thought that the higher the voltage, the lower the amperage needed for a given task. And that amps were what killed you, not volts, was I wrong?

Well you were right actually, on both points.

If you require a certain amount of power to perform a given task, then a higher voltage would result in a lower current flowing into the load. Power = voltage x current. So you got this one "the higher the voltage, the lower the amperage needed" right.

(well this is a simplification actually for DC conditions but if you want to know how it really works you're into University level physics/electrical engineering and it's too much for this discussion)

It is also indeed current that kills or causes damage to body parts also. The relationship here that's most relevant is voltage = resistance x current. A body looks like more or less a constant resistance under any given condition (some conditions much lower resistance than others) and it's a fairly high number of ohms - several tens of thousands usually if you're not wet. So we can see that there is a direct relationship between voltage and current, yes it was the current that kills you (so you got this part correct also: "amps were what killed you"), but it was a voltage that caused that current to flow.

Whenever thinking on this level, water is a good analogy to electric power for explanation. On point about current killing rather than voltage, say you got hit by a jet of water under very high pressure and you died as a result. Was it the pressure that killed you or the water? Clearly, it was the water. In the analogy, water pressure is the analog of voltage (measured in volts). The water itself is the analog of current (measured in amps).
 
Those peripheral systems like the lights work on 12V. However, they get their juice not from the battery, but from a DC-DC converter. 360V DC from the battery is converted to 12V and from there used directly. The 12V battery is tiny in an i3, only 20 Ah AFAIK, much smaller than in an ICE. It would in principle be possible to eliminate the 12V battery altogether, but this is needed for some tasks like backup for emergency lighting in case of a 360V failure.

Frank
 
fdl1409 said:
The 12V battery is tiny in an i3, only 20 Ah AFAIK, much smaller than in an ICE. It would in principle be possible to eliminate the 12V battery altogether, but this is needed for some tasks like backup for emergency lighting in case of a 360V failure.
In practice, it's probably not possible to eliminate the 12 v. battery because the many control systems that boot when the Start/Stop button is pressed depend on the 12 v. battery. The DC-DC converter likely also depends on the 12 v. battery, so it doesn't provide 12 v. until it boots from the 12 v. battery. Kind of a chicken and egg problem.
 
fdl1409 said:
Those peripheral systems like the lights work on 12V. However, they get their juice not from the battery, but from a DC-DC converter. 360V DC from the battery is converted to 12V and from there used directly. The 12V battery is tiny in an i3, only 20 Ah AFAIK, much smaller than in an ICE. It would in principle be possible to eliminate the 12V battery altogether, but this is needed for some tasks like backup for emergency lighting in case of a 360V failure.

Frank

We could discuss the chicken/egg but all will make up their own mind. i.e. what is the source of the power for the accessories: the HF battery, the DC-DC converter, the 12V battery.

In the end the 12V battery provides an important function - it can't be replaced by, say, a few 33,000 uF caps. Specifically, it provides a significant store of energy which is electrically isolated from the HV battery.

In answer to the original question - where does the power for the headlights come from - the answer is clearly the 12V battery because the car can lose the DC-DC converter and the HV battery and the lights will still work. The converse - losing the 12V battery but not the converter or the HV battery and seeing that the lights still work - is not true.
 
alohart said:
fdl1409 said:
The 12V battery is tiny in an i3, only 20 Ah AFAIK, much smaller than in an ICE. It would in principle be possible to eliminate the 12V battery altogether, but this is needed for some tasks like backup for emergency lighting in case of a 360V failure.
In practice, it's probably not possible to eliminate the 12 v. battery because the many control systems that boot when the Start/Stop button is pressed depend on the 12 v. battery. The DC-DC converter likely also depends on the 12 v. battery, so it doesn't provide 12 v. until it boots from the 12 v. battery. Kind of a chicken and egg problem.

The converter requires nothing on the low voltage side to operate.
 
The 12V battery is usually just sitting there and doing nothing. All the 12V power comes from the DC-DC. The battery is only a backup for certain conditions like failure of the high voltage battery or DC-DC. I agree it is also important when the car is switched off or while systems are booting up. When the 360V has been to 0%SOC it is needed to start charging. It may also balance voltage when there are short bursts of peak power. I think however that far more than 90% of 12V power comes directly from the DC-DC without touching the 12V battery. Because of that it can be reduced to minimum size in an EV.

Frank
 
wayne325 said:
The converter requires nothing on the low voltage side to operate.
If true, then the connection from high voltage circuit through the DC-DC converter to the 12 v. circuit must be always exist which I doubt. If it works like my Honda hybrid, a 12 v. relay closes when the Start/Stop button is pressed thus completing the DC-DC converter connection between the high and low voltage circuits. So without a 12 v. battery, this relay could not close and the 12 v. battery could not be charged by the traction battery.

A definitive test would be to disconnect the 12 v. battery and try to enter the Ready state. I would be very surprised if this works.
 
fdl1409 said:
I think however that far more than 90% of 12V power comes directly from the DC-DC without touching the 12V battery.
I would be surprised if this is true. Without a wiring diagram, I'm guessing that the 12 v. battery is always connected to the 12 v. circuit as in all cars. The DC-DC converter serves the same function as an alternator in a normal car. I.e., it is feeding the 12 v. circuit only when the 12 v. battery's voltage is a bit low or the 12 v. load is high. Running the DC-DC converter continuously when the car is in the Ready state isn't the most efficient way to operate.

Monitoring the voltage at the 12 v. accessory receptacle would clarify the DC-DC converter's operation. If the voltage is always >14 while the car is in the Ready state, then the DC-DC converter is running continuously. But if the voltage jumps between 14.x and 12.x, then the DC-DC converter is being turned on and off as necessary.

fdl1409 said:
Because of that it can be reduced to minimum size in an EV.
The main reason that the 12 v. battery can be smaller in an EV is that the large current draw of an electric starter motor, especially in cold weather, doesn't exist.
 
fdl1409 said:
The 12V battery is usually just sitting there and doing nothing. All the 12V power comes from the DC-DC. The battery is only a backup for certain conditions like failure of the high voltage battery or DC-DC. I agree it is also important when the car is switched off or while systems are booting up. When the 360V has been to 0%SOC it is needed to start charging. It may also balance voltage when there are short bursts of peak power. I think however that far more than 90% of 12V power comes directly from the DC-DC without touching the 12V battery. Because of that it can be reduced to minimum size in an EV.

Frank

The fact that ultimately all the power used by the accessories can be traced back to the converter is not a proof that the battery is "doing nothing".

The battery is a component in a circuit and it's a very important one. Sometimes current is flowing in and sometimes current is flowing out; that's not "doing nothing".

Someone else pointed out the fact that because the battery has no starter to provide power for, it can be sized quite small compared to an ICE.
 
alohart said:
fdl1409 said:
I think however that far more than 90% of 12V power comes directly from the DC-DC without touching the 12V battery.
I would be surprised if this is true. Without a wiring diagram, I'm guessing that the 12 v. battery is always connected to the 12 v. circuit as in all cars. The DC-DC converter serves the same function as an alternator in a normal car. I.e., it is feeding the 12 v. circuit only when the 12 v. battery's voltage is a bit low or the 12 v. load is high. Running the DC-DC converter continuously when the car is in the Ready state isn't the most efficient way to operate.

Monitoring the voltage at the 12 v. accessory receptacle would clarify the DC-DC converter's operation. If the voltage is always >14 while the car is in the Ready state, then the DC-DC converter is running continuously. But if the voltage jumps between 14.x and 12.x, then the DC-DC converter is being turned on and off as necessary.

fdl1409 said:
Because of that it can be reduced to minimum size in an EV.
The main reason that the 12 v. battery can be smaller in an EV is that the large current draw of an electric starter motor, especially in cold weather, doesn't exist.

The converter is far more complicated than "being turned on and off as necessary" or operating like an ICE car with a simple regulator circuit. There's a bunch of SW running in a controller in there, it'll be far more complex than you've described.
 
fdl1409 said:
Because of that it can be reduced to minimum size in an EV.
The main reason that the 12 v. battery can be smaller in an EV is that the large current draw of an electric starter motor, especially in cold weather, doesn't exist.[/quote]

Whoa. With all that back and forth, my head is spinning :? . I was originally speculating that the headlights were coming from the HV battery out of a step down transformer, hence, the testing of driving without headlights in very bright big city close traffic conditions where the LED "U" lights seem to be plenty bright for drivers to see you (I never get flashed using just those). But then again, I don't see any range GOM deterioration when I switch them on, so I guess not an issue.

However with all the posters comments, I'm actually kind of surprised that I have not seen the question - "what kicks on the REX when it comes on?" I assume it's got a starter and not a pre-spun rubber band so its' cranking load has to come from somewhere... squirrels under the floorboards? However motorcycle batteries are pretty small and kick over motorcycles pretty easy. The REX is a BMW motorcycle engine right?
 
DeafSoundGuy said:
fdl1409 said:
Because of that it can be reduced to minimum size in an EV.
The main reason that the 12 v. battery can be smaller in an EV is that the large current draw of an electric starter motor, especially in cold weather, doesn't exist.

Whoa. With all that back and forth, my head is spinning :? . I was originally speculating that the headlights were coming from the HV battery out of a step down transformer, hence, the testing of driving without headlights in very bright big city close traffic conditions where the LED "U" lights seem to be plenty bright for drivers to see you (I never get flashed using just those). But then again, I don't see any range GOM deterioration when I switch them on, so I guess not an issue.

However with all the posters comments, I'm actually kind of surprised that I have not seen the question - "what kicks on the REX when it comes on?" I assume it's got a starter and not a pre-spun rubber band so its' cranking load has to come from somewhere... squirrels under the floorboards? However motorcycle batteries are pretty small and kick over motorcycles pretty easy. The REX is a BMW motorcycle engine right?[/quote]

You don't want to drive without the headlights at night to save power. If you're involved in an accident it will allow whoever else is involved to say that they didn't see you because your headlights were off. Plus it'll get you a ticket - it's illegal.

You are right, in the case of the REX version cars the 12V battery will be driving the (small) starter motor - yet another reason the 12V battery is required - and it's not "doing nothing". The converter will not be able to source all the current needed by that motor. And the battery is needed also to reduce the voltage spikes when that motor is engaged and disengaged.
 
[/quote]

You don't want to drive without the headlights at night to save power. If you're involved in an accident it will allow whoever else is involved to say that they didn't see you because your headlights were off. [/quote]

I'd like to repeat that although I may look and act like a moron sometimes, especially with the too-hard-to-resist rampant torque bursts from the i3 :twisted: , that the headlight test was just that - a test, to see if I could see any difference in battery GOM usage, which I could not with any certainty.
 
wayne325 said:
The converter is far more complicated than "being turned on and off as necessary" or operating like an ICE car with a simple regulator circuit. There's a bunch of SW running in a controller in there, it'll be far more complex than you've described.
Of course, but since none of us has the source code for this software, we can't describe its behavior exactly, so I was stating only its likely behavior. For the past 12 years, I've driven a car with a DC-DC converter between its high and low voltage circuits. Its DC-DC converter performs the functionality of a conventional alternator (i.e., keeps the 12 v. battery charged under various 12 v. loads) but whose behavior is almost certainly more complex than that of an alternator, voltage regulator, electronic load detector, etc., just as it is in the i3.
 
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