Why does the Prius need a 12V battery at all? They could use Flash memory for storing all the settings and the clock can run for years from a small coin cell. Given that Flash memory is less than $1 per GB, it seems like it would be cheaper to design it to not have a 12V battery at all.
Touche to cwerdna's response and also I would just generally say that most car makers have contracts with people who make parts in cars. If they were to completely remove the "Car battery" Which is in every car in america...they would potentially start a trend where other new cars may come out without 12V batteries. Causing them to be unhappy and in turn toyota making less money. Just work it in to the process and boom everybody happy. (Cept gas companies )
You need power to energize the high-voltage relays. I'm not sure how I'd feel if the traction battery was the only battery in the car, and we had 201.6V nominal floating through the start button...
They could add series resistors and/or fuses to limit the current, so a short circuit would not do damage. They could also mechanically link the button a distance from the actual switch. Bear in mind that in 240V countries, most people have no problem with 240V in every light switch...
From a technical standpoint, there is no need for the 12V battery. All 12V power could be provided through a DC to DC converter. From a practical standpoint, using a standard 12V battery allows Toyota to completely isolate the HV battery when the car is powered off. This is good from a safety standpoint, and also helps protect the HV battery from accidental discharge. In other words, Toyota could have forgone the 12V battery, but common sense and market pressure suggested to keep one, at least in an auxiliary role. Tom
But that would mean you potentially have 200+ volt wiring running all over the car instead of being isolated to specific known locations documented in the emergency response guides. What you suggest would also imply way more potential points of failure. Homes don't get into crashes nor do they have 200+ volts being supplied by batteries.
Not necessarily. The DC to DC conversion could be done inside the battery box, and only on demand. Similar systems are used for inverters on boats. There wouldn't be any more points of failure, and there certainly wouldn't be "200+ volt wiring running all over the car". The downside, and there always is a downside, is that the HV battery would always be under some load, even if small. This would increase the chances of draining the HV battery, and recharging the HV battery is a lot more complex than putting a charger on the small 12V battery. As I have stated before, there is no technical or safety reason that requires the use of a separate 12V battery. It's just a design decision made by the Toyota team that at this point in hybrid technology a separate 12V battery is worth the extra cost and weight. A few years down the road and we might see a different approach. Tom
In a serious crash, safety demands that the main traction electrical systems be de-energized. Would your current-limited switch be normally open, or normally closed? One method would be defeated by a severed wire, the other defeated by shorted wires from slashed insulation. Crashes produce both sorts of damage. Using external 12V to power an internal relay means that, in addition to the car's own safety responses, emergency responders can insure that the high voltage voltage is isolated by using the same familiar methods as on non-hybrids: cut a 12v battery cable, then wait a few minutes for capacitors to bleed down. This adds more reliability and crash damage issues. Bear in mind that in the U.S., most emergency responders to motor vehicle crashes have little 240V experience. There was already plenty of FUD going around, including in the ER community. Limiting the high voltage to only those loads that couldn't be run from 12V was a good step towards limiting the fears of the nay-sayers. Much of this could change as experience of all parties expands.
There are significant advantages to higher voltage devices, especially those requiring significant amounts of power. In the boating world, some of the larger yachts are now fitted with high voltage DC systems. This allows for much smaller cable, reducing weight and cost. Initially this move was controversial, but is now generally accepted as good practice. It's similar in the automotive world. We haven't traditionally used higher voltage DC because the batteries are too expensive. A simple 12V battery works well and is relatively cheap. With the Prius, all that changed. Now you have a HV battery, and the 12V battery becomes an additional expense. But Toyota was smart enough to realize that hybrid vehicles are a transition. The high voltage already scared people, even without running it to accessories. They took a safe and simple approach. It works well enough now, but I suspect it's just a transitional solution. Eventually the 12V battery will go away. Tom
My 2 cents: a battery can function as a relatively cheap, reliable, and stable voltage regulator. A DC-DC converter (converting traction battery voltage to 12V) can always have failure modes that pass the high voltage through to the output, and it's more complicated than an inverter to charge the 12V battery from the traction battery. Using a 12V battery the way it's used in the Prius seems reasonable to me.
Except all the problems posted indicate that it's not that reliable. Not cheap either. It's also very unlikely for a full bridge converter to fail in a way to connect the input to the output as long as double insulation standards are followed, no more likely than in a common mains operated power supply. (Buck and linear DC/DCs can do that, however. On some wireless routers, there was a voltmod hack that involved gluing an extra heatsink to the RF power amplifier and breaking off a feedback resistor to supply the full 5 or 3.3V to the amplifier. Then up the transmit power drive in DD-WRT and it gives wicked speeds.) Given that a 1F, 18V capacitor is only $30 retail from Fry's, they very well could have just used large capacitors to do the filtering. 1F would likely be massive overkill considering that large server power supplies, some as large as 1.5kW (125A), don't need anywhere near 1F to meet very strict transient response requirements.
Yes, those are reasonable points. I guess I didn't mean to imply that the design as it is is the "best" way to do it, but, it seems at least "reasonable" to me (i.e. not obviously stupid and unacceptable). But, I know something about electronics but I'm certainly not an expert.
