78 volt systems?

[ex-x-fire]

On another thread I commented on a electric waterpump, this got me thinking about all the auto manufacturers we're going to put 78(?) volt systems in cars. This was 12-15 years ago, the higher volts was to run each accesory (a/c,p/s,w/p,ect) when only needed. Seems like they put alot into developing it, but never used it. I guess the gains were not worth the added weight/complexity. Do you guys think they(OEMs) should look at this technology again?

[dcb]

It was probably just too challenging to push out 78 volts to the masses given how prevelant 12 volt stuff is. Most electric and hybrid vehicles even have a 12 volt system onboard.

[NiHaoMike]

Why stop at 78v? The Prius is 200v.

[RobertSmalls]

The voltage has to be high enough to drive high-power devices with reasonable amperage, but not so high that it presents a hazard to your mechanic.

Another big application for higher voltage systems is large starter/generators. That would allow the engine to stop at idle, and start up in a few hundred ms so the system is not disruptive. A really big alternator also allows you to do most of your charging while the driver wants to slow down. The underwhelming Chevy Malibu Hybrid did this with an auxiliary 48V NiMH electrical circuit, which means that system required two batteries, plus a 48V->12V DC-DC converter, both of which add cost.

The downside of changing the voltage is that everything electrical on the car will have to be reengineered. New part numbers for every bulb on the car, you'll need a 48V radio and instrument cluster, and even the throttle position sensor and ECU will need to be revamped. If you want a 12V power socket, that will require a small 12V power supply.

[Clev]

I've never heard of 78 volt systems, but there was talk about a 42V system (essentially, 36V nominal, 42V charging). I think it ended up being a cumulative cost problem (i.e. EVERYTHING would need to be retooled, from dashboard lightbulbs to starter motors.)

Too bad, too; 36-42V would allow for a powerful enough starter that auto-stop would pretty much be a standard feature, and even some very mild hybrid regen/boost would be feasible.

[Piwoslaw]

Originally Posted by RobertSmalls

The voltage has to be high enough to drive high-power devices with reasonable amperage, but not so high that it presents a hazard to your mechanic.

I believe that 50V is the "safe limit", that's why we see RE systems (PV, wind) up to 48V, but not many at a higher voltage.

IIRC in the old days cars had a 6/7V system, which got beefed up to today's 12/14V (can't remember which year). In the 1990's there was talk about using 36/42V for all of the "power" accessories (windows, steering, heating). The higher voltage would also open the way for mild hybrids. There would still be a 12/14V system for many things, as lighting is supposedly more efficient at lower voltages.

Extra weight of a 36/42V system? Many cars already have 2 batteries (first example off the top of my head is the Peugeot 607) as 1 would have a hard time with all the electric/electronic gadgets. One more battery in a 2-3 ton luxury car or SUV will hardly make a difference. In fact, the extra 20-40 pounds would hit FE hardest in city driving, where the gains from an engine start-stop or mild hybrid system would be largest.

[Clev]

Originally Posted by Piwoslaw

Extra weight of a 36/42V system? Many cars already have 2 batteries (first example off the top of my head is the Peugeot 607) as 1 would have a hard time with all the electric/electronic gadgets. One more battery in a 2-3 ton luxury car or SUV will hardly make a difference. In fact, the extra 20-40 pounds would hit FE hardest in city driving, where the gains from an engine start-stop or mild hybrid system would be largest.

Provided you need that much more battery, of course. You can always build a battery with 3 times as many cells, but with each cell 1/3 the size.

[ex-x-fire]

Yeah, you guys are right about 42 volts systems, I don't know why 78 popped into my head. Anyway the OEMs gave up on it, though you do see that electric p/s is more common, maybe the R&D wasn't wasted.

[busypaws]

50V is the upper limit for safety. Anything higher then 50V DC and it can kill you. Yes everything has to be reengineered for 36/42V power but once that is complete everything gets that much lighter. Higher voltage means less current to do the same work. Less current means smaller/lighter wires. I think most motors/actuators would also get smaller/lighter. In the couple of years it would take to convert over the DC/DC converters can be used on the components that haven't been converted yet. I.E. spend the money in the first year converting over the radio and lights, everything else (door locks, windows, fans get run with a DC/DC converter. Next year redo these subsystems. Now start working on the new things like Water pump and AC converting them to 36/42Volts.

[Duffman]

I think 2 things happened, the electonics became much more energy effiecient (along with more LED lighting) and the generation capacity is soaring on vehicles now. Who would have ever thought 15 years ago we could get 200 amp alternators on an automotive application. I remember reading the automotive SAE magazine and there was a belief at the time that the consumption was going to surpass generating ability.