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To Power the Future, Carmakers Flip on 48-Volt Systems

nytimes.com

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Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#11

Post by twic »

sowbug wrote: post #8
saltcured wrote: post #6

Earlier quoted context omitted.

You want a higher voltage to allow more power without huge cables due to high current. You want low voltage to avoid high-voltage safety issues which increase engineering, assembly, maintenance, and emergency costs. The 48V system sits right at the boundary where you can deliver enough power for more auxilliary systems and some mild-hybrid techniques including frequent start/stop systems, brief acceleration boost, an…

Thanks. I had always understood PoE's relatively high voltage as helping deal with the attenuation from long runs of thin copper wire. I couldn't find a succinct explanation in the article why cars would benefit from this spot in between merely sending information (digital levels like 1.8, 2.2, 3.3, 5.0, etc.) and using electricity to do "real" work (120V, 240V, 480V, etc.). As I understand your answer, yes, they'd l…

The specific number of 48V is presumably inherited from telecommunications:

https://electronics.stackexchange.com/questions/10851/why-do...

There's lots of telecom equipment that works at 48V, so it's presumably cheap and easy for carmakers to find suppliers.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#12

Post by Gibbon1 »

saltcured wrote: post #6
sowbug wrote: post #3

There is a TLDR for this article, but as far as I can tell it cannot be derived from the article itself. Will someone please explain why 48-volt systems are newly desirable in cars?

You want a higher voltage to allow more power without huge cables due to high current. You want low voltage to avoid high-voltage safety issues which increase engineering, assembly, maintenance, and emergency costs. The 48V system sits right at the boundary where you can deliver enough power for more auxilliary systems and some mild-hybrid techniques including frequent start/stop systems, brief acceleration boost, an…

> Yet, it's not high enough to require switching to a different electrical wiring safety paradigm.

This is the tldr. Also once you ditch lead acid batteries for higher voltage hybrid batteries there no reason to keep using 12VDC.

The math bit is that the required wire size is related to the inverse square of the voltage. At the same time reduced current means smaller switches.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#14

Post by cameldrv »

maxerickson wrote: post #2

This article is kind of weird. The push for 48 volt is mostly about secondary systems that need a lot of power and doesn't have much to do with "mild electric" or whatever it is implying. The 21 HP direct boost mentioned in the article is going to be for performance rather than running on electric, the vehicle comes with a 1 Kw-h battery (~5 minutes at full output).

Whether it's for performance or economy depends on how it's tuned and how the engine is sized relative to the equivalent pure gas car. The attractive things about these mild hybrid systems is that they're cheap, they don't weigh much (so you're not hurting efficiency by hauling around a bunch of batteries and electric motors), and they give you a lot of what a full hybrid (non-plug in) offers.

If you're not plugging in, you don't need a big battery, because the hybrid only needs to store the energy from braking, and then deliver it when the car speeds up again. Even though the motor is only 21hp, it provides a lot of low-end torque, which lets you have good acceleration from a stop. This is something modern efficient gas engines are generally bad at, and the immediate acceleration in a lot of modern cars is even worse because of aggressive upshifting to save fuel. The electric motor papers over this deficiency, so you can be more aggressive about using a smaller engine with less low-end torque. The electric motor is also much more powerful than a normal starter motor, so it allows the car to more aggressively shut off the engine at stop lights and even when coasting, and turn it back on very quickly.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#16

Post by dogma1138 »

maxerickson wrote: post #5
sowbug wrote: post #3

There is a TLDR for this article, but as far as I can tell it cannot be derived from the article itself. Will someone please explain why 48-volt systems are newly desirable in cars?

The higher voltage means lower amperage, which means smaller components and so on.

You can use thinner cables or have the same cables with less loss. As for actual component size it’s not that simple the higher the voltage the more clearance you need between conductors. So that is bigger airgaps in PCBs and higher rated insulation where gaps aren’t possible.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#17

Post by bsder »

PhantomGremlin wrote: post #10

It's too bad that automakers didn't make this change a long time ago. One thing the article didn't mention is that power losses in a wire are a function of the square of the current. So, let's say one needs to deliver 48 watts to a load. In a 48 V system, that's 1 A of current. Using 12 V, that's 4 A of current. Power loss is 16x greater at 12 V for same diameter of wire, loss = (4 A * 4 A) * resistance of wire.[1] T…

You're not wrong, but you should probably tell the whole story if you're going to post on HN.

Primary Objection: Um, Ohms law gives V=IR. And P=IV. So, P=I^2R. Except that P ALSO can be calculated as P=V^2/R. So, we got squared losses anyway and R should go the other direction. WTF?

The answer is that a copper wire is a distributed system, not a lumped system. So, you derive the power loss from the generated heat across a differential slice of the line and then integrate over the whole length of the line--which gives you Power=RI^2 in the limit (and V->0 for any individual slice as the differential ->0).

It is one of those (un?)fortunate happenstances (like the Bohr hydrogen atom model) where the system in the limit sums to something we already know and leads to a bit of misunderstanding when you confuse two things which appear to be related but really aren't...

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#18

Post by bigiain »

saltcured wrote: post #6
sowbug wrote: post #3

There is a TLDR for this article, but as far as I can tell it cannot be derived from the article itself. Will someone please explain why 48-volt systems are newly desirable in cars?

You want a higher voltage to allow more power without huge cables due to high current. You want low voltage to avoid high-voltage safety issues which increase engineering, assembly, maintenance, and emergency costs. The 48V system sits right at the boundary where you can deliver enough power for more auxilliary systems and some mild-hybrid techniques including frequent start/stop systems, brief acceleration boost, an…

From my reading of the article, 48V has been chosen to comfortably fit in under some regulation that defines 60V as “high voltage” which required special (read “more expensive”) components.

Keep in mind there’s no such thing as a “48V battery” or a “60V battery” if you have hard requirements on those voltages. If they’re using lithium polymer batteries, they’re rated at 3.7V per cell nominally, but will be at 4.2 or 4.3V when freshly charged - so a 48V lipo battery will probably be 13 cells and 48.1V nominal, but possibly as high as 55.9V fully charged (and one more cell in a 14 cell pack would hit 60.2V fully charged). Even lead acid batteries do the same thing, your 12V battery today is six 2V nominal cells in series, but you’ll commonly see 14V when it’s fully charged, so four current tech car batteries with a nominal 48V rating will have 56V fully charged.

I don’t think there’s anything “optimal” about 48V apart from it being as high as you can risk going using available battery technologies without bumping into the 60V “high voltage” regulations when your battery is fully charged.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#19

Post by maxerickson »

cameldrv wrote: post #14
maxerickson wrote: post #2

This article is kind of weird. The push for 48 volt is mostly about secondary systems that need a lot of power and doesn't have much to do with "mild electric" or whatever it is implying. The 21 HP direct boost mentioned in the article is going to be for performance rather than running on electric, the vehicle comes with a 1 Kw-h battery (~5 minutes at full output).

Whether it's for performance or economy depends on how it's tuned and how the engine is sized relative to the equivalent pure gas car. The attractive things about these mild hybrid systems is that they're cheap, they don't weigh much (so you're not hurting efficiency by hauling around a bunch of batteries and electric motors), and they give you a lot of what a full hybrid (non-plug in) offers. If you're not plugging…

I buy what you are saying but that particular gas engine is 330 HP.

Re: To Power the Future, Carmakers Flip on 48-Volt Systems

#20

Post by bsder »

sowbug wrote: post #3

There is a TLDR for this article, but as far as I can tell it cannot be derived from the article itself. Will someone please explain why 48-volt systems are newly desirable in cars?

Other people talk about the power, but there is also the fact that lots of actuators want more voltage to work with. So, any motor, servo, solenoid, relay, etc. works much better at higher voltages.

In addition, it is also much easier to convert between high voltages because your transformer/inductor connected to your semiconductor switches is so much smaller.

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