I didn’t see this mentioned–the alternator also runs from a belt, which can also fail.
This is out of necessity, to avoid running the engine to keep AC working and not for efficiency of using an electric A/C compressor in itself.
As pointed out above conversion from mechanical to electrical and back to mechanical energy is not 100% efficient and has losses.
This is a compromise they make on hybrids because they gain efficiencies by not running the gasoline engine but is less efficient than a direct drive would be.
12V could run these pumps but at very high current with large enough cabling. Automobiles actually went to 12V to save money and cost on wiring compared to the older 6V systems. As higher currents require thicker more expensive wire that also has a much higher weight which would also have an impact on fuel efficiency these days.
According to this source, a typical OEM automotive alternator can provide 65-100 amps of current. At 12 volts, that’s 780-1200 watts, or 1.05-1.6 horsepower. So what’s the power consumption for various accessories?
heated seats: 45 watts (each)
heated steering wheel: 20 watts
Headlights: 70-110 watts (depending on whether HID, LED, or incandescent)
rear window defroster: 250 watts
fuel pump: 100 watts
engine ECU: 25 watts
Fire up all of that stuff on a cold winter morning, and you’re asking the alternator for 550 watts (and if the engine is at idle RPM, then the alternator isn’t able to make that much power, so you start sucking it out of the battery instead). The remaining scraps of capacity are enough to run the various other lights on the exterior and interior of the car, the sound system, and whatever you’ve got plugged into the cigarette lighter.
Bottom line, yes, the alternator has enough capacity to power all of the things the designers could reasonably foresee it might need to, plus a small margin. But they didn’t design it with a gigantic excess of capacity; that would have been a waste.
In the case of the fuel pump there are practical reasons for using electric in-tank pumps on fuel injected motors.
- They get the system up to pressure before the engine is started so you don’t have to run the starter as fuel pressure builds.
- High pressure pumps suffer from cavitation that dramatically reduces their useful life when they have have much of a draw.
- The fuel in the tank is used for cooling, if you know someone who has often has fuel pumps fail ask them if they tend to run near empty a lot and there is a good chance they do.
There are always trade offs in complexity, cost , reliability or efficiency.
FYI, cars are moving to 48 volt systems for the very issues with 12v mentioned here.
Interestingly it’ll be a 12/48 volt hybrid system with the 48 v system complementing a normal 12 v system .
Finally someone touches on this …
This is precisely why electric water pumps are a common upgrade on high performance engines. Because the flow can be tailored to meet demands instead of moving in lock-step with the engine RPMs. This is also why we have electric radiator fans as well. Because the old paradigm of driving them off the crank often had radiator fans working hardest (thus using the most energy) when they were needed the least.
[Moderating]
SamuelA, the manliness of cars is not a question for this thread (or, indeed, for this forum), only the efficiency.
High RPMs and large HP and thus heat make this more important but at a typical lower HP level with a flow rate of 10GPM at 20PSI at 75% water pump efficiency and 90% drive efficiency you need about 0.15 HP to drive a pump no matter what the drive source is.
If you have a M3 with a 8000 rpm redline it becomes problematic to make a pump that will operate efficiently from idle to redline.
If you are producing a passenger car which will stay in the “eco mode” RPM ranges for EPA tests or most of it’s run time in that range the advantages start to go away. If you don’t have special needs like wide operating parameters or ultimate performance despite the cost of the more complex systems; higher losses from energy conversion and weight may not be beneficial.
The decision is application dependant.
I believe that is for safety reasons. 48 VDC is a shock hazard, so for interior electronics the circuit protection would probably have to incorporate ground fault interruption. Also, 48 VDC can strike and maintain an electrical arc which is a fire and shock hazard.
Makes sense. This, by the way, creates the absurd scenario where Tesla, Bolts, Volts, and Prii all have large capacity high voltage battery packs, yet one of the most common failures requiring a service call is a failed 12v lead acid battery.
I have kinda wondered if they couldn’t just run the DC : DC converter all of the time and use a bank of ultracapacitors instead.
Essentially the architecture would be in these vehicles that there would be an efficient, low idle power draw DC : DC converter that would convert from the 200-400 volt main battery packs all these vehicles use to 12 volts. The capacitors are just there to keep the bus voltage up for a few minutes if the DC : DC converter fails. (say, while driving down the road, since brake booster pumps and power steering pumps on these vehicles are all 12 volt)
DC-DC conversion is pretty inefficient, but the 12v battery failures are because keyless entry, alarm systems and other parasitic draws are typically on that system and the batteries used typically lack undervoltage protection and 12v batteries are typically purchased for the lowest cost.
If they put those parasitic draws on the larger, far more expensive batteries without undervoltage protection the cost of not plugging your sitting car into a battery maintainer would but much much more expensive.
The majority premature 12v car battery failures is caused by the loss of water from recharging charging due to the lack of maintenance etc…
Being a premium product hybrids are far more likely to be equipped with those bells and whistles too.
Why not use an electrically-driven alternator?
(Runs and hides.)
that may be true for Tesla, since they beat the snot out of their 12V battery. but it’s not clear it’s true for anyone else. “Key-off” current draw is a big deal for the industry at large.
because cars sit for extended periods, and even in full sleep modules draw some current. I’d wager a handful of ultracapacitors the size of a 12V lead-acid battery would be drained fairly quickly.
yet nobody is doing this. I wonder why…
Note that with an “ordinary” car, saving $1 here and there adds up so to keep costs down simpler, cheaper, lighter systems are employed. For something like a Tesla, the owners don’t seem to care about the cost of the car so what the hey, go for it.
Also Tesla’s don’t have to worry about things like emissions. And optimizing things for a gas powered car leads to lower emissions.
So the reason a Tesla or hybrid has X while a gas powered sedan doesn’t can be attributed to a variety of reasons.
Nitpick: There’s plenty of energy available for free. It’s free energy that isn’t free! 
Worse than the radiator fan being tied to RPM as I understand the biggest problem was that at highway speeds the fan not only was unnecessary but actually added significant drag and fuel consumption - essentially running a large propeller for no productive reason.
My 2018 volt has pretty much all electric accessories im pretty sure the power steering and AC are electric.
My 2006 Chevy Equinox had an electric power steering pump.
Several cars have used electric water pumps for years, e.g, the BMW 3-series E90 and F30 models.
There are lots of aftermarket conversions for electric water pumps.
Any of the main accessory drives are theoretical candidates for pure electric operation, such as power steering pump or HVAC compressor.
Electric power steering which eliminates the belt-drive hydraulic pump has been used on various cars for years: Electric vs. Hydraulic Steering: A Comprehensive Comparison Test - Feature - Car and Driver
In most of those the steering system is still mechanical, only using electric vs hydraulic assist. I think the Infiniti Q50 uses steer-by-wire, but still has some kind of fail-safe mechanical backup.
An electric HVAC compressor is particularly challenging on an internal combustion car because it requires so much power. A regular alternator is NOT “generating plenty of electricity”. A normal alternator might output 1100 to 1400 watts (peak), which is about 1.5 to 1.9 hp. A conventional mechanical HVAC compressor might draw 5 hp mechanical power from the engine, so you obviously would need a hugely upgraded alternator and electric system.
Supposedly automotive alternators are 70-80% efficient and an electric motor driving an HVAC compressor might be 90% efficient, so right off the bat you’re down to about 70% power transmission efficiency. Some automotive electric HVAC compressors require AC, so you also must add in DC/AC inverter efficiency, which might be 95%. Multiplying those efficiencies, the end-to-end efficiency for electric HVAC might be 68%.
If the electric HVAC compressor required 5 hp (or 3,700 watts) mechanical drive, this would require about 5,400 watts electrical output at the alternator.
If you redesigned the entire accessory drive system you might also redesign the alternator, compressor, HVAC drive motor, etc for better efficiency. Obviously the wiring and power distribution system must be vastly upgraded. Unless all cars of a certain model have that feature, it would take two different manufacturing configurations.
Obviously the technology exists, as proven by electric vehicles, but they have no choice and all accessory drive components are designed from scratch for electric operation.
which to me seems like a solution in search of a problem.