How much fuel does a gasoline car use for electrical needs?

[dnix71]

We are just getting out of a supply chain fuel shortage in south Florida. Port Everglades, which supplies 40% of the state's fuel lost the ability to fill gas station tankers because it rained too much. I know that sounds like a lame excuse in Florida, but we had 30 inches of rain dumped in central Broward from crisscrossing air streams that lasted over 3 days. There are only 12 pumps to fill road tankers at the Port and they were under water from someone not thinking about this ahead of time. The state has a lot more people than they did 5 years ago and many people drive around on 1/4 tank or less. If the public panics and tries to fill up all at once, it just crashes the system. The nearest pipeline from Port Tampa terminates in Orlando. The Colonial supplies the northern part of the state.

I have a 2008 Honda Fit with a 10.8 gallon tank. It won't go far around town on that. Highway mileage is almost double city in real life, espcially with tha a/c running.

There is actually plenty of room inside for mods. like an extended range gas tank, but I wondered about adding a battery and dc-dc buck converter to eliminate the engine load from the alternator.

That should not be hard to do, aside from tweaking the boost charge set-point so the car's computer does not realize it's not the alternator carrying the load. There is no room under the hood for any mod but wiring. I can't fill the radiator overflow bottle without a funnel and cannot reach to check the transmission fluid level without raising the car well off the ground.

[joestue]

Its pretty minimal.

If you measure the load you can then determine if its safe to drive with the alternator disconnected for short trips. 10 amps or so, plus headlights.

A 3 cell lithium battery and a diode will hold up the lead acid without a bms needed to prevent overcharging. Or a 4 cell lithium that you can connect to the car's system to, but use the 12v lead acid for starting. This is easy to do, just disconnect the cars electrical system from the lead acid but leave the starter cable on the battery.

Removing the alternator to save weight and belt friction is the next step. Or move to an electric water pump, pump up your tires to 60psi, remove the passenger side mirror.

Add sharp edge boat tail to the rear for air drag improvements.

Put an air dam in front of the tires. Etc

[dnix71]

Tire pressure makes a big difference in mileage, but this Fit is a tin bucket with a rough noisy ride as it is.
The battey is expensive but small and fragile. It was replaced this year simply because it went sour from sitting awhile without starting. I had a battery solar panel maintainer but the power port is only enabled when the key is in the on position. My 1995 Toyota Corolla had a power port that was always live. I keep the battery for months waiting to sell it with a solar dash maintainer plugged into the port.
We don't use daytime running headlights here is south Florida like they do in Canada.

[mab]

I've been thinking of trying pretty much the same idea - i try to drive as economically as i can but there are times when you have to use the brakes,  and without regenerative braking that energy is lost.

 So like you i was thinking of using a small lithium battery to take the electrical load off the alternator,  then possibly when braking, switch to using a boost converter to load the alt and recharge the lithium battery. I.e. turning the car into a very mild hybrid.

From looking online i got figures of 2 - 4% electrical load, but it obviously varies a lot depending on if you're city or open road driving and what electrical loads you have: in my case the car has an electric power steering pump and in the winter i can be using lights, heater, window demisters, etc.

I've been trying to log my actual miles per gallon in order to have a baseline to compare with, once I've installed the battery, but haven't got any further yet. When i do I'll post the results.

[dnix71]

It's annoying when you are behind one at a traffic light, but many cars shut the engine at a stop and run from the battery. The Honda Fit battery won't handle that. It's way too small.

I saw a Cadillac CT5 than went dead in a parking lot when the 5 year old battery failed. That battery was a beast of a starting/deep cycle. A lot of cars now will refuse to run unless the battery is within specs. This one only had 4 volts on it and wouldn't come back up with a boost.

The simplest battery boost would be to add a switch on the dash. Brake until you stop and then click the switch. In south Florida, a/c, engine cooling fans and wipers use a fair amount of power in city traffic, depending on the weather and when you are stopped the engine rpm is not ideal for your alternator anyway. Unloading the alternator at idle will make it last longer and your car run cooler.

If you are towing a trailer, a self-contained brake/light feed will save you something. Florida does not require e-brakes on small trailers, but the module that senses turn signals and brake/running lights must either be powered from an extra battery or the car's battery. My Fit has one of those. I connect one sense wire and plug a 12v positive feed wire into the dash port. Down side of that is that it only works with the engine runing or the key in the on position.

[JW]

I am assuming that you are talking about a car alternator so here goes.

80 x 14.5 = 1160 watts ( despite the 12V battery the alternator output is nearer 14.5v)

1160 watts = 1.55 Hp if 100% efficient. An alternator is nearer 55%.

You therefore lose approximately 2.75 Hp driving the alternator if it is under load.

[MattM]

We get a lot of fog in northern Florida, so daytime running lights are common on vehicles.  Lots of numbnuts forget to use headlights in fog conditions, so it is very much warranted.  I'm surprised they don't offer a quasi-hybrid option for cars rather than full EV.  American made cars get a $3750 tax incentive while American-made EV batteries get another $3750 tax incentive.  You no longer need the full EV experience for the tax credits.  The vast majority of travel is during daylight hours where an integrated solar roof of 100W would probably handle 80% of your loads.  Regenerative braking is an idea that should have been incentivized a long time ago since its a small part of a price tag when average cars are in the high $40k these days.