To save gas, I want to disconnect my car’s alternator and then connect the car’s battery to a battery charger in the evening. I know I won’t be able to go long distances, but I should be able to go at least 30 km (21 miles)? I know because the alternator belt broke one time and I was able to drive for quite awhile without it.
Tell you what, we’ll go ahead and send you 4.00 to make up for the gallon of gas that you’ll save every month, and you keep your alternator connected. 
But seriously, I really doubt that it’s worth it. Your battery wasn’t built for that kind of drain-recharge activity, and it’s not very good to be subjecting the electronic components in your car to ever-weakening voltage like that. I can’t answer your question directly, but I can tell you with a fair amount of certainty that your gas savings will be almost unnoticeable.
I agree, last year my alternator failed and I drove some distance; a couple of days later I replaced my battery because it would no longer maintain a charge.
The amount of fuel you would save would be negligible. Under normal conditions, you alternator uses a fraction of a horsepower. You will save more fuel by just doing the basics; slower speeds, gentle acceleration, proper tire inflation, no extra weight, minimize AC use, etc.
Replace the belt with a new one so you have good grip on the gears, then convert your car to propane. The drag on your engine when your battery starts to run low will use more fuel than leaving it alone.
Oh yeah; and when you replace the starter after it starts all those times, you will pay a lot more in the long run.
This is a great penny-pinching idea – as long as you don’t factor in the cost of home electricity. I hope you come to the realization that the cost of electricity for your spark plugs, etc, is about the same whether you generate it yourself with gasoline or purchase it ready-made from the power company. There are no savings.
The electricity in your house is generated from coal, natural gas, nuclear fission, wind, solar, hydro, and if they use heating oil, they don’t pay federal and state road use taxes on it. Also, electric power is generated using some of the most efficient engines known to man which turn some of the most efficient alternators known to man.
I agree with Mr_Josh and Craig58. Your battery wasn’t designed for the kind of ‘deep-cycling’ that you’ll be subjecting it to. If you do try your experiment, make sure to include replacement batteries in the overall cost-benefit equation.
This is a joke right? What you’re describing is a bad idea. Now if you had a diesel it would be doable (if not labor intensive). There are far easier ways to save gas.
I’m going to abandon this idea, primarily because: 1) as noted by the diligent responders, lead-acid batteries just do not hold enough energy and are not made to deep discharge regularly; 2) alternators don’t use much energy - the energy use drops when the electric load on the alternator drops. I’m just going to have to wait for a real electric car with lithium batteries or something equally as powerful.
To hotpockets: I’m not sure how this would affect the starter - it runs off the battery normally anyway.
To SteveF: Actually a US gallon of gasoline contains about 36.4 kwh of energy, or about $2.55 worth at $0.07/kwh here in Victoria, BC. A US gallon of gasoline costs about $5.56. Also gasoline engines only run at about 15% efficiency, while car alternators and household battery chargers work at about 90% efficiency. So it is much, much cheaper to get energy from household electricity than from gasoline.
It is almost the same, but that is because it is so small either way. The actural cost per unit of energy is much higher for the car than for your home. Consider what it cost to power a home with a generator vs the grid. No one is going to want to power their home with a generator when they can get on a commercial grid.
To hotpockets: I’m not sure how this would affect the starter - it runs off the battery normally anyway.
I’m not hotpockets, but he’s probably referring to the fact that using the starter at a lower voltage, due to the battery not being recharged while driving, will cause the starter to run hotter and thus fail sooner. Basic Ohm’s law here. To turn the engine over to start requires some fixed amount of energy, to get the same energy from lower voltage means more current must be used. More current, more heat. Low voltages can fuse relay and solenoid contacts, too.
What would save energy is if the alternator is only energized (charging battery) during braking/coasting, or if the battery gets low. This may already be in us on hybrids.
One more point: you may have to keep reprogramming the radio! You’ll be wasting gas while you’re sitting there doing that.
You will most likely damage sensitive electronics in the car if you do this. Most of the electronics are designed to run on the 13.5 to 14.5 volts supplied by the alternator, not the 12V from the battery. You’ll not save any appreciable fuel doing this, and the cost to replace just the ECM will make you wish you never thought of this crazy scheme.
I assume you mean all this running and without the car moving.
To find out how much energy is used, torn all this stuff on, measure the exact voltage, the total current, and calculate. The formula is simple and is everywhere.
I’ve driven just over 20 miles with a busted alternator belt. With the lights, radios, and all accessories out. But I wouldn’t recommend it on a modern car because of the electronics. Nor would I recommend disconnecting the battery every night for the same reason.