Old Battery & Alternator

The alternator has to charge the battery. That takes power. If the battery has gotten old enough to need any more power to charge it than a new one, it wil not start the car after it sets overnight. Even at that it will not take enough extra gas to notice it

Bad batteries do this. It doesn't matter if the batteries are new or old.

One horsepower is about 750 w. If you alternator puts out about 30 or amps (assuming 50% inefficiency), you'll lose about 1 HP. This probably amounts to about 0.3 mpg or something like that.

Then I don't see any reason to replace them. It's whether or not they're bad, not if they're old (although old batteries are more likely to go bad).

That's not true. The output of the alternator is more than it takes to power the battery and the other electrical things.

The problem with old batteries is not the amount of energy it takes to recharge them. The problem is that they can't hold charge or have problems like broken or shorted out plates.

And it doesn't have anything to do with setting overnight, except the engine is harder to turn over when it is colder.

You won't notice it, but it will use a little more gas.

Jeff

Batteries should be checked every fall prior to the bitter winter starting season. We've managed to get 8 years out of a battery. But the test will tell you what shape it's in, i.e. how much voltage and amperage it is able to put out. If it doesn't pass muster, replace it. It's not worth your peace of mind. New batteries can be had for under US $100 for most Corollas, I got my last one for 39.

Charles of Schaumburg

The battey's age is not a facter as much as it's condition,a three minute charge test will tell you if it is not reaching it's fullest charge allowing the alternator work harder attempting to bring battery up to full, charge at high rate for 3 minutes with a voltmeter attached if the battery voltage goes over 16 volts the battery is due for replcement,it may still start the engine but it is doomed for failure. I have seen "new" batteries fail this test,If it does return the battery .