Automotive: Battery age effect on fuel economy?

Factual Questions
1

My fuel economy has been trending downward for the past couple of months. My battery finally gave it up yesterday and I had it replaced. In the poor battery’s defence, it has been stupid cold out (-35C) for the past week.

Could it be the two issues were related?

I’m not sure how the electrical system works in modern cars, does the alternator run everything, and the battery just serves to start the car? Or does the battery run everything and the alternator keeps the battery charged/charging continuously? It’s a 1995 VW Golf 2.0l at 121000km/75185mi .

The old battery was between 4 and 10 years of age. I could not find a manufacture date anywhere on it, but is was an Audi/VW battery so it either was the original factory battery (in which case I’m AMAZED it lasted near 10 years!). or somebody who owned the car before me (I’ve had it 4 years) had the battery replaced by a dealer.

The car is otherwise in proper shape, gets tuneups and oil changes at required intervals. Tire pressures checked weekly. The last time I had fuel econo. go bad was when I had a vacuum leak in the brake booster, that was sealed back up in the early fall.

The only way I can think that the battery would have an effect on fuel economy is if either the ignition spark was weaker than normal, or else the alternator had to do more work cramming juice into a battery that wasn’t up to the task anymore, and so put more load on the engine through the alternator belt. Would either of these be true?

I’ve considered that the cold snap is also a reason behind poorer fuel economy, but like I said, it’s been heading down over a longer period than it’s been cold.

Thanks in advance for your replies.

2

I’m not sure I would focus on the battery that much. I suppose the real test would have been to see if voltage really was the problem or the alternator was able to supply what was needed during that time.

Your alternator runs things above a certain engine rpm that is usually very low. And it does take power to charge a dying battery. But the difference between charging a dead/dying battery and a healthy one seems to me to be very small and hardly noticeable from a casual standpoint.

Weak spark is another thing where it could be happening, but the voltage would have to be very low and the spark very weak to cause a problem. If your alternator is working well enough to power the computer on the car, most likely your spark is more than good enough.

Most likely you have an issue where gradually decreasing tire pressure might be the culprit, or some other engine management problem. Cold weather starting and warmup is definitely going to reduce fuel economy significantly as well.

3

I agree with what Una Persson said. In 30 years of professional auto repair, I have never heard it suggested, much less demonstrated, that a weak battery would affect fuel mileage.