Starting battery age affect on gas mileage.

[Ryland]

My starting battery is getting old, I've been putting it on the charger every month or two, this morning after not driving all weekend it wouldn't crank over with it 15F outside, but the weather is getting warmer and the days are getting longer so I'm not going to have the lights on as much and it's not going to see a voltage drop from the cold... but should I go ahead and replace it anyway? as soon as the weather warms up I tend to coast down my short drive way and pop the clutch then only use my gas car for longer trips as my electric car comes out of storage.
In electric cars with lead acid batteries as the batteries age the watt hours per mile drop, you can end up charging the batteries longer and get fewer miles out of them, so I was wondering how that works on a gasoline car, does anyone have a solid number of how much their mileage went up when they got a new starting battery?

I suspect the starting battery is at least 6 if not 8 years old, so I know I should replace it anyway, but my driving habits vary enough that my mileage numbers are not consistent to test this my self.

My other thought is of course being cheap and just plug it in to a 1.5 amp charger when I know I'm not going to be driving it for a day or two, I'm sure I could get another year or two out of the battery that way if it's not going to help my mileage.

[Frank Lee]

I've had fe drop due to a bad battery- the alternator must have been working harder to try to keep it charged.

[MetroMPG]

No hard numbers for you, but anecdotally I've heard another credible member note a drop with a weak 12v battery (krousdb - not active any more).

[Cobb]

My sidekick was dead the other day so I swapped out the group 51 starter battery for a group 24 deep cycle and pre charged it. Man, my sidekick felt like it had 15 mo hp by the seat of the pants dyno.

Ive also found a low or bad 12 volt battery lowers the mpg of the Insight and hinders the ima system performance too.

[Miller88]

A bad battery will definitely hinder the fuel economy and "feel" of the engine. I had a bad battery in the Cherokee over the summer. In addition to starting hard, the alternator whined, voltage seemed to be a bit low and it felt sluggish (and also stalled every once in a while).

Once I installed the new battery, it felt like it had more pep, voltage was normal, FE went up a bit and the alternator stopped whining.