[gecko]

It'll reset the engine control unit (ECU), which will cause your car to run rough constantly as the computer has to relearn how to adjust the engine tuning from scratch every drive. You'd be better served using a trickle charger.

[RunningDroid]

Resetting the ECU will also cause the vehicle to fail inspection unless it's driven long enough beforehand. (Here in Upstate NY, anyway)

[garyfirestorm]

that is not how ECUs function Some Transmission Controllers will adjust to driver behavior, however engine controller default behavior shouldn't cause 'rough' experience I worked in automotive testing for over 10 years and constantly measured and analyzed vibration data on vehicles. Many prototype vehicle batteries die due to early release software that causes parasitic drain and I have never noticed any rough behavior just because the battery died and got replaced. There are many sensors and closed loop control systems that monitor strange engine behavior and adjust timing. This happens very quickly within few engine rotations. If there was an issue you would notice it for at the most for couple of seconds.

[mianos]

Yes, it's a closed loop. The O2 sensor in the exhaust rapidly adjusts the mixture.

The whole 'learning' the air fuel mixture to adjust the stoichiometric ratio over a longer period of time does not make any sense. You have the air mass, O2, fuel, temperature and sometimes more, you can calculate the mixture near instantly.

Maybe someone way back said you have to warm the car up and that takes a few minutes. Modern fuel maps achieve a fair idle from near dead cold. It's a solved problem.