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Er, well, assuming your jam has one of them infernal combustion engines in it, I don't think changing the battery should make any difference to the way it runs. I think what we might have here is a bit of coinkydink. It's possible that while you were rummaging about trying to find where to put the new battery, removing unwanted pieces of pedestrians, bicycles and the like from the engine compartment, you may have temporarily improved a dodgy electrical connection by jiggling it.However, you might also consider submitting your question to these geezers -http://www.cartalk.com/content/showpodcast/index.html
I did wonder if the new battery may have had an effect on the engine management system ?..I just do not know !
Quote from: neilep on 07/02/2011 20:36:51I did wonder if the new battery may have had an effect on the engine management system ?..I just do not know !(Thinks - that's a good point.)Ah! You could be right.When you disconnected the old battery, the engine management system "forgot" everything. When you installed the new battery, the EMS reverted to it's default settings. It will now be learning new settings and will probably get back to the same settings that it had before you disconnected the battery.The default settings are not optimized for fuel economy, so the fuel/air mixture is probably a bit "rich" at the moment, which would explain improved performance. However, your MPG is probably not as good as it was before.
If the battery is poor it will draw more current from the alternator than a new fresh battery.The increased current increases the force required to rotate alternator shaft: stronger current => stronger magnetic fields in the alternator, (cf magnetic brake). The increased force required to rotate the alternator will increase the load on the engine (a little), which will then reduce car performance (a little): poorer acceleration, increased fuel consumption.
I doubt that you would notice the effect of the alternator load on the performance. The maximum power it could take from the engine is probably under one horsepower.
IBTW when you disconnect the battery, always remove the "earthy" side first and reconnect it last. These days this will almost certainly be the negative side.
OK what about a low battery only producing a poor spark leading to incomplete combustion and consequently poor performance ? ...
Surely once the engine has kicked off the alternator provides the spark and current is flowing into the battery and not out of it.NOW...put a strong magnet on the negative lead to the battery and your mpg will go up!
Really ?...will putting a magnet on the negative really affect the mpg ?
Quote from: neilep on 09/02/2011 13:42:03Really ?...will putting a magnet on the negative really affect the mpg ?Oh dear Neil! You are a sucker for those snake-oil salesmen, ain't ya? 
This voltage "sag" is a function of the internal resistance of the battery, the current drawn by the starter motor, and any voltage drops in the supply and return wiring and connections.
Quote from: Geezer on 09/02/2011 18:08:45This voltage "sag" is a function of the internal resistance of the battery, the current drawn by the starter motor, and any voltage drops in the supply and return wiring and connections.For this reason my old car - a Vauxhall Viva - had a ballast resistor in series with the coil when running/cut-out of the circuit when starting. I could see the sense in the idea, but the best thing I ever did was rip that blasted resistor out! All the cold starting problems went away when I did.
I think the ballast resistor was also intended to prevent you from frying the primary of the ignition coil in the event that you left the ignition turned on while the engine was not running. It was a bit like Russian Roulette though. That would only happen if the points happened to be closed - probably something like a 50/50 chance.
You could take the battery out once the car has started.