I am buying a lightweight battery 11lbs and also want to purchase a an underdrive pulley asp should i get the overdrive pulley for the alt to help in charging this smaller battery

 

Personally I would do one, or the other. I have see people do both but if you have any real electrical demands or live in a cold area, it maybe an issue. Also certain LW batteries have an issue that if ran dead, do not take a charge, I forget the brand but pls post what your looking at.

 

Stock charging system on my car gets weak when
at idle RPM, in hot weather with A/C on the line
starts to sag. I'd do the under/over combo unless
you have stripped the car and don't DD it. Or have
bumped up the idle to where it stays over 13V at
full accessory load (A/C + lights + stero + ...) with
the engine bay full hot (seems to aggravate the
field current limit of the alt regulator).

 

Good question OP.
I have A4 with HID, 500 w Alpine amp, and am planning pullies (over & under), and wonder is a lightweight battery is a bad idea for me??

Sorry to

 

for a little peice of mind i would overdrive the alt. i have a m6 and i still see a little dip when stereo is going and all lights and other crap are running.

 

I ran a pulley with two different deep cycle batteries and on a hot day i'd get close to the bottom of 13 volts at idle with everything on. Alt output even for aftermarket ones isn't the best at idle, so i'd suggest an overdrive pulley. I believe the factory ratio on the alt/crank is 3/1 and the underdrive knocks it down quite a bit.