Quote Originally Posted by podcake
not ENTIRELY true chaps, if you connect something DIRECTLY to the battery while the car running ( depending on how much power you are drawing) it is possible to take power off the battery
ie: if alt will supply for argument 10 amps ( i know this wouldnt occur) and you draw 20 amps you will actually draw power out of the battery assuming the alt dont blow up

if you are really fussed about what is happening to the battery you COULD put a second one in the car with a switch that will swap between the two ( so you only charge one at once but you do have the possibility of using two) if you get me
You are kinda right, but if you are drawing more than the alternator can supply eventually you will end up with a dead battery.

The alternator has to keep up with all the stock equipment + keep the stock battery charged. Then you add to the work load by adding amps computer or what ever. Then an extra battery to charge. You must have an alternator that can put out enough to take care of all that, pulling more power that the alternator can supply can only result in a battery that is not being full charged.