slight problem, you still need the power at idle where the lights
usually
dim. for some aftermarket alts. they actually put out less power at idle
which will make the problem worse not better.

That's why cars have batteries. If you have flicker at idle, it's
because your power and ground wires at the battery and alternator are
insufficient are insufficient.

At idle, your alternator is still producing ~14v. So a 1.5 volt drop down
to battery voltage can still produce a noticable dimming effect. Most alts
deliver relatively little current at idle, so it's often not difficult to
get that 1.5v drop. Depending on the severity of the situation, a cap can
often alleviate the dimming effect rather substantially. The power and
ground wires at the battery and alt, as well as the general "health" of the
battery and alt should always be checked out first though.