I live in the Tampa Bay area and it has been pooring down rain for weeks, so I have to run a lot of accessories: wipers, headlights, defrost, radio, radar detector. Anyways, when I get stuck like that at a light, the battery gauge drops to around 13 and rises back up when get moving again. The battery is new....So is this normal/okay?

 

It normal for the guage to drop a little at idle like that. Especially if you are running a high load - wipers, headlights, defrost(+cooling fans), radio, radar detector.

 

Sounds perfectly normal. The problem with any alternator, it needs to spin to generate volts. At idle, they are nearly dead, and thats when the battery has to do its job. This is why its important to put in a battery with as many CCA's (Cold Cranking Amps) and reserve minuts as can fit under your hood.

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Rob
'86 TA 305 TPI
Gutted MAF, Ported Plenum
TB Bypass, Home made Cold Air Intake,
Edelbrock T.E.S.