Impala Battery Light

Didn't see this addressed...dealer can't (won't) help: very intermittently the battery light will stay illuminated after starting vehicle. When this happens, shift lever can be moved but transmission will not engage. No power assist to the steering wheel. It's as if the car does not know its motor is running. Only way out is to shut car off and restart. Happened with some frequency at first, now it happens about once every 6, 7 months. Dealer says "Well, the battery light does come on when you start the car!" Well, duh, I told them, it just does NOT go out at all when this happens unless I shut the car OFF. They refused to run diagnostics while car was under warranty without "just cause" (ie, the lazy schlubs didn't actually see it happen, and/or wanted to get paid real dough). It's a 2001 LS, 3800 V-6.