Battery light comes on when car stopped and idling.

Voltage Regulator. I don't know what it looks like on an 94 Sentra, but you should replace that. They are not too expensive. If you have a voltmeter you can test it. Check the battery voltage when the engine is not running. The voltage should be a little over 12 volts dc. Start the engine. If it isn't real hot outside, the voltage should go up to between 13 and 14 give or take. If it stays close to 12 your voltage regulator is bad. If that doesn't fix it then maybe you have a bad connection in the charging system. Check the following for corrosion and or damage: 1) alternator positive to battery positive, 2) battery negative to chassis, and 3) engine ground to chassis. If none of that works, well, brand new alternators have been known to be bad right out of the box.

see if you can raise the idle a little bit, the engine isn't spinning fast enough.