My 1996 cadillac eldorado says battery no charge now hard to turn but it cranks right up

Most likely you need a new battery but do this first. Remove the battery ground connection and the positive connection. Clean them with soda water(coke is sticky but all sodas will work). Scrape or sand the inside of the cable connectors and the outside of the battery posts till they are shiny. A low battery light usually means it is not fully charged or taking a charge optimally but effective troubleshooting the battery and starting system starts with clean,secure connections and a fully charged battery. If your battery has electrolyte filling caps undo them and check the level. If they need topping off use distilled water only. Charge the battery with a car battery charger. It should read about 13.4 Volts under no load. Check and clean the battery negative cable connection to the block and the positive connection to the starter solenoid. Check for good connections to the charging output from the alternator. Inspect all wires for kinks, cracking, shorts, ect. A good once over will reveal probem wiring. Reinstall the battery and cables. Giv'er a crank and see if that is not stronger. Drive it for an hour or so and crank it again. Any noticable degradation in cranking power means you have a bad battery, alternator, diode pack if applicable, voltage reguator, or any combination of the three. Take your alternator and battery to your auto parts dealer. They will usually test them for free. Aslo, starters can wear after time but this is most likely not your problem based on the symptoms.