1978 Chevrolet El Camino Won't start

If the new battery is down, take it off and get it charged. Make sure battery cables and connections are good. When you get the vehicle started, check the charging system.

Another possible issue is a battery drain, with everything off. The old way to check for a drain, is to take a battery cable loose, use test lite between cable and battery post. If the lite comes on, you have a drain. A common drain is a lite left on somewhere? Best time to check is in the dark, exterior--interior--under the hood.

You can pull/replace fuses one at a time, when you pull the fuse on the faulty circuit, the drain goes away. Sometimes it can drain through your alternator.

You can't do any testing unless the battery has a full charge.