I was really psyched this afternoon when my dead battery pyramid scheme actually worked. Here is how I swapped a few 10 year old totally dead car batteries for 3 year old gel based units that are in good shape.
1. Collect dead batteries from friends and neighbors.
2. Go to your local autoshop and ask if you can swap them their dead cores for your dead cores.
3. Use a multimeter to test the batteries at the autoshop. Anything with a 12.4V or higher is worth taking as is. A lower voltage means you need to crack open cells to measure specific gravity and individual cell voltage.