This is a revised video of the selfcharger with a new more efficient design. This is the 2 to 4 selfcharger located at https://www.youtube.com/watch?v=uEcj7oNWYY8 (https://www.youtube.com/watch?v=uEcj7oNWYY8)
What type are the batteries? NI-MH? Lithium?
The four being recharged are enercell rechargeable 1.2v, but any kind for the most part will do for the four in parallel so long as they are rechargeable. The main two batteries in series powering it all dont matter since they just drain eventually as the four in parallel recharge.