From the ARC4 advanced instructions:
Supported battery configurations - the numbers in parenthesis are the
nominal open terminal voltage:
Primary cells - non-rechargeable:
• Li-SO2: 1 cell (3.0V), 2 cells (6V)
• Li-MnO2: 1 cell (3.2V), 2 cells (6.4V)
• Li-SOCl2: 1 cell (3.6V)
• Alkaline: 2 cells (3.3V), 4 cells (6.6V)
Secondary cells - rechargeable:
• NiCad: 2 cells (2.5V), 3 cells (3.75V), 4 cells (5V), 5 cells (6.25V)
• NiMH: 2 cells (2.6V), 3 cells (3.9V), 4 cells (5.2V), 5 cells (6.5V)
• Li-ion/polymer: 1 cell (4.2V)
Me wants one too! Very nice.