The Guppy input voltage range, for 2 x AAA batteries in series, is 2.4 to 3.6V. 2 x AAA batteries in series is achieved with the standard battery holder.

This implies:

• 1.5V AAA batteries must be used.
• 3.7V LTC batteries cannot be used.
• 1.5V Lithium Iron Disulphide batteries are ideal. Examples of these are the Energizer Ultimate AAA. These have good temperature ranges and are reliable.
• Alkaline batteries will work, but failure rates are higher, and the temperature range is lower. They will work down to 2.4V (1.2V per cell open-circuit, 1.15V at 106 mA), which is near the low end of their capacity.