Set information from Peeron.
According to lego it is a 1F (as capacitors go that’s huge by the way) 2.5V capacitor.
- Charging voltage maximum 4 V
- Maximum charging and discharging current 0-250 mA
- Red LED indicator when fully charged at 2.5 V
- Protected against reverse polarity; capacitor will not fully charge
- Charged to 2.5 V, the capacitor will store about 3.125 joules of energy
It is not waterproof - so precautions should be taken.