Supercapacitor
A supercapacitor is a high-capacity capacitor in your Vantage Pro2 or Vantage Vue sensor suite that retains electricity generated by the solar panel to power the transmitter at night or during periods of little or no sunlight. It can take and provide charge considerably faster than batteries and can withstand many more charge and discharge cycles than rechargeable batteries.
Supercapacitor from the Vantage Pro2 ISS Transmitter