As the air-conditioning units on hybrids have to work with the engine switched off, the compressors contain an electric drive medium, which was introduced with the second-generation Toyota Prius. Unsurprisingly, these devices operate at high voltage and are filled with special POE oils with a high electrical resistance. The use of an incorrect oil can cause either permanent damage, or else the climate control ECU can deactivate the compressor. The power supply is provided by separate batteries. The third-generation Prius introduced a solar-powered ventilation system as an optional extra, with air-conditioning that could run for up to three minutes before the engine was started.