Experimental investigation of refrigerant charge determination for heating performance in a CO2 heat pump system for electric vehicles

•The inefficient charge level is more sensitive for performance deterioration than the excessive level.•The insights for discharge pressure control strategy with charge determination are offered.•Optimal valve opening is more preferred for a maximum COP than a maximum cabin air supply temperature.•Effect of valve openings, climate conditions and compressor speeds are studied for charge determination.•∼ 20% charge loss from the appropriate level can usually provide more than 12% COP drop under the same discharge pressure.