Most used variants of rechargeable batteries in different sizes and shapes. They are fitted as standard connector type Tamiya or its smaller variant Tamiya mini.
NiMH batteries are specified primarily with these positive and negative qualities. Advantages are high-capacity, relatively hard source (a small internal resistance, especially at high capacity), ruggedness, ease of use, friendly to the gun. Negatives are bad volume ratio of weight vs. capacity compared to Li-Pol, while strong springs unusable cell capacity, high self-discharge. Please note that too high battery voltage = high cadence that can wear out piston wheel in the gearbox or motor. Cadence weapons must always be "healthy".