When evaluating R20 (D size) rechargeable batteries, prioritize NiMH (Nickel-Metal Hydride) or Lithium-ion (Li-ion) chemistries over NiCd due to environmental regulations and energy density. Key specs include Capacity (mAh), where high-performance D cells should range between 8,000mAh to 10,000mAh; Cycle Life, aiming for at least 500 to 1,000 charge-discharge cycles; and Self-Discharge Rate, specifically looking for LSD (Low Self-Discharge) technology which retains up to 80% charge after one year of non-use.