Feel free to correct me - with regards to plugging it in. I believe you're okay if you have it set to Alkaline in the settings, as this dictates the charging and charge characteristics (as ni-mh and ni-cd have different charge timers and voltage ranges for depletion) and presumably Alkaline would mean no attempt at charging. Haven't used my 651 personally to charge batteries, so I'm not 100%. The yellow holder only exists to possess the third contact necessary to pass current to the batteries, just as a redundant safeguard. It can absolutely be used as the black holder can.