BATTERIES - 1.2 volts vs 1.5 volts?

AAA and AA rechageables are only 1.2 volts. AAA and AA non rechargeables are 1.5 volts altho both are exactly the same physical size and both share the same size designators (AAA or AA) - WHY? My Pentax Optio S40 suddenly will not work with the 1.2 volt rechageables altho they have performed well for 1000's... show more AAA and AA rechageables are only 1.2 volts. AAA and AA non rechargeables are 1.5 volts altho both are exactly the same physical size and both share the same size designators (AAA or AA) - WHY? My Pentax Optio S40 suddenly will not work with the 1.2 volt rechageables altho they have performed well for 1000's of pix.- it works fine with plain old non rechargeable Duracell 1.5 volt batteries - WHY? Do they manufacture single 1.5 volt AA and AAA rechargeable batteries other than the CR-V3 which has two 1.5 volt AA's? If so, where can they be purchased?
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