In practice, the alkaline batteries and the rechargeable batteries can be used interchangeably in sets. They have only different voltage characteristics. It is given by their different chemistry. Primary cells gradually drop in voltage from use, they start at 1.5 volts, drop to 1.2 and continue to 1.0 where the appliance stops working. Secondary cells operate more uniformly even with only 1.2 volts, they have flat discharge where they pretty much stay at 1.2 volts until depleted and then drop off very quickly to below 1.0 volts.