I always hear the recommendation that we should always put batteries of the same brand and type in an appliance to power it. Is this recommendation valid?
Yes, it is very important that at the time of use the batteries have the same electrical characteristics. However, since different cells of different brands and different usage times have different characteristics, they can cause different types of problems when powering an appliance. For example, if one of the batteries is worn or branded with higher internal resistance characteristics, all internal resistance of the assembly will be changed and the current capacity of the assembly will be compromised. This compromise can cause better batteries to wear out more quickly and can no longer be used. For this reason we should always place batteries with the same state of use in the device, avoiding to take advantage of one that has been used in another device and that therefore already presents a certain wear.