Electrical question to increase battery life?

So let's say I have a large battery and a smaller battery, do I place a high amp diode in between the big and smaller battery and wire a device after the smaller battery to get longer battery life? I know you are supposed to use a diode to prevent the smaller battery from causing problems with the larger... show more So let's say I have a large battery and a smaller battery, do I place a high amp diode in between the big and smaller battery and wire a device after the smaller battery to get longer battery life? I know you are supposed to use a diode to prevent the smaller battery from causing problems with the larger battery just making sure I wire it correctly.
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