I mainly use rechargeables. When the voltage level is at about 1.2 V itīs time to change. I gathered experience to judge the remaining runtime from the voltage level (also for batteries). When I canīt afford poor battry performance I change before I go otherwise I deplete the battery. That saves managing batteries with different charge levels. OTOH rechargeables can easily be recharged when it seems sensible so different charge levels are no real problem.
I never take partly depleted batteries as spares.
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If it isnīt broken, it doesnīt have enough features yet.