It sounds like you may have gotten a bad unit on the minimote, it’s one of the most popular devices and most people get several months of use at least out of a charge. It can happen with any electronic device, every once in awhile you get a bad one.
From the manufacturer:
How long does my battery last on a full charge? (Recommended settings)
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The battery should last about 3 months on a full charge assuming you are controlling something or tapping its button 20 times per day depending on the health of the battery itself, or how often you use it. We highly suggest disabling any Wakeup Interval that your gateway may set to this controller in order to conserve your battery life. (Having the Minimote is not necessary for its use and can be seen as a waste of battery).
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You can set the wakeup interval depending on your gateway, each gateway is different, and not all will let you set this if it allows you at all.
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We suggest: Wakeup Interval = 0 for the maximum battery life you can get.
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If you are seeing battery life less than 2.5 months, you may want to consider setting the Wakeup Interval in case that it may be the culprit.