'''++Paul Nowack++''' there really is not enough information on here. It sounds like a bad battery. It may not supply the required voltage needed and may fail under a load. Let us know what exact model this is and what you have checked.
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=== Update (04/07/2019) ===
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Thanks for the updated information. Would have been great to know ahead of time :-)
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Post some good images of the switch PCB. Possibly some bad component on it and you should try to use a multimeter to see where the switch circuit is on that PCB.
'''++Paul Nowack++''' there really is not enough information on here. It sounds like a bad battery. It may not supply the required voltage needed and may fail under a load. Let us know what exact model this is and what you have checked.