Casio keyboard can be battery operated or plugged in. This is over 30 years old as was mine as a child, but my son has been learning to play on it for the last year and has just progressed to a larger one.
Still works fine (have just tested it) though we have had a couple of random occasions when a couple of the lower keys sounded odd. I also just noticed that one of the prongs on the adaptor has broken off (see photo) but also tested and still works.
Casio keyboard (Thames Ditton KT7)
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