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Either your Garmin Nuvi has to be wired to a continuous 12 V source, direct from the battery (with at most a fuse on the line, of course), or else you have to wait a moment until you see the 'about to power down' message appear on the screen, and tell it, 'No', to not power down.
There is no 'disable power down' setting in most older Garmin devices - so simply you must either tell it to not power Off just before it is about to, or not interrupt the 12 V power to it.
But tell me your Garmin Nuvi's exact model # - if it is a recent model, newer than 2012 or so, there might be more to tell you, or a way to disable power-Off/power-save mode.