Good advice as some people do not realise not all Mini or Micro USB powered device require the same Amperage/Wattage.
30watts is about 10 Amps, so for the two devices you mention that only require 1.0Amp it's pushing it
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Spoiler: AC Power Adaptor
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I use all sorts of AC power supplies ,you just have to make sure the output is compatible with the device ,larger screen Garmin devices require more power 2.0 Amps and more to maintain function.
From my experience the nuvi 13xx & 14xx are known for USB port issues ,normally a resolder fixes it so might be worth opening them up to see?