Right here!
If the plug for the Kindle is a micro-USB (has the USB logo on it), it will almost certainly be the right voltage as all micro-USB adapters must conform to the USB standard. As for the current, if it's less than or equal to what the Motorola stock charger is, you're going to be fine. You may suffer some touch screen interference while using it during charging due to the power not being properly filtered for 60hz ripple and RF interference.
If the current is higher than the stock charger, then as dezymond said, it will shorten the battery life over time. Using it every so often when the stock charger is not convenient won't have a measurable impact on its life. Charging at a rate faster than the stock charger will boost the battery to a near full state quicker, but will take about the same time overall to complete the charge to 100%, so using it to give a quick replenish is fine but don't think using it will result in a faster charge overall.
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