Power supplies-adapters can get very complex when used outside of their designed purposes. Most of these adapters use "switching" power supplies to reduce the voltage to some level. In other words the output is "pulsed" to provide the average desired voltage or current. This means two nasty things happen when you are taking measurements. With no load the voltage may not be regulated to the set point. Your cheap meter may just be seeing and displaying peak voltages and not the true RMS value of what is likely to be a nasty waveform. 

Also for motor heating 5 minutes is not much of a test.  By some definition continuous motor operation is considered 3 hours or more. This is used in all motor and branch circuit feeder calculations. 

Most likely your plan will work but why not just get that fan connected to the battery output? It may just be time to replace the charger. 

Dennis

	
-----Original Message---- 
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of WILLIAM WITZ
Sent: Friday, March 4, 2016 1:07 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Voltage Question

Let me be clear. the Battery Charge section of the Battery Charger is working fine. It is charging at 12 V. at 2,10, 20 amps. The section that puts out voltage to the 12 V. Muffin fan stop-ed working so I found a small step down adapter that is rated at 12 V. to power the cooling fan for the Charger.The adapter is putting out 17.V How long will the fan last at this Voltage? I ran it for five minutes and measured it with a I. R. gun and it was not over heating.Thanks Again, Bill 84 Westy