Applying 17 volts to a 12 volt device is, in general risky unless the device is sophisticated enough to have over voltage protection or a voltage regulator. Digital electronics might smoke right away, or misbehave while dying slowly.  A fan motor might just run faster with a shorter life, but there is some risk of overheating and fire.

An important question remains unanswered. Was is the voltage across the fan while it is running?

Sent from my iPhone

> On Mar 4, 2016, at 1:45 PM, Larry Alofs <lalofs@GMAIL.COM> wrote:
> 
> I will repeat my question:
> 
> Was the fan connected to the power supply when you measured its voltage?
> The open circuit voltage of a power supply is usually considerably higher
> than when it is under load.
> 
> Larry A.
> 
> 
>> On Fri, Mar 4, 2016 at 1:06 PM, WILLIAM WITZ <pbwitz@sbcglobal.net> wrote:
>> 
>> 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
>>