Go to autozone and ask them to use a DC volt meter (DMM-Digital Multi-meter) and measure the voltage just like I explained here:

Take a volt meter and put it on DC, when the car is running the voltage across the battery should be around 13.00-14.5V or somewhat in this range, when the engine is off, the voltage should drop to around 12-12.5V.

If it is above 14.5V, you have a problem. 17V is very high!

It has absolutly nothing to do with a relay!

Either your grounding is not good (But this would be unlikely since your voltage is high) or your alternator rectifier stage is failing and now you are getting some AC that raise your average voltage (Mean) to high. It means you need to replace your alternator.

There two and only two voltage sources in the car. 1)Battery. 2)Alternator.

When the engine is running, the baterry is charging and the alternator which is a AC generator with rectifing stage which makes it a DC generator.

When engine is off, the battery supplies all power / voltage to all the devices.

If indeed you have a 17V DC comming out of the alternator, you will start seeing systems burn out.