If your battery voltge is reading 13.7 volts when the engine is NOT running, then either one of two things is happening, because a fully charged battery should read about 12.7 volts.

(1) If you are taking the reading of battery voltage immediately after shutting the engine is shut off, then you are getting a false reading. This is normal, and is caused by a phenomenon called "surface charge". Let the vehicle sit for at least 30 minutes after shutting the engine off, and then take a reding of battery voltage. If it is now reading close to 12.7 volts, then the surface charge, not dissipated, is what was causing the high reading.

(2) Your voltmeter could be off (i.e., reading incorrectly). If, after wating the 30 minutes I mentioned above, the battery voltage is still reading 13.7 volts, then I would suspect that your voltmeter is reading too high. If that is the case, then the 14.7 volts that you read with the engine running is probably really about a volt lower also.