14.4V is normal, anything below 13.5V is marginal in that you're battery won't get recharged on short trips. 15.5V is very high though, the only thing it will damage is your battery (it will prematurely wear out.) Double check the readings with a multimeter across the battery terminals. I suspect the your guage is not getting a good reading from the ECU power wire. If you confirm those readings than have the voltage regulator checked, it may be bad.
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