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Could be your alternator, bad battery, battery current sensor, or high resistance in your charging circuit…13.9 to 15.5 is a good charging voltage so if you have no electrical diagnostic knowledge I would seek out an auto electric shop in your area…if your battery is discharged you charging voltage will be higher and you will see it decrease as you drive…you want to check your charging voltage AT the battery.
I would start by taking a voltage reading directly on the battery terminals and compare that to your gauge reading. If there is a difference start looking for voltage drops.
When you first start your car should be around 14v or so then slowly voltage will come down as the battery charges to your 12.8 to 13.2 normal range. My vote is normal.
