How did you arrive at that figure? A fuel gage is on an instrument regulator, isn't it? That's because alternator voltage varies, depending on the load and the temperature, for examples.
nor used it. I later discovered that my other battery in my car is fine, something was wrong with my car's charging system. Any way, I thought I wouldn't be needing this new battery, now I am stuck with their battery.
A spare battery can be useful. If you keep it charged in a cool place it should last many years.
Current+Voltage will jump from 12.80v to 13.76v, also to 15.45volts,
Alternators can jump like that. 15.45 sounds a little high in warm weather. Off the shelf, a battery may have to sit on a charger a day or two before it will work right. The problem could also be corrosion in a connection or a faulty alternator/regulator.
this? I put a current & volt meter on their battery and that's how I know.
You said you hadn't opened the seal on the battery. You could turn on your headlights with the engine off. If the battery is fully charged, the voltage should be about 12.6. If the lights don't flicker, the current flow is constant. If they flicker, the problem may be in the electrical system.
pump depends on a constant flow of current.
Do you know your fuel pressure fluctuates? I imagine it should be constant despite the voltage variations you described.