Does the alternator have a 3 pin plug? 'smart charge' type?

When you start the car, does the light go off then comes on 20secs or so later?

If you remove the 3 pin plug, the alternator should revert to a default charge setting around 14 volts, check across battery terminals.

If it does, the alternator is fine, its a fault with the smart charge circuit - wiring,ECU etc.

Officially yes should have silver calcium battery on it.