For lead-acid chemistry, as in a car battery, think 13.6 volts DC.
No, you cannot charge an 8 volt battery with a 6 volt charger. The voltage of the charger must match the voltage of the battery.
i=v/r can be used to it
yes more voltage
With the battery fully charged (12.68 Volts) and a volt meter connected to the positive and negative battery posts, run engine at idle with no accessories on, voltage reading should be aprox 14.2.
With the engine running at idle check the voltage at the battery with a digital voltmeter set to the 20 Volt DC scale. You should get a reading of 13.5 to 15.5 Volts if the alternator is functioning as it should. If you get less than 13.5 V or more than 15.5 V the alternator is suspect. If the voltage reading is good then suspect the battery is defective.
Usually 12 Volt
12 volt
No, the battery charger has to match the voltage of the battery it is to charge.
12 volt of an ordinary inverter battery
The voltage gauge on the dash refers to the charging system voltage. What this most likely means to you is that your alternator is not charging properly. If the battery is new and all battery and alternator connections are good and corrosion free you can be relatively sure the alternator has failed.
This is not advised! The 9-volt charger might have a current limiter so that nothing burns up, but it any case if left on it will destroy the 6 volt battery by overcharging. A 6-volt charger is not expensive.
Some have a switch for 6 or 12V - if not you will "cook" a 6 volt battery using a 12 Volt charger