The charge indicator in the speedometer only comes on when the battery is no longer charged. You must then take immediate action to find the cause. The cause is in the dynamo/alternator or in the voltage regulator and has nothing to do with the battery.
If the dynamo/alternator is charging at too high a voltage, the battery is overcharged, which is not a pleasant experience for the battery. In this case, the indicator does not light up. If this goes on long enough, it can also damage the electrical consumers in operation.
With a voltmeter you are protected against this. As long as the meter shows between 12 and 14 volts, everything is fine, if the meter shows more than these values, you should immediately investigate the cause. If the meter does not reach 12 volts, large consumers may be active at that time, if not, the cause is often the poor condition of your battery and you should consider replacing it. Buying a voltmeter is not an absurd measuring instrument, it warns you before you spontaneously stop.
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