This only means the voltage (not power) of your Bioenno battery is not high enough to allow 5W output mode.
When a battery voltage is greater than ~ 11.5 volts the 5W output is enabled, when a battery voltage is less than ~ 11.5 volts, the 5W output is disabled. It does nothing with a battery capacity, only with a voltage.
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Serguei VE3KDE
