Sennheiser Set 820 Instruction Manual page 31

Charging the battery
Place the receiver into the transmitter's
charging compartment, with the operating
control at the front. The left LED on the
transmitter must light up.
Charge the battery for 24 hours before you use
it for the first time. The normal charging time
is 4-5 hours. A fully charged battery has an
operating time of approx. 4 hours.
Always store the receiver on the
transmitter to ensure that it is fully charged.
The transmitter automatically switches to trickle charging so that the
battery cannot be overcharged. The receiver can remain in the charging
compartment even when the battery has been fully charged.
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