Siemens S5-135U/155U System Manual page 111

After Replacement
The recharging time for the battery may be up to 46 hours, depending on its
state of charge. Please note that backup via the battery is only possible within
limits during this time.
If the rechargeable battery does not work after replacement, there may be the
following faults:
"RB low" LED does not go off after
6 minutes,
"RB low" LED goes off but the
relay does not pick up,
Replacing the
Rechargeable
Battery in the Fan
Subassembly
System Manual
C79000-G8576-C199-06
Central Controllers and Expansion Units Power Supply Units
IF...
the rechargeable battery is incorrectly connected
or faulty or the charging circuit in the PSU is
faulty.
the battery has failed and the yellow "MB low"
LED lights up or the battery compartment is
faulty.
Rechargeable
battery
THEN...
Fan
sub-
red
assembly
Hole
black
4-45