Hi,

please check manual

SIMATIC S7-400 S7-400 Automation System Module Data

 Check chapter 3.4 Operator controls and indicator.

The backup voltage is either supplied by the backup battery or fed externally into the CPU or
receiver IM. In its normal state, the level of the backup voltage is between 2.7 V and 3.6 V.
The backup voltage is monitored at the lower limit. Violation of the lower limit is indicated by
the BAF LED and reported to the CPU.

 Regards,

Towome

Hi,

if you measure less than 3.6 volts on a new battery but have been in storage for a while, close it short:

Please check this:

Why is a battery fault signaled when you insert a back-up battery, even though the battery is new?

Regards,

Towome