Hello Jack,

I'm not a S5 expert. [:(]
But maybe the following information's I found in the manual SIMATIC S5-135U/155U System Manual will help to answer your question.
Capture 4.3 is about "Power Supply Units". There they describe the following:

• RB low: Rechargeable battery voltage below preset limit (3V)
• MB low: Lithium battery /external battery voltage below preset limit (3V)

So if you change the external battery the MB low LED should not light anymore.
If RB low is lightning the Rechargeable battery must be changed.

In the manual the description is better than my words.

Best regards
Sabine

Hi Hulzhex,


I wonder that why there is a such warning since I never used external battery.

I will try to connect a external battery.

Thank you very much for your kindly help.