@Paramji,
I expect the system the batteries are connected to, to automatically detect a low voltage battery condition and do a switch-over to the spare. I say this because of the presence of the spare you mentioned in the system. The spare is useless if the system cannot automatically switch over when required. So I guess, your question is how to configure your system to be able to do this for the power plant. In which case, you need to provide details of this system including name of device, manufacturer, model, serial number ....it's possible as is always the case, that someone here have worked on such equipment before.
Cheers