Siemens Simatic RTU303xC Series Operating Instructions Manual page 353

A completely equipped battery module with its full fittings is made up of the following
components:
● Battery module including battery holder and connector (see below)
● Two battery expansion modules including battery holder and connector (see below)
● Six batteries (2 for the battery module + 2 x 2 for the expansion modules)
For information on the batteries refer to the appendix Battery (Page 356).
Module changeover
When two battery modules are used, module 1 initially supplies the RTU (in the figures the
right-hand module).
When a critical voltage value in module 1 is reached, the RTU switches automatically to
battery module 2 (on the left in the figures).
After replacing the batteries of module 1, the RTU can switch back to module 1 after a critical
voltage value status is reached in module 2. This guarantees that the batteries of a module
are always completely emptied before switching to the other module.
Note
Reduced power output with high current and low temperatures
With a high current in conjunction with very low temperatures the power output at the 12/24
V outputs is reduced because the battery voltage drops considerably at very low
temperatures. Under these conditions it is recommended to add at least one battery
expansion module to each battery module.
This increases the battery capacity and reduces the disturbance caused by high current
peaks particularly at low temperatures.
RTU303xC
Operating Instructions, 06/2019, C79000-G8976-C382-06
Accessories
C.1 Battery module
353