Overview of the 7900-127 Charge Controller

The 7900-127 DC-to-DC charge controller provides several additional functions:

  • Charges the 12 VDC backup battery.
  • Switches to backup battery power to keep the Sutron 9210 system running if the system power supply fails.
  • Converts a 9-30 VDC input to 12 VDC output to power the Sutron datalogger.
  • The backup battery should provide 1 to 3 days of continuous operation for a basic biomet system.

Note: The backup battery is designed to power only the Sutron 9210B datalogger. Do not attempt to power the gas analyzer, sonic anemometer, or other components with the backup battery. Doing so may damage the battery.

Operation Summary

The charge controller will switch to battery backup power if the main power input falls below 7.8 V or rises above 31.4 V. The datalogger will turn off when the backup battery input drops below 10.6 V or rises above 15.8 V.

  Low (falling) turn OFF threshold Low (rising) turn ON threshold High (rising) turn OFF threshold High (falling) turn ON threshold
Main 7.8 V (battery backed) 8.1 V (on main) 31.4 V (battery backed) 30.5 V (on main)
Battery 10.6 V (Sutron off) 10.9 V (Sutron on) 15.8 V (Sutron off) 15.4 V (Sutron on)

If the main input voltage drops below 7.8 V, the system will switch to the backup battery and turn off the high-draw sensors, such as the heater for the self-calibrating heat flux plates and CNF4 heater/ventilator. The system will switch back to main power from backup power when the main power voltage goes above 8.1 V. The charge controller signals the datalogger with a 1 to indicate main power and 0 to indicate backup battery power.


The battery/charge controller includes the following:

  • Vented Enclosure (retrofit kit only): Allows hydrogen gas to dissipate. Hydrogen gas may be emitted if the backup battery is damaged.
  • Battery: 12 VDC lead acid battery (7900-128), 7.2 Ah with 2-pin terminal strip connector. Powers the datalogger and biomet sensors if the main power supply fails.
  • Charge Controller: DC-to-DC charge controller (7900-127) provides a regulated 12 VDC power supply to the Sutron datalogger and other sensors that have an upper limit near 12 VDC.
  • Terminal Strips: A 2-pin terminal strip for the battery cable, a 3-pin terminal strip for Power In leads, and a 5-pin terminal strip for Power Out leads. See Installing the Backup Battery
  • Thermistor: An external thermistor measures the ambient air temperature while the battery is charging. The charge controller uses this measurement to optimize the charging voltage provided to the battery.
Component Quantity LI-COR Part Number
Backup Battery Kit    
12VDC Lead Acid Battery 1 7900-128
DC to DC Charge Controller 1 7900-127
2-pin Terminal Strip 1  
3-pin Terminal Strip 1  
5-pin Terminal Strip 1  

Indicator LEDs

The charge controller has 3 LEDs:

  • Main Power: On when the device is powered from the main power supply.
  • Battery Power: On when the device is powered from battery power.
  • Charging Indicator: On when the battery is charging.

It has three terminal strip connections and a thermistor:

Note: Use caution when installing terminal strips—plugging the Power In or backup battery into the Power Out terminal may damage the charge controller.