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Hi, we are building an instrument that should be lowered to the bottom of a 350m borehole on a glacier. There will be one logger and power supply (battery/solar) on the surface, and one logger connected to the sensors on the instrument in the borehole. The borehole logger is powered from the surface and sends data to the logger at the surface by RS-485. The voltage drop from the surface is too much for having reliable 12 volt at the bottom. My workaround is to use a dc/dc converter and bring up the voltage to 48v at the surface, then another dc/dc from 48v to 12v in the borehole. The problem is that the loss and self consumption in the dc/dc converters is huge compared to what the loggers drain. Now I am thinking about controlling the power to the borehole logger with the SW12 port on the surface logger. Every 10 min we turn on the borhole logger for 10 sec and make the measurements, and send the data to the surface logger. Note that there are no scan in the borhole program, just a BeginProg/EndProg. Se programs below. 

This will of course not be beneficial for the internal lithium battery. Therefore I plan to change the internal battery with a Tadiran SL- 2790 (35000mAh) https://actec.dk/media/documents/8CFFD7EC7720.pdf How many years can I expect to get from a big battery like this?

 I have this setup running on my bench, and it seems to work well. I understand that this is not a recommended way of running the logger, but is there any obvious risk with this setup that I do not see? Is there a smarter way to work around the problem?

'Borehole logger
Dim BH_PTemp 'Can be Dim, will not show in the surface loggers Public table anyway
Dim BH_BattV
Dim BH_BattV_Lithium

DataTable (Borehole_data,True,10000)
  Sample (1,BH_PTemp,IEEE4)
  Sample (1,BH_BattV,IEEE4)
  Sample (1,BH_BattV_Lithium,IEEE4)
  TableFile ("CRD:Borehole_",8,-1,2000,0,min,0,0)
EndTable

BeginProg
  PakBusClock (1) 'Syncronize clock with surface logger

  PanelTemp (BH_PTemp,50)
  Battery (BH_BattV)
  BH_BattV_Lithium = status.LithiumBattery
  
  CallTable (Borehole_data)

EndProg

'Surface logger
Public PTemp
Public BattV
Public Result
Public ForceCR6_On As Boolean

DataTable (Surface_data,True,10000)
  Sample (1,PTemp,IEEE4)
  Sample (1,BattV,IEEE4)
  Sample (1,Result,FP2)
  Sample (1,ForceCR6_On,Boolean)
  TableFile ("CRD:Surface_",8,-1,2000,0,min,0,0)
EndTable

'---Copy from BH logger---
Dim BH_PTemp
Dim BH_BattV
Dim BH_BattV_Lithium

DataTable (Borehole_data,True,10000)
  Sample (1,BH_PTemp,IEEE4)
  Sample (1,BH_BattV,IEEE4)
  Sample (1,BH_BattV_Lithium,IEEE4)
  TableFile ("CRD:Borehole_",8,-1,2000,0,min,0,0)
EndTable
'---Copy from BH logger---

BeginProg
  ForceCR6_On = False

  Scan (600,Sec,3,0)
    PanelTemp (PTemp,50)
    Battery (BattV)

    SW12 (SW12_1,1) 'Power up borehole logger
    Delay (1,10,Sec) 'Wait for borehole logger to start and make meassuremnts

    'Get data from borehole
    GetDataRecord (Result,ComC7,2,2,0000,0,5,1,Borehole_data,1)

    If TimeIntoInterval (0,1,Day)
      ForceCR6_On = False 'Set ForceCR6_On to off at midnight (if forgotten to change)
      ClockReport (ComC7,2,2) 'Syncronize clock with borehole logger at midnight
    EndIf

    If ForceCR6_On = False
      SW12 (SW12_1,0) 'Shut down borehole logger after meassurements
    EndIf

    CallTable Surface_data
  NextScan
EndProg

The normal clock battery is 2.4Ah. It can last 2-3 years with 12V disconnected. In hot conditions, that battery might only last 1 year.

Do note that you can use the battery charger built into the CR6 as a DC to DC. The input range is 16-32VDC. If you have any 12V loads on the logger, it would help to put a capacitor into the 12V input to smooth the output power if a12V battery is not present..

Thanks for the answer Jacob. The Tadiran SL- 2790 is probably a bit overkill, but I have one laying around so I could as well use it. If we get 4-5 years out of the instrument we would be very happy.

I like your idea to use the built-in charge regulator. We would only run a couple of I2C sensors and 4 weatstone bridges with strain gage sensors. How big of a capasitor do you recommend?

You don't se any problems with de-powering the logger between measurements?

Thanks,

John

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