Really small system voltage discrepancy

cptdondocptdondo Solar Expert Posts: 99 ✭✭✭
I have a single 300W panel connected to a Victron 100/30 charge controller, with 2 AGM 6V batteries.

The Victron consistently reports much higher battery voltage than the system monitor. 

For instance right now it's reporting 12.3V and the system monitor 11.9. Earlier the difference was even greater; 13.3 for the Victron and 12.4 for the system.

I am not running heavy loads; my total current is between 5-10 amps continuous. 

Any ideas whats going on?


  • mcgivormcgivor Solar Expert Posts: 3,815 ✭✭✭✭✭✭
    What gauge wire are you using and how long ? At 12V voltage drop is a more pronounced issue, if the system monitor is connected to the battery there will be a difference in voltage between it and the controller, the value of which will be dependent upon the size of the conductors and the current passing through them,the higher the current the greater the voltage drop, the battery is the load in this case.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • PhotowhitPhotowhit Solar Expert Posts: 5,576 ✭✭✭✭✭
    Where and what type of "system monitor" are you using? How is it connected to your system?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • cptdondocptdondo Solar Expert Posts: 99 ✭✭✭
    I did not wire this trailer, so the following are guesses.

    About 20' from the Victron to the batteries using 10ga wire.  The system monitor is a SeeLevel gauge panel, wired in to the control panel. That panel is a snake's nest of wiring, most of which looks to be 12-14 ga, probably also 20' from the batteries as the wires runs.

    The control panel runs through a fuse panel.

    Power usage was minimal; maybe 5 amps, 10 at most. Most of that is two wifi access points, a satellite dish,, and a TV. Not sure if the control panel is hooked into the same circuit as my electronics.

    I'll dig the meter out and measure directly at the batteries.

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    20 feet, 10 AWG, 10 amps, 12 VDC:

    The drop is 0.48 volts

    Same as above with 14 AWG wire:

    1.24 volt drop

    So, from one end to the other you could see an almost 1.7 volt difference.

    With 12 volt wiring, you need short/heavy connections to keep voltage drop low... For loads., I would suggest 0.5 volt drop max from battery to load.

    From charge controller to battery bank, around 0.05 to 0.10 volt maximum drop.

    My first suggestion would be to move the solar charge controller "next" to the battery bank. The controller needs short/heavy wiring to the battery bank to keep voltage drop down. Also, the controller should roughly the same temperature as the battery bank (if lead acid batteries). A remote battery temperature sensor is also a good option to have.

    If you are interested--It sounds like you should think about the revisiting the wiring and clean the mess up. Also check connections to make sure they are clean and tight. Look for browning of insulation (and even dust marks from warm air circulation near wiring connections) to ensure nothing is overheating.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cptdondocptdondo Solar Expert Posts: 99 ✭✭✭
    OK, I took some time to try and sort out the mess.  The wiring from the controller to the batteries is 8AWG - so plenty big enough for the 0-15 amps it’s carrying. 

    The culplrit turned out to be some cheap fuse holders inline, hangning outside below the batteries, exposed to road salt and grime.  Completely corroded and showing a significant voltage drop.  Cut them out, replaced with new crimp terminals, and now everything is close.  I still have about a 0.2V unaccounted for, but that’s better than the 1+V I was seeing earlier.  

    At some point I’ll need to add fuses again, but for now I need all the solar juice I can get.
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