Pwm showing different v than multi meter

Streatwise
Streatwise Registered Users Posts: 7 ✭✭
in General Solar Power Topics #1
Just installed basic pwm controller and solar panels in my van. The battery is fully charged and is showing 13.8 on multi meter, tested on wires going into pwm. The pwm display is showing 12.7 v.
Any thoughts if this is a faulty pwm or if its a setting wrong?
Thanks for any advice 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #2
    Take the voltmeter to your car and see if it around 12.8 volts with the engine not running and around 13.8 to 14.4 volts when the engine is running.

    Your voltmeter is probably fine, but I have seen DMM (digital multimeters) give some very strange readings if the battery is near dead (and no on-screen battery warning warning).

    Some higher end meters do allow for adjustments to meter readings.. But very few lower end system meters/solar charge controllers do--And a 1.0 volt difference is not something to 'paper over'.

    Sometimes, you can get variations in meter vs actual readings--Some vendors will display a temperature corrected battery voltage meter reading for the battery bank... For example, if there is a -0.005mv/C/Cell offset, and your system is at 35C (95F):
    • -0.005 volts * (35C ambient - 25C standard temp) * 6 cells (12 volt battery) = -0.30 volt "depression" in charging voltage for "hot" battery
    But this is not anywhere close to your 1 volt error.

    One other slight chance for errors... You always want to connect the DC Battery to the controller first, then connect the solar array. If you connect the array first, and then the battery bank, the charge controller can get confused during booting (and in some cases a connected/in sun solar array without battery bank connected can damage the charge controller). So, to reboot the charge controller: Disconnect solar panel first, then disconnect battery wiring to charger--Wait 30 seconds--And then connect battery bank first, then solar array second.

    Assuming your DMM is correct and the solar charge controller is wrong--It sounds like you need to call the vendor for more help or a replacement. At this point you do not know if this is a "display issue" (LCD is telling you the wrong voltage) or a problem where the controller is "seeing" the wrong voltage and will not properly recharge your battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    #3
    Hi Bill

    Thanks for the advice. Pretty sure my DMM is correct but will test again against a battery. 
    All being well will disconnect battery and pvc from the controller then reconnect a x test again see if it resets. 
    Will let you know how it goes, weather permitting as its broken today and raining. 

    Cheers
    Aaron
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  • mike_s
    mike_s Registered Users Posts: 156 ✭✭
    #4
    Streatwise said:
    The battery is fully charged and is showing 13.8 on multi meter, tested on wires going into pwm. The pwm display is showing 12.7 v.
    Not sure what that means. You measure 13.8 at the terminals which connect to the battery on your PWM controller? You say "wires going into pwm", which is ambiguous - the battery connection is logically thought of as going _out_ of the controller, although you could also be referring to the wires physically connecting to it. Or you may be referring to the wires from your panels going into the controller.

    In any case, the input and output of a PWM controller may not be a simple DC voltage. There can be a PWM component (basically, AC on top of DC), which may confuse a simple meter.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #5
    What Mike _S talks about could be an issue if there are bad electrical connections or too small of wire from charge controller to battery bank.

    Checking the voltages when the sun is "down" should show both Meter and DMM with the "same numbers". And you can do a quick check with the system charging (current flowing) by using the DMM to measure both the terminal voltage on the Vbatt output of the controller, and on the battery(ies) terminals themselves. In an ideal system, with full charging current, the voltage drop from Vatt-terminals to Vbus should be 0.10 volts or less (for a 12 volt system).

    There can be issues with "simple" digital meters that "sample" once a second or so the voltage vs True RMS (root mean square) DMM meters. With a PWM output, it is possible for the meter to be sampling "peak" voltage of the PWM (pulse wave modulation) vs the "average (RMS) voltage. It is not a common issue--But if combined with "problematic wiring" is certainly possible (checking voltage at night would bypass this issue).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    #6
    Hi Both
    Well following my last post I have reset controller... disconnected solar array, then battery, left 1 min and connected back up again. Have float set to 27v and no load attached, only used to keep battery charged for when needed. 
    Have attached pictures of afterwards. 

    1st pic is display on PWM, the 2 probes are my DMM on the wires to the battery. So PWM is reading 12.6v.
    2nd pic is my DMM show a reading of 13.03v (it did fluctuate between 13.2 and 13.5 while probes were in place)
    3rd picture is probes on the solar array input 
    4th pic is the reading from this (have 2 20v pv panels in parralell) sun was going down so is quite a low reading. 

    So after resetting I still get a different reading on the controller display of float charge state to the reading onmy DMM.

    Maybe this is just because the controller is a cheap model, but thought it woukd still give an accurate votage reading. 

    Anyway any thoughts much appreciated 
    Thanks 
    Aaron
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #7
    Thy replacing the battery in the DMM, this may affect readings and I believe the meter is telling you to do so by the battery icon 
    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.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #8
    Did you measure the battery terminal voltage on the batteries themselves?

    The solar panel voltage should be a little bit higher than the battery terminal voltage (to ~20 volts). Current flows "down hill"--So the panels have to be the same or higher voltage than the battery terminals.
    And as McGovern says, try a new meter battery.
    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    #9
    Thanks guys
    Will try battery replacement on my DMM and see how that goes.
    Yes I did test on tbe actual battery terminals and DDM gave same reading as above 13.03v 
    The reading from my panels was low as this picture was half hour before sunset, during the day they normally show as giving between 15 and 20v.

    Cheers
    Aaron 
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,113 ✭✭✭✭✭
    #10
    "Well following my last post I have reset controller... disconnected solar array, then battery, left 1 min and connected back up again. Have float set to 27v and no load attached, only used to keep battery charged for when needed. "


    Why is the float set to 27 volts?

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    #11
    I have set the float to 27v as didn't want to over charge the battery, seeing as the metre is reading a higher voltage than the controller I was erring on the safe side until I figure out what's going on... 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #12
    I think the question was more 27 Volt Float with a 12 volt battery bank? Nominally float is around 13.6 volts or so for a 12 volt bank...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    edited March 2021 #13
    Sorry my mistake... That meant to say float set at 12.7v not 27v...
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #14
    No problem... Everyone just wants to make sure to keep safe.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Streatwise
    Streatwise Registered Users Posts: 7 ✭✭
    #15
    Ok so update on this issue..
    Replaced battery in DMM, also got new probes as the ones in use were a bit knackered.
    Now only showing a +0.2v difference to the charge controller display...
    Keeping an eye on it but hopefully this was the issue..

    Cheers all
    Aaron
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #16
    Probably the tollerance of the metering is responsible for the  deviation, I would think the meter is more accurate because it likely has a higher resolution and tighter tollerance than a charge controller, good to know you've sorted it out.
    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.
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