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Sense wire Anomaly

This morning when i disconnected my sense wire to feed it through something i checked the voltage on my battery bank against the MPPT screen and VRM , the batteries were half a volt lower . I left the sense wire off to see when the charge controller would go from bulk to absorption and there on to float . The charge controlled did it at exactly the correct voltage shown however the batteries weren't ready , they were half a volt lower , my concern now is that the batteries will be undercharged . I dont actually need a sense wire as my cables are short and more than sufficient size and the voltage reading at the unconnected controller end of battery cables and sense wire are identical to the battery side to 1/100 volt so there's no noticeable loss . I had left the sense wire connected to the charge controller end so out of curiosity i put the meter on the other end and there was no voltage shown at all coming from the sense wire connection in the controller . Now comes the really weird thing , i connected the Meter to the batteries and placed it so i could read it and touched the battery side of the sense wire to the positive lug on battery bank and watched the voltage increase to the same as shown on the MPPT screen and VRM , it rose almost half a volt in a few seconds , taking it off and voltage dropped , it did this every time , up and down like a yoyo just by touching and removing it for a few seconds with batteries . After that i have my doubts that the higher reading is the true battery voltage , no idea why the voltages increased as it did , cant believe its possible to provide sufficient current to raise the voltage it had raced up in seconds from a cable that had shown no voltage and was only 18 awg (75 mm) . Batteries almost new AGM , Sense wire correct size and correct polarity and everything else on the charge controller functioning perfectly . Even though the sense wire isnt necessary to compensate for voltage loss in cables i have to have it connected or i undercharge batteries , even with it connected i am not confident the battery voltage is what is shows .

battery system voltage
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Alexandra avatar image Alexandra ♦ commented ·

The sense wire, is it the temperature sense? There is temp compensated charging on lead batteries

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ako answered ·

No Alexandra , the Temperature cable is in the T Sense which is right next to the V Sense and temperature compensation is spot on for any given temperature .

Last night after charging has finished i compared the Controllers LCD screen , VRM and battery voltages and they are ALL basically the same , connecting and removing the Sense wire from the battery which made such a difference before while charging now has no effect .

This morning early in the charging cycle i again tried touching and removing the Sense wire from the positive battery lug while at the same time watching the True battery voltage and the LCD screen , it appears to do the opposite to what it is designed to do ,battery voltage was around 0.2 lower than the screen and VRM but each time i touched the sense to the Battery they both very quickly dropped to match the battery voltage . Noticed as charging/voltages increase the differential between batteries and Controller increased up to 0.05 volts . I understood a sense wire was to raise the Charge controller voltage to compensate for cable loss but for me its the reverse , it lowers Charge Controller voltage to match batteries . End result is that the sense Wire is needed even when voltages taken at batteries and controller end of cables are identical with zero loss as without it the controller seems to progressively add up to half a volt to itself and regulate on that rather than Battery voltages while charging ..

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