Battery Monitor or Charger

Dcline

What device would have the correct percentage of charge my Schneider Battery Monitor or my Midnight Charge controller?

Sometimes their readings are close and sometimes they are far apart. Don’t know what to believe. 

RCinFLA

Assume you are referring to state of charge percentage.

Only columb counting is reasonable for state of charge tracking  Controller just keeps tally of amps in or out verses time.  

As to accuracy.  Accuracy of shunt resistance and controller calibration accuracy on reading small voltage drop across shunt.  This is likely fairly good although some controllers have the ability to adjust (or mis-adjust) the calibration on controller shunt voltage reading for a reference amperage reading you enter.  Many monitors have a shunt type setting which can be 50mV at max current or 75 mV at max current.

More then likely you have one of the several default settings different between monitors.

% is based on its tally of in-out amperage vs time based on what you set for total battery AH capacity.  So check you have both set with same AH capacity.  You can set AH capacity anything you want to. You might want to hold some reserve margin in the readout by setting a value like 80% of battery actual AH rating or 50% of actual to get max longevity from lead acid batteries.

Monitor determines what is 'full' by what you set for charge voltage 'full' value.  Make sure they are set the same.  If you don't recharge above this point for a long time the monitors are working on 'dead reckoning'  meaning errors will cummulate over time.  'Full' reset trip resets monitor to its 100% reference point and clears cummulative errors.  Cummulation begins to work from this reset full AH point based on AH capacity you set up.

Some monitors have a recharge efficiency factor. This is likely one of the default settings that may be different between controllers.  It is a derating factor it applies to charging current based on given battery chemistry efficiency.  Lead acid may be the default and it may be set to something like 85-90% derating for recharge current cummulation counting.  Lead acid recharge efficiency is better below 85% state of charge and gets much worse above 85% state of charge.  This is one source for cummulation error depending on where in the SOC you spend most of your operating time.