Electrical conundrum

[keithgdg]

Anyone have an answer to this. I have fitted a sterling pro, 200w of solar panels linked through a mppt controller that is linked through the sterling. I recently fitted a nasa bm2 monitor and have noticed that the net charge/discharge tab always shows a gradually increasing discharge of around an amp or two per day yet the 350amphr bank is always fully charged (on shore power) so why the net discharge?

Help because it's really annoying me that I don't know why.

 

[Bilgediver]

Is the connection from the solar panels to the correct side of the shunt and so being measured by the BM2?

 

[keithgdg]

Yes I'm pretty sure it is as all negs went through the shunt but I will check.

 

[tri39]

I think the Nasa monitor can be diff to set to zero! Mine will always register a slight discharge (or charge).

 

[GrahamM376]

Mine seems to drop a little for no reason. Although it's only supposed to register what passes through the shunt, I suspect that's not entirely correct.

When setting up the NASA, it's supposed to be connected to fully charged batteries with nothing connected to charge or discharge them. If the charger was disconnected but the batteries weren't left to settle to their 12.8? volt state when the monitor was connected, it will think (say) 13.8 volts to be 100% and anything less therefore below 100% capacity remaining.

As the sun disappears or the charger goes into float mode, the voltage drops from the charge voltage and the monitor may think this is because of something discharging the battery. Two hidden small discharges can be the car radio permanent live for memory and the gas alarm, both of which are often direct to the battery.

 

[keithgdg]

Thanks for the info. I understand what is meant by your 13.8 volt scenario and it makes sense. I think I will attempt a complete reset after get the excess voltage to settle to a realistic 100% level.

I have to confess that mechanics I understand but electricity, even 12v, is still to some extent "white man's magic" though I'm getting better.

Thanks again to all who gave me their thoughts.

 

[keithgdg]

Thanks for the info. I understand what is meant by your 13.8 volt scenario and it makes sense. I think I will attempt a complete reset after get the excess voltage to settle to a realistic 100% level.

I have to confess that mechanics I understand but electricity, even 12v, is still to some extent "white man's magic" though I'm getting better.

Thanks again to all who gave me their thoughts.

 

[VicMallows]

The zero adjustment for the Amp hour counter on the BM1/BM2 is notoriously unstable. It varies greatly with both ambient temperature and actual battery voltage. Over a few days it doesn't matter too much, but after a couple of weeks the reading is meaningless. I just zero the A/hr counter when I get to the boat . If you have PV panels or mains charger the batteries are likely to be fully charged. Otherwise zero the counter when you judge the batteries to be fully charged such as after several hours motoring.

Yes, poor design, but adequate, and price is attractive