Phantom charging

[FairMaid]

I have a NASA BM-1 battery monitor connected to my domestic battery bank (2 batteries and correctly installed as per the manual) - recently (i.e. it has not always been the case) I have seen a positive charge of up to 2.5 amps at night when there is no possibility of any charge coming from my solar panels and when not connected to any other means of charging (and when the main battery master switch is set to domestic). I have re-set the "zero current" and the "battery capacity" as described in the manual but the same phantom charge appears. Any ideas ? It can't be coming from the engine starting battery (which I am about to replace) ?

 

[Nigel-in-Oxford]

Could be from the starter battery, after having my my microwave on for too long, paralleling the batteries gives a charge to the domestic of about 6 A.

 

[Puggy]

Is the NASA shunt on the positive or negative side of the circuit? Does sound like the engine battery is still connected to the domestic, and I suspect the regulator for the solar panel may be the culprit. They are not very well separated (especially on the negative side), and the engine battery is back charging through this. Usually solar panels are directly connected to the battery (fused hopefully) so that the panel will still charge when the battery switch is off. You might want to look at a regulator with dual outputs that are diode protected to stop this.

 

[FairMaid]

Thank you.

The shunt is on the negative side of the domestic batteries (as the NASA unit is supposed to be set up). The (2) domestic batteries are connected permanently in parallel and are effectively one battery. The negative terminals of the domestic "battery" and the engine starting battery are also permanently connected.

The solar panels (with control unit) are directly connected to the domestic "battery" so as you say they will charge when the battery switch is off - generally when leaving the boat - the solar panels then maintaining an effective trickle charge.

I will experiment by disconnecting the engine starting battery when I am seeing the phantom charge.

 

[VicS]

Thank you.

The shunt is on the negative side of the domestic batteries (as the NASA unit is supposed to be set up). The (2) domestic batteries are connected permanently in parallel and are effectively one battery. The negative terminals of the domestic "battery" and the engine starting battery are also permanently connected.

The solar panels (with control unit) are directly connected to the domestic "battery" so as you say they will charge when the battery switch is off - generally when leaving the boat - the solar panels then maintaining an effective trickle charge.

I will experiment by disconnecting the engine starting battery when I am seeing the phantom charge.

This is not right.

The only thing connected to the neg terminal of the domestic bank should be the BM shunt
The only connections to the battery end of the shunt should be the white and black wires to the BM

All other negative connections must be made at the other end of the shunt ( It is useful to fit a small busbar here) This includes the starter battery negative and the solar panel negative.

Although it does not show the orange wire to monitor the starter battery by later BMs the diagram below should explain.

See also the wiring diagram ( Figure 2) in the BM installation instructions at http://www.nasamarine.com/images/file/BM1+-BM2+.pdf

 

[VicMallows]

Mine behaves exactly the same and I believe it is probably an inherent design defect. When left for several days even with everything definitely disconnected it will indicate a charge or discharge of many Ah. It seems that the zero point (when setting the offset in the ENG function) varies depending on the actual battery voltage at the time. This suggests inadequate voltage regulation within the device. Sometime I may get around to investigating the problem, but for the moment I just reset the Ah counter whenever I turn the domestic supply on. (Anyone got a circuit diagram for the BM-1 ??).