Weird things with a battery monitor

[Toryboy1]

I fitted a NASA battery monitor to my boat in the summer. I have a fairly conventional 2 battery system controlled by a 1-2-both switch, no diodes or VSR. there is a Sterling battery charger with two outputs, one to each of the battery banks. The shunt for the monitor is in the negative lead from the "house" battery before it picks up the engine starting battery negative. When the battery charger is connected, it is showng a substantial discharge, over 8 amps when first switched on, then falling back to 2-3 amps- discharge remember, even when there is no load on the system- i.e. all circuits, lights etc off. It seems to be the case whatever position the 1-2-off switch is in. When the battery charger is disconecterd, the monitor shows 0 amps. The volts readout seems to be behaving as expected. Any idea what is going on here?

 

[VicS]

You have the connections between the battery monitor and the shunt reversed with the result that the charging current is showing up as a discharge?????????

If that is so then a load on the house battery (with the charger switched off) wiil show up as a charge.

 

[peteandthira]

First, check that your diesel heating is not running. Often they are installed to bypass your battery switch so that you can't suddenly interrupt their operation.

Secondly, remove the small cover from your battery charger, the bit that covers the LED lights, then try again. It is possible for the negative return line to your charger to touch this small cover, which might produce the results you are describing.

This cover is not necessary and can be chucked away.

All IMHO by the way. Beware: switch off your chargers first in case you short something, and note that most battery chargers have smoothing capacitors on their outputs and can hold quite a charge. (Even connecting batteries to the charger output, with the chargers switched off, can produce quite a scary spark as the output capacitors suddenly charge up.)

 

[peteandthira]

Vic

I was just coming to that one too! Bugger...!

 

[peteandthira]

Toryboy

From VicS "If that is so then a load on the house battery (with the charger switched off) wiil show up as a charge."

It will also show Ah's being consumed whilst charging, and start to count down to zero again with the chargers off.

I think!

Pops (PS - I'm not "Lakesailoring" there, am I???)

 

[hubert]

I could not get mine to read charge current. I sent a sketch of my system to nasa. The told me exactly where I had gone wrong. The fix was simple and now it works perfectly. I suggest you contact nasa.
Best regards.

 

[crossbones]

I think it is unlikely the shunt is reversed. This would show charge when on discharge and vice-versa, which would be very obvious. I suspect the wireing is not exactly as you think it is. I agree with Hubert, a call to nasa is the best bet.