MEP-805b current draw

[baxter462]

Very interesting.

The schematic shows the circuit breaker on the on the alternator side of the current transducer, which is definitely not how mine are. The only way that would make a difference though is if the wire is supposed to go from the alternator to the circuit breaker, then up through the current transducer before going to the batteries. The gauge doesn't read backwards, it is just biased by negative 6 amps. When severely discharged, it will read positive until the batteries charge, just not as high as before the modification. I am running regular flooded car batteries, not optimas though.

It seems like the tech note for the upgrade mentioned replacing the backplane with a newer design with surface mount components. Maybe the new back plane has a different value resistor somewhere in the BCT circuit to the I/O module to calibrate it differently?

The BCT definitely had two wires going through it before (charge wire and control system wire, and it read appropriately that way, and afterwards it only has one (as per instructions). In both cases, the direction of the wire is from alternator downward through BCT to current protection device, to batteries.

 

[baxter462]

Okay, so I've been thinking about this during my morning commute, and I realized something. The way it was wired before was charge wire from alternator downward through BCT, to inline fuse, to battery. The control circuit went from battery through BCT to control box. I didn't pay attention to the direction the control circuit went through the BCT because I knew it wouldn't anymore after the upgrade. So, my realization this morning is that there are two possible scenarios for the original wiring. Either A) the control circuit wire was ran through the BCT "backwards" so the it effectively "cancelled out" the current control system current already being measured through the charge wire, or B) the control system wire wasn't "backwards," and the BCT was actually double measuring control system current (once via the charge wire and a second time via the control circuit.

When it gets warmer outside, I'll do some testing with an accurate low current inductive clamp. I know I can get it to read correctly, but it will require me to run the control circuit wire through the BCT along with the charge wire like it was before. I'd just have to try it both directions because I didn't note which way it was before. Of course again, this will only work if both actually fit (it'll be tight at the very least)

 

[Guyfang]

Please post your findings. I too am intrigued, with this problem. I know I fixed this problem once or twice, but its simply been to long ago. Perhaps if I had been kinder to my brain cells, they would be kinder to me now.