question

@christo64, the BMV registers charging current through the negative shunt, so if it's not registering your alternator that's most likely because your alternator ground (usually a chassis ground) isn't running through the shunt. It's worth noting that this doesn't mean your battery isn't being charged, it just means that the BMV can't see it being charged because the charge method is bypassing the shunt.

According to a fairly generic spec sheet I found on (theoretically) your alternator, it's listed as 90A output; this is of course at full-bore and you'll never see that much current out of it for very long; if you ran it at full-bore you'd burn it up in a matter of hours. So let's call it 40-45A under normal operating conditions, at (usually) around 14.4v... Then the ArgoFET will add a voltage drop of roughly .02v at low-current and roughly .01v at high current... negligible, in other words. Under perfect conditions, then, you should be seeing roughly 45A at 14.2-14.4v into your deep-cycle IF it's been significantly depleted; if it's relatively full, however, of course the current in will be less.

Keep in mind, however, that this is not taking into account any voltage drop occurring in your cabling. You didn't specify distance or cable gauge used, but both these specs are also critical in calculating your actual charging current into the aux batt, because undersized cabling for your run to the aux batt could give you voltage drops of .5v or more, not to mention generating heat along the way.

Overall, I'd say you should just make sure your chassis ground and all other grounds in the system run through the shunt so that the BMV can start seeing that charge current and making full use of the features of the device. I know it's a PITA and will likely cost a fair bit in cabling and lugs, not to mention the time, but in the end you'll be glad you did because you'll then be getting truly accurate readings from the BMV, which will make you feel better about the money you spent on that as well!