High side shunt question

[mythbuster88]

So with the AM solar kits the Victron BMV shunt is on the return side (-). Why not have a shunt on the high side (+) and float the current monitor? Then you could eliminate the DC/DC converter.

[rmkrum]

Circuit is much easier to design with the shunt in the negative lead...reliable isolation is hard to do on the hot (+ lead).

[mythbuster88]

Originally Posted by mythbuster88

So with the AM solar kits the Victron BMV shunt is on the return side (-). Why not have a shunt on the high side (+) and float the current monitor? Then you could eliminate the DC/DC converter.

Bad question Mythbuster!

The DC/DC converter has nothing to do with the shunt used for current monitoring.

The reason for the DC/DC is to combine a travel trailer using LiFePO batteries to a tow vehicle charging system made for lead acid batteries.

Where is the "everything you need to know about a lithium battery upgrade" thread? And what else don't I know?

[rmkrum]

Actually, thinking about battery charging hardware, components, and the like, I don't know of ANY that use a shunt on the high side (positive with respect to chassis ground).

Pretty much everywhere I've looked for battery monitors the shunt is on the negative side...

I'm guessing there is a pattern here--a few millivolts above ground is a lot easier to sense that a few millivolts bunches of volts above ground.

I worked in the datalogger and temperature measurement business years ago for a little company called Doric Scientific. We had systems that could sense precise millivolts while the thermocouple junction was up to 200 volts above or below earth ground. It had a nasty circuit inside the box that automagically took the signal conditioning box up to the common mode voltage. Company called it "AutoGuard".

We techs and engineers that worked on it called it "Auto Grab" because if the ground safety spring that was supposed to ground out that isolated section failed to make good contact, the circuit would go hard-over to at least +200 volts DC, and, yes, grab you and NOT let go. Once grabbed, forever vigilant...

So yeah, I like working closer to ground with the inputs...and I bet most other designers do to. Isolating stuff across a high voltage accurately is a real pain in the butt, just sayin'