Where to connect Mazda6 2017 external smartcharger negative lead for battery sensor?

I'm planning to attach external battery smartcharger permanently on my Mazda 6 2017 because of short driving trips. But where to connect charger negative lead so that battery sensor (iStop) and car computer will notice charging? I suppose not straight to minus pole because of the sensor in negative battery cable? Is it just somewhere on chassis ground or is there a specific spot on Mazda?

I have tested couple of times with negative clamp both in chassis ground and minus pole, but with OBD2 Forscan app I can see BATT_SOC battery charge% does not rise after charging. Overnight voltage is ok like 12,6V though. I have used Bosch C3 and a little older charger.

With Forscan app I can see that iStop does not work because of low SOC charge like 60%. After smartcharging about 3-4 hrs to ready, SOC has not risen a bit, so battery sensor/computer somehow does not recognize the external charging? With car alternator works ok after driving enough.
 
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