I’m not sure I understand the problem so apologies if what I am about to say is of no help!

I have installed a NASA Battery Monitor (about £100) on Playtime and it displays total amp hours (the sum of charge and discharge current over time) as well as instant amps, instant volts and a couple of other bits of data. You can reset the amp hours total when required so you can get a ‘clean’ start for your measurements.

However, it is not really a ‘portable’ device; it needs a largish shunt to be permanently wired into the main battery feed. It actually measures the voltage drop across the shunt (of known resistance) and calculates the amps through the shunt using Ohm’s Law if you remember your school physics. If you want to measure specific circuits you would have to turn everything else off. That said, I find it very useful for giving the aggregate discharge amp hours (at anchor or crossing the channel, for instance) before charging is restarted.

Is this the sort of device you need or have I missed the point?