If you can probe cell terminals themselves, voltage drop from cell to cell through contacts and busbar will tell you watts dissipated in the metal. More difficult to decide if heat is flowing into cell or transferring to air (unless convection, emissivity etc. are part of your vocabulary) but it should give a qualitative understanding.
Down to whatever millivolts your DMM reads, unless you have a fancy bench DMM with microvolt sensitivity. We're using op-amps or instrumentation amps to resolve microvolts - in our case to determine ripple. That lets us use scopes with sensitivity a bit better than millivolt. You might be able to use an eval board or breadboard as preamp to get to microvolts.
I can use my 4 wire Yaorea meter (I also have a Fluke 87 V). It can measure quite low resistance, the biggest problem is being able to get at the terminal surface with a busbar in place. I do know some of my temperature rise is due to my testing with 40 amps with 10 gauge wire, just started switching to 8 gauge. Using Ancor wire
and crimp connectors (Ancor again)
. Being anal retentive, I had to order the Ancor crimp
tool to go above 10 gauge, probably my 10 gauge crimp tool would work, but it would bug me if not perfect. Of course you can also use battery lugs
and even heavier gauge wire (or even lugs with 8 gauge wire). Ancor makes some very high quality products, and everything is already tinned.
Like you say, how much is transmitted due to the wire, and how much is from the cell itself is the question, hopefully I can get some more accurate readings soon.
But I don't have any welded studs to test with. I will likely order some soon, my initial objective is to be able to run a 12,000 BTU mini split heat pump from 3:00 PM to 6:00 PM Monday through Friday (SRP charges a lot extra then, it accounts for 50% of my electric bill in the summer).
Eventually I want to be able to run it for at least 8 hours, before having to fire up the generator during any extended power outage.
No solar yet, but already planning on grid tied solar next year. Load shifting is a bonus.