Looking at raw telemetry data provided by NASA from the ISS (here's a nicely formatted version: https://isslive.com/displays/spartanDisplay1.html), the current output of the photo-voltaic panels are shown as a negative value. Maybe there's a simple explanation for this, but I haven't found any information on it (one NASA "teaching guide" I found talked about calculating PV power output from this data and essentially ignored the negative sign). I can see why voltage might be negative, but not current--after all, the PVs are generating power, not consuming it. Any thoughts?
If this Reddit thread is to be believed, the currents are representing the current draw of the ISS batteries. Negative indicates that the solar panels are charging the batteries (driving current into the battery).
Looking at the Spartan Console handbook (the system from which you are looking at data on ISS Live), we can see that the data we are looking at is from each of the 8 battery controller channels:
In order for the ISS to continually maintain electrical power, the SPARTAN flight controller oversees a set of batteries for each of the eight independent power “channels” (one for each solar array).
Current measurements taken for a battery will have one sign represent charging, and another sign representing discharging. As to which one is which, you would need more information.
Assuming the data for each is up to date, then at the moment of writing this answer:
- ISSLive shows a negative current
- ISSTracker shows that the ISS is currently on the night-side of the planet
As the ISS is on the night side of the planet, it's solar panels will be dark. That means that it must currently be running on battery power, i.e. the batteries must be discharging.
Based on this a negative sign probably indicates discharging of the batteries.
It may be a convention where positive current means a load (ie. consumer of electricity) and negative current means the opposite of a load, ie. source of electricity.
All needed to find out if the system is balanced, charging or discharging is to add the numbers.
Historically Energy consumed, dissipated or measured in a load is + ve therefore to satisfy KCL and be consistent source current must be - ve.
But typically we say a PS sources positive current or sinks positive current from the perspective of THE LOAD.
NASA clearly does it correctly using KCL at the node. Although KCL may apply arbitrary assumptions for initial reference directions and still work, this is the convention of using Kirchhoff’s Laws.