# Why is ISS photo-voltaic current output shown as a negative value?

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?

• Turn the ammeter round, and current is negative. No problem. Perhaps they do it to see if anybody is reading their reports. – Neil_UK May 26 '18 at 16:16

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.

• In 20 minutes or so, we can test the theory. The current should go positive. – Tom Carpenter May 26 '18 at 16:31
• Never mind. Current stayed negative. Voltage jumped up to 160V though meaning panels are in full sun. – Tom Carpenter May 26 '18 at 17:11
• I'm looking at it now on a position tracker and it's only just got into daylight and it is very south so maybe it's still not aligned properly. It's about 1200 km west of Chile at the moment and at latitude -48. Current in 2B is -46 amps. Good theory whether you are right or wrong! I also think the solar panels maybe can't get perfectly aligned yet - the words on the screen link in the question seem to imply 41 deg and 319 deg where I believe for best alignment it sould be 0 and 360 ??? – Andy aka May 26 '18 at 18:36
• Maybe the excessive current overnight are due to heaters and maybe the current shown is truly load current and not indicating net charge in or out? Current has definitely fallen to an average of low 40's over 5 minutes. – Andy aka May 26 '18 at 18:39
• Yes, I too thought that it might be "negative when running on batteries" and "positive when charging/running on solar" (or vice versa), but after looking at lots of data, it's always negative (as far as I've seen). Hmmm. It is possible that in its current orbit and with the current position of the sun, etc., that ISS is never in Earth's shadow? – eric May 26 '18 at 18:46

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.