I keep doing these things, getting bizarre readings and then failing to explain it properly. I have another problem.
I have a single source of 24V DC which is running 2 DC/DC bench PSUs.
One is set to 13V and is floating a lead acid. The lead acid is powering the network switch and the internet/wifi router (and 2 USB hubs).
The other is set to 19.50V and is running the work laptop docking station.
The trouble is, the currents are very suspicious. The lead acid's loads have been pulling pretty much exactly 1.3 Amps for the past 2 weeks. This morning the current out of the battery fell to 1.0 Amps.
The PSU powering the laptop started out at 4.5 Amps, topping up the laptop battery and fallen to around 1 Amp. Which is "about right", 20 Watts or so.
So where did the other 300mA go?
As a "finger in the air" I connected the multimeter to the "-" pole of the lead acid and via the "Amp" range to the copper radiator pipe beside it. Hmmm. 0.5 Amps, falling over a few seconds to.... 300mA.
At the weekend I tried to power a monitor from the PSU which is powering the work laptop and I ended up disconnecting it pretty quickly.
The monitor came on fine. Worked perfectly, but when I look at the PSU running it it said "0.00A" and the other PSU was showing 2 Amps more. I could even unplug the "-" from the PSU and the monitor continued to run.
So 2 amps which was not returning to the PSU that made it. The only other path from the monitor to anywhere was via the HDMI and DP cables, from there t the USB hub and back down the "-" rail via the battery and back untimately to the 24VDC that provided. I didn't like the sounds of 2 Amps on the lowside of 19.50V "leaking" that way, so I shut it down.
This mornings oddity is different. Those 300mA appear to being "added" to the circuit, not re-routed.