I have S1000 with WKM + iOSD and S900 with WKM + iOSD. Both have the same settings.

When flying, I have 0,35V less voltage reading on the iOSD than external battery monitor shows. But when on the ground, voltage is the same.

On S1000 I have the same reading as external monitor in the air and also on the ground.


I think there is some loss in the S900 power board?

try to calibrate the wkm voltage monitor .35 volts lower and see if that helps in flight.

Just for reference, I don't see any voltage drop on my S900. In the air, my battery plug in volt meter reads within 0.1 volt of what my Naza and A2 reads throught my IOSD and FPV monitor. Are you using a different battery on the S900? Those 150 connectors are large and it takes a lot of heat to get a good solder connection. You may want to inspect the connections to make sure you don't have a cold solder joint. You can also check for warmth right after landing. If you have a resistance point that point will be warm to the touch with such a large voltage drop.
If your voltages read the same on the ground, they should be very close to teh same in the air.