Technically not an multirotor/FPV question, although I am an FPV pilot most of the time and this may help others with the same RX unit... 

I've just changed the receiver in my RC saiboat from a FrSky D8 (Radiomaster R86C) to an ELRS pwm unit, in the hope to get some more range, since I already use ELRS on some of my quads and planes. The unit is a really basic HappyModel EPW5
https://www.happymodel.cn/index.php/2021...ixed-wing/

I use a LifePo battery in the boat that gives about 6.6v when fully charged. On the old RX, the telemetry would give me a correct battery level on my TX16S (I think it was the "A1" telemetry value).

The new ELRS reciever binds and operates the boat controls fine, but the RxBat telemetry value is weird. Shows around 4.2v when I first plug in and visibly drops through the voltage range, through 3.x and gets to the high 2.x within a few minutes. The odd thing is that if I take the battery and plug it back into the original Radiomaster receiver, my transmitter correctly shows 6.6v. Plugging it back into the ELRS rx, it goes back to about 4.2, then starts dropping. All the time, the boat works fine (since the boat has no motors and only sails with the wind, a charged battery usually does about 6 hours of sailing). 

Showing 3.5v


Immediately plugging the same battery into the old D8 receiver, it shows 6.6v, so I'm confident the voltage hasn't dropped:


Any ideas? I can't work out what that RxBt value is supposed to be telling me.

ELRS version is 3.1.2

The EPW5 does not have a voltage sensor. The ADC pin is probably left floating, hence your variable readings.

The EPW6 does have a voltage sensor, as do many of the Matek PWM receivers. Look for a "Vbat" or similar pad on the receiver.

Ah! Thanks!. That makes sense. In that case, I may stick this on a small RC plane and get a better receiver for the boat.