OK, this isn’t really a racing quad or FPV question but I figure the people that hang out in this section of GA should be able to help out!
I’ve recently replaced the RX unit in my RC yacht. I had a Radiomaster PWM RX (FrSky D8 protocol) and was getting RSSI warning around the far end of the race courses we use - the antenna is inside the hull which is often submerged in water so it’s understandable I guess. I had the great idea to stick an ELRS PWM rx unit in there so I could basically sail to the moon.
All works fine. Except for the telemetry value for RxBat. I’m running a LifePo (I think) 2-cell thing with a standard servo plug on it only. When charged it runs at about 6.6v. On my TX16s, connected to the D8 RX, the telemetry (A1 variable) seems to give sensible readings:
When I power the ELRS receiver from it, the RxBat value starts off giving me about 4.5v, then drops down tthrough 3.x reasonably quickly and after about 5-10 minutes of faffing around with EdgeTX settings on my controller was reading about 2.9v.
I’ve tried different bandwidth settings, refresh rates, telemetry ratios etc. Doesn’t seem to change anything.
One thing that strikes me is that on first plug in it goes straight to 4.2v. I wonder if the ELRS software is trying to do some conversion/scaling to represent typical LiPo cell voltage. That said, it then starts dropping visibly and gets to 2.x quite quickly (all the time the actual voltage if checked via the D8 RX is 6.x volts).
I did come across this which seems related but doesn’t really answer the question
Depends what you mean by ‘regular’ ELRS rx. It’s a a normal receiver with PWM outputs, instead of just serial RX/TX (although it has those too).
I bought it because it was nice and small and also about £10. Radiomaster do a PWM ELRS RX for about £20 which looks a lot better made and more likely to make sense. If I don’t get any joy from this I’ll use it for a little foamboard plane and get one of the Radiomaster ones for the boat.
I have set an alarm on my tx for the lipo battery on my drone and its always worked. Sometimes there might be a bit of delay, but mos of the times it matches up with what I see on the osd…
(its a spi receiver on a happymodel fc)
Maybe LifePO has a different behaviour on the telemetry?
I tend to use a PWM board soldered to a serial RX (ELRS or Crossfire) such as a Matek CSRF-PWM-6 (Matek CRSF To 6ch PWM Converter | HobbyRC UK) and use the + on the LiPo balance plug to feed the Vbat pin. Your battery doesn’t have that but it would be easy enough to make up a little adapter cable that outputs battery voltage on both the main connector and a flying wire.
Always found the value sent as RXbat telemetry to be spot on when checked with a multimeter.
Not an answer to your question, but maybe worth investigating as an alternative solution?
ELRS RX is £9 for a Jumper one, and this little board is £5 so not expensive to try in any case.
After some research, it looks like the reciever I have doesn’t have any voltage sensing on it, so that number I’m seeing is just some residual/floating nonsense. I expect I’ll bung it in a small foamboard plane for flying around the park (actually I’ve just realised what it would be perfect in!) and get something else for the boat.
Either the solution you suggest or something like this which does it out of the box and has all the right plug n play connectors straight out the box