Battery voltage via ELRS (PWM) telemetry

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.

Old vs new RX:

All working and bound nicely.

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.

Immediately after taking the photo above at a reported 3.5, I plugged the same battery back into the D8 reciever and the TX16S was showing 6.6v, so the battery is still charged.

WTF. How am I supposed to monitor my battery? What does 3.5v even refer to? I wondered if the RX has some kind of BEC circuit, but even so the battery readings don’t really make sense.

This is the RX. Says it needs 5v to run but will accept up to 8.4v:

Any clues? Anyone used ELRS on a PWM receiver?

Maybe something with the bandwidth configuration you have set up? What are the settings you have?

Although this wouldn’t necessarily explain for it to show a lower value. Could be something else entirely as well…

Realized after the post that you are not using a regular elrs rx. Did some digging and came across this Battery Telemetry (RXBT) for PWM Receivers · Discussion #1464 · ExpressLRS/ExpressLRS · GitHub
Seems to be somewhat related to your problem, but I didn’t go through it in detail…

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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

lol… we ended up in the same place

Ha, just posted the same link!

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.

Thanks. Yeah, that’s an idea.

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

Yep, that should work fine too, Radiomaster is good gear.


Did you end up buying the radiomaster receiver? I believe I have the same battery (and possibly the same boat) and this discussion seems to be the only place where this issue is discussed.

I got the Radiomaster ERA5C and seem to have the same problem you had: the battery voltage reported on my TX16S is currently 0.8 volt. On the 16s you can enter an offset, but not sure if that is really the solution. As the off set would have to be pretty high.

I didn’t buy the Radiomaster reciever, I bought a Matek receiever with a dedicated battery sensing pin which some Google searching suggested would do the trick. That said, it’s still sat in the packet waiting for me to solder some pins on it, so I haven’t tested it yet. Will report back here as soon as I do - hoping to get out sailing in the next couple of weeks if work allows.

Pretty sure this is the one I bought:

Thank you. Looks like you do indeed need the sensing pin for the battery monitoring to work.