ELRS Rx brownout

Merv

Moderator
Moderator
I just tried a HappyModel ELRS Rx in my plane & have experienced several brownouts. When I'm about half of my normal flight time, I lose telemetry, have no control and the plane crashes. When I let the pack recover for a few minutes, I can plug it back in & everything works. When I run the motor on the ground, it will run for 20 seconds or so, then quit, when the Rx browns out again. I never had this issue with my old Rx.

Anyone else having this problem?
Any ideas to solve the problem?
 
Last edited:

LitterBug

Techno Nut
Moderator
Sounds like an ESC, BEC, or battery issue.

I don't have any HappyModel RXes. I have Matek and RadioMaster PWM RXes and have not had any issues like that.

LB
 

Foamforce

Elite member
I have one HappyModel 6 channel PWM ELRS receiver. So far I’ve only had one flight on it, which lasted about two minutes. No problems that I’ve noticed yet. While I was flashing and binding it, I had it hooked to a storage charged (3.8v) 1s battery and it didn’t have a problem, so it seems to be able to handle low voltages fine.

It has voltage telemetry built in, so maybe you could use that to see if you’re getting low voltage. I believe that if you don’t connect the bat+ voltage telemetry pin, then it just reports the BEC voltage back. Put a widget on your main screen for that, I think it’s RXBt. It should stay at 5v. If it drops, then probably a bad BEC.
 

LitterBug

Techno Nut
Moderator
I have had batteries with one marginal cell that would act like that. They could even have all cells showing a nicely balanced battery, until under load, the bad cell will drag the whole thing down. I have a couple of chargers that can read the resistance of each of the cells. If cells are all pretty close, no problem. But the flakey batteries always have at least one cell with a much higher resistance. I will mark those with red permanent marker "BAD CELL" so they don't accidentally end up in the air. I'll use them to bench test till they are mostly discharged then run them the rest of the way down with a dummy load and dispose of them.
 

Merv

Moderator
Moderator
Update. I did some bench testing. I took a flight pack which was not charged, 11v. To my surprise, the Rx work fine. I connected my volt meter & had 5v from the BEC. I started the motor it ran for about 1 minute, gradually slowing as the voltage dropped. I ran the battery down to 8v under load. When I shut the motor off, the battery immediately jumped to 10v. During this stress test the BEC voltage would fluctuate but the lowest reading I saw was 4.5v but only for a moment then it would jump back to 5v. The Rx never had a brownout, even though I was moving all 4 servos.

I was about to add a standalone BEC which outputs 6v, but now I doubt that is the problem.

When I crash Thursday, I checked the battery voltage, it was 11.8. No idea what the voltage under load was, but the motor was running much faster than today under my stress test.
 
Last edited:

LitterBug

Techno Nut
Moderator
Hmmmm, The BEC shouldn't be dropping that low or fluctuating with 8V....


And HappyModel rates the RX from 5V to 8.4V, so I think you may have your answer.


Screenshot_20240928-141950.png
 
Last edited:

Merv

Moderator
Moderator
Update, more bench testing. I taped into 1 cell of a flight pack, it was at 3.68v. Thank for the idea @Foamforce . The Rx worked, but when I moved the servos, I lost telemetry one time & it immediately recovered. The servos were sluggish & glitchy which I suppose should be expected.

Two ideas: First, add a capacitor to smooth any low voltage. Second go with the stand alone 6v BEC. Both are on hand.

What say ye?
 

Merv

Moderator
Moderator
Repeated the low battery test. Battery was at 11v, under power 10v, quickly dropped to 9v. The BEC was at 5v, dropping to 4.8v with 4 servos constantly moving & under full power.
 

LitterBug

Techno Nut
Moderator
I recommend not going below the rated 5V for the RX. Do everything you can to prevent it. It does not take much to upset the processor on some of these newer RXes causing them to reboot. The other thing you should do is make sure the RX defaults to the same packet rate as the TX module, so in the event that it does reboot, it will link up faster and recover from failsafe quicker.
 

Merv

Moderator
Moderator
I just got back from a fly. I added a standalone BEC. This appears to have fixed the issue.

The standalone BEC runs at 6.2v & will drop to 6.0 when the servos move.
 

LitterBug

Techno Nut
Moderator
Here is what I mean about matching the TX packet rate to the RX packet rate. This is configured using the SYS --> Tools --> ExpressLRS.
IMG_20240928_191039_9.jpg


The TX packet rate is what will ALWAYS be used when the RX connects to the TX when it boots up. So as an example, the screenshot below shows my TX is using a 333HZ packet rate.
IMG_20240928_190247_186.jpg


To see what the RX is configured to, scroll down and select >Other Devices

Your RX should then show in the list. Select it and hit enter. In my current setup, it is a RadioMaster ER5A.
IMG_20240928_190322_062.jpg


Now I see what the default packet rate is set to for the RX. As you can see, it is set to 100Hz.
IMG_20240928_190348_056.jpg


The problem with this is, when it boots up, it will go through a round robin test of EVERY packet rate till it finds that the TX is listening at 333Hz. This takes several seconds per test. (100Hz ,50Hz, F1000, F500, D500, D250, 500Hz, 333Hz) In this case, it takes 20-30 seconds before the RX and TX are talking. THAT IS AN ETERNITY when you want to regain control of a plane in the air!

After Matching my RX to the TX packet rate, the connection is almost instantaneous.
IMG_20240928_192318_1.jpg


Same thing is true if it seems like it takes a long time for the RX to connect when you plug in the battery. If it doesn't connect almost instantly, check that the RX init rate matches the TX!

Cheers!
LitterBug
 

LitterBug

Techno Nut
Moderator
As far as adding telemetry, once your RX and TX are bound and connected, Go to Model --> Telemetry --> discover new
IMG_20240928_185239_670.jpg


You should now have a nice list of data being passed back from the RX to the TX

IMG_20240928_185302_949.jpg


This will allow you to use other scripts and alerts for battery voltage, TX power, RSSI, link quality, etc.

IMG_20240928_210604_283.jpg


I need to do some research on my ER5A RX, because the RXbat is reading WAY low....

EDIT: Appears my V1 ER5A does not have battery monitoring so the pin is floating. The V2 supports it.
 
Last edited: