LiPo voltage warnings

[Tim Kearsley]

I'm trying to set up some realistic (and useful) LiPo battery warnings in a Taranis X9D+, running OpenTx 2.3.11. I'm doing this by monitoring the battery voltage telemetry reported back from a Neuron ESC. Now it's child's play to set up a logical switch and special function to announce a warning based on the battery voltage dropping below a certain threshold. My question is, what should that threshold be? The problem, as I see it, is that in normal powered flight the battery will be under load and the battery voltage will therefore sag by a certain amount, due to the battery's internal resistance. If the load is removed, the battery voltage will recover.

So, bearing the above in mind, what do you think is a reasonable voltage per cell to set as a warning threshold? I don't want to set it too low and risk damaging the battery, but equally setting it too high will likely result in spurious warnings, particularly if the throttle is opened up.

Any thoughts gratefully received!

[PeterF]

The voltage should depend on the max current drawn and can vary from model to model. I believe the Neuron ESC can also log current, does it report mAh used. On my Jeti set ups I use mAh used as the main battery monitor with voltage as a back stop, often set around 3.2 to 3.4V per cell depending on how much current the model draws.

[Bob Cotsford]

Depends on the expected maximum draw! If it's a power hungry model I use 'low battery' at 3.3v for 2 seconds with a 'battery critical' at 3.1v on the lowest cell using the lipo sensor for individual cell levels. You could use a little higher voltages on lower current setups or if you want packs to last longer. It's all a bit of a compromise between battery life and flight length.

Peter, yes the Neurons report mAH used but voltages are a nice backup to detect failing packs.

Edited By Bob Cotsford on 01/02/2021 16:57:08

[Dickw]

As you say, the battrey voltage will vary under load, so it all depends on the battery size/internal resistancen and the load you are going to apply.

For most normal applications I would start by experimenting with a setting of 3.6 volt per cell and see how you get on.

I say "normal applications" because I have some models where I regard 3.3 volts per cell under load as being a very good battery freshly charged, and also a small 150gm aerobatic thing that will sometimes show 3.5volts per cell from the 2s 300mah pack.

One option might be to set up an alarm that shows "off load volts" and triggered if you shut the throttle.

Dick

[Frank Skilbeck]

You could always set up a logic switch so say it only alarms if the throttle is below x% and the voltage is below y volts.

Problem is, as you note, off load 20% left is around 3.7v, but if you were say pulling 25C amps, the alarm could go off when the battery was still at 90% capacity.

On my models I use mah used as the main alarm, with a 3v per cell as a back up.

[Tim Kearsley]

Thanks chaps. I should have said that I'm already using mAh consumed as the primary indicator of battery state, so voltage monitoring is a backup. Frank, I have considered your idea of only alarming when throttle is low and might go that way. Indeed, I already use that method in an F5J glider where I haven't got current monitoring and the model is at zero throttle for 99% of the time!

[Dad_flyer]

I only have voltage telemetry, not mAh, on my receivers. I set to about 3.3V or 3.5V initially and then adjust to different models - if I find I am landing with the batteries nearly full. These are non-aerobatic flights, so the throttle is pretty constant most of the time. Doing low passes or repeated landing practice the alarm goes early as you pull back up, but then you know that is a pre-warning not the real one.

[robert chamberlain]

My HITEC automatically sets low NIMH & NiCad rec batteries (5 cell) at 5.6, so it is looking at pack voltage and not each cell. It is an older trans and ,I guess, is better than nothing.

Who is running NiCad anyway?

[Andy48]

After a great deal of trial and error, I use a very similar setup to Bob, with voltages of 3.2 and 3.0 per cell. I get very few spurious warnings, and with the individual cell monitoring it gives a very early warning of a battery declining in usefulness. Normally I use consumption as the best warning of how long to keep flying.