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Lipo and FrSky SPort Telemetry

What settings for Open TX alerts

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charlie holdford01/03/2019 19:03:55
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16 forum posts
27 photos

Hi All,

Hi This is my first post as a newbe.

Quick intro, I've joined a local club and have an instructor. Just had my second outing to the field. I have a FMS Kingfisher as my trainer with 11.1v 2200mAH 3 cell batteries. The first flight we flew 10mins and had 45% left in the battery. Weather was nice about 15c at ground level. So it was suggested to set the timer for 14mins for the second flight. This worked OK.

Given this I added a current and voltage sensor to the aircraft. Which work well.

So to the issue. I set an alarm at 11.30v and after about 2 mins flying it went off ! today the Weather was colder 10c but with a 12mph wind chill (felt cold)

Result we came in and it ended the training...

A few tests at the pits indicated the motor draws max 26A the cells ended at 3.79 3.80 3.81

I checked the logs at home and the starting voltage was 12.53v when it hit 11.30v it recovered back to 11.80v soon after and then gradually went down to a low of 10.80v and back to 11.40v before we landed.

So the Question, I clearly have not set the alert correctly. Should it be less 40% and should I have a long delay before the alert sounds?

PS. The GPS module takes ages to lock can it be powered separately?

Cheers

Charlie

Frank Skilbeck02/03/2019 08:27:08
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4430 forum posts
101 photos

Charlie, this is why voltage monitoring to check Lipo capacity in operation is not very reliable as the voltage will drop under load, so an alarm at 11.3v (3.77v per cell) is fine for a battery not under load. You say you have a current sensor, can you not get a mah readout and then set the alarm when you've used 60% of the mahs (1300 mah) with a battery voltage alarm set at say 9.6v (3.2v per cell) as a back up alarm in case the battery is below par. This curve shows a typical lipo discharge under different loads.

charlie holdford02/03/2019 09:29:57
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16 forum posts
27 photos

Hi Frank,

Excellent suggestion, thank you.

Charlie

Peter Christy02/03/2019 09:56:37
1517 forum posts

I use a countdown timer on my electric models, triggered by the throttle being opened. (I don't bother with the throttle percentage option, just open or shut.)

With a new model, I fully charge the battery, set the timer for 5 mins and land at 5 mins. I check the capacity by seeing how much it takes on re-charging. Those gadgets that purport to show a percentage reading are really only useful as a rough guide, because, as Frank points out, the voltage reading can vary with conditions.

If after 5 mins, re-charging only supplies 50% of the nominal capacity (for example), you know its safe to increase the timer to six or maybe seven minutes. I don't like discharging below about 30% capacity.

It takes a couple of flights to establish a safe motor duration, but once done, it will give you a much more reliable indication of remaining duration than a voltage monitor.

An ESC will usually cut out when the voltage reaches a certain minimum, and that should leave enough power in the pack for the radio to allow a dead-stick landing. (The radio draws almost nothing compared to the motor!)

However, don't rely on the ESC cut-out. This will often be set to a voltage lower than the safe discharge level for the LiPo. Yes, you will have enough to power the radio for a safe landing, but you risk over-discharging the battery.

--

Pete

Frank Skilbeck02/03/2019 10:21:09
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4430 forum posts
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Peter, yes, but if you can measure mah then that is even better than a throttle controlled timer. On the amp sensors I use which give mah the amount the charger puts back in is within a few mahs of what the sensor says I've taken out.

BTW if you want an all in one voltage, amps, mah, plus vario, height and rpm sensor the Unisens E sensors are very good.

Geoff Sleath02/03/2019 12:18:50
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3297 forum posts
251 photos

I do more or less what Pete writes. I don't think I've ever reached the esc battery cut off voltage because I always land long before it cuts in. The model I'm currently flying most is my Wot 4 mk2 on 4S 4AH Lipos and I usually land at about 8 or 9 minutes (that's enough for me) with about 30% or more left in the battery according to my battery checker.

Because of the very flat discharge curve voltage is not a very good method for estimating battery energy levels but it's the easiest by far and does give some indication. One problem with using voltage as the capacity measurement is that its value is dependent on load. If the voltage on load goes too low and the esc disconnects (or reduces) the feed to the motor, the voltage rises, the esc reconnects the motor hence, the voltage falls again and so on. My hearing aids are the same. I get a warning that the battery is exhausted and that happens repeatedly because the voltage rises again after it disconnects, then falls

Geoff

Attilio Rausse02/03/2019 13:13:10
101 forum posts

Hi Charlie

I would like to give a different viewpoint to what is suggested above.blush

I use a Frsky MLVSS voltage sensor which measures the voltage for each cell in the pack, I have the alarm set so that when the lowest cell reaches a voltage of 3.6v for 3 secs or more the alarm (voice readout) is activated and it works really well. I decided against measuring the amount used for various reasons the main one is that if one cell starts to play up (can happen any time even 3 mins into flight your calculated consumption will lead you to think all is well) you could end up in real trouble. At the end of the day voltage is the only reliable way to warn you when its time to land.

yes

Edited By Attilio Rausse on 02/03/2019 13:15:15

Denis Watkins02/03/2019 13:22:39
3803 forum posts
52 photos

Geoff is very concise that voltage is not a measure of capacity

All we have for alarms is to set the voltage, but volts, as has been explained, is not the full picture

A cell quite low on capacity can still display a deceptively high voltage, which drops with a vengeance under load.

The purpose of the comments is just to be aware that voltage does not give the full picture.

Bob Cotsford02/03/2019 13:26:57
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7926 forum posts
436 photos

With Frsky you can combine these methods. I set my capacity warning for 75% with the capacity used announced every 15 seconds from that point, the lowest cell value is set for a 'low battery' warning trigger at 3.3v for a 2s duration and a 'battery critical' alarm at 3.1v for 2s. Then I use a timer so that I don't hog the flightline - I have had twenty minute + flights out of my Wot4 on 4500mAh packs face 1

Edited By Bob Cotsford on 02/03/2019 13:28:00

Attilio Rausse02/03/2019 13:41:30
101 forum posts

Voltage is the full picture, you cannot measure capacity only what has been used. Low cell voltage (not battery voltage) is the warning to land and always occurs as you use the battery. Also when a cell in a battery is nearing 'end of life' the voltage can drop very quickly no matter what the capacity is. Sorry but voltage measurement is always the best way.

Peter Christy02/03/2019 13:52:58
1517 forum posts

Frank: Yes, in theory you are quite right, but that method depends on you using the same throttle setting that was used to measure throughout the flight. It does guarantee a good safety margin, but if the model is overpowered, you may end up landing with less than half the pack used.

A good example is my Durafly Spitfire, flying on 4S. The only time I hit full throttle is for take-off - and then only because the scale wheels make long take-off runs impractical on our strip! For most of the flight, the model is on half throttle or less. If I measured take-off power and based duration on that, I would be landing with 70% or so left in the pack. Basing the timings on the consumption during a typical flight gives a much better idea of capacity used, IMHO.

Of course, if you can get an airborne telemetry sensor that measures mAH used, that is ideal. But I've never used one, nor found the need for the added expense. (Plus there wouldn't be room for it in the Spit, which is a very tight fit already! wink )

--

Pete

Chris Bott - Moderator02/03/2019 16:23:35
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Posted by Attilio Rausse on 02/03/2019 13:41:30:

Voltage is the full picture, you cannot measure capacity only what has been used. Low cell voltage (not battery voltage) is the warning to land and always occurs as you use the battery. Also when a cell in a battery is nearing 'end of life' the voltage can drop very quickly no matter what the capacity is. Sorry but voltage measurement is always the best way.

I'd say voltage is an unreliable guage for capacity remaining. Voltage can decrease a fair bit just by opening the throttle. You can get to a point where you have an alarm with throttle open and it stops with throttle closed.

As a pack ages it's internal resistance will slowly increase and the above effect will change with the age of the pack.

A mAh reading will tell you how much energy you've used. This gives you a fair idea of what's left. But again, as the pack ages the mAh total available will decrease, therefore knowing what you've used doesn't tell you what's left.

So using either in isolation is not as good as having both.

I rely on mAh used, mostly. But I also have a voltage alarm that will sound if I'm pushing a pack too hard or taking it too low for comfort. I pretty much always land at around 75% pack used and don't ever hear a voltage alarm. All the way through a flight I know exactly how far through my mAh I am, no matter whether I've been cruising at low throttle or thrashing the model.

If I get a voltage alarm then I know I need to check to see if the pack has lost capacity etc. So I guess I'm agreeing that voltage is the last thing to rely on to show we're right on empty, but mAh gives us a nice linear fuel guage that can be referred to at any point in a flight. Using both together allows us to "know" our batteries.

charlie holdford03/03/2019 17:55:51
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16 forum posts
27 photos

Thanks everyone.

I have now configured my Open TX to display the cell voltages with an alert if the lowest gets below 3.3v for more than 3 seconds, as a final let's get down now.

i have also set a display of remaining mAh that has a 20% reserve. With an alert at 10% remaining.

With your input this should give me plenty of time to do a circuit and land with a good 20% left in the Battery.

I will also set a timer for 10 mins.

Thanks again for all the info, I really get a lot out of this forum

cheers

charlie

Attilio Rausse03/03/2019 18:42:02
101 forum posts

Charlie 3.3V is too low, I have mine set at 3.5V when this is triggered there is then a readout every 20 seconds of lcel, if lcel stays at 3.5v you need to land pdq, I am speaking from experience. I would advise that for a new model you start at 3.6v for safety. its up to you of course.

Peter Christy03/03/2019 18:52:30
1517 forum posts

I also think you've set the reserve mAH too low. I would set the alarm for 35%, and aim to not drop below 30%. LiPos don't like a deep discharge, so its good to err on the cautious side.

LiPos go *very* suddenly when discharged to the limit!

Setting the reserve higher gives you more opportunities for a "go round", if required!

--

Pete

charlie holdford03/03/2019 19:57:07
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16 forum posts
27 photos

Many thanks Attilio and Pete

I will adjust accordingly.

last thing I need at the moment is to think about battery failures.

There is enough to keep me occupied trying to fly the thing....

cheers

Charlie

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