6s lipo - low voltage warning setting on Tx

[MikeQ]

Hi

I’m flying a Sebart Wind 50E.  Its got a 6S lipo with the Futaba R3008SB Rx on a separate NimH flight pack.

 

I have a feed from the 6S lipo motor battery to the Rx so i can set a telemetry alarm on my Tx when the motors 6S voltage gets below a certain threshold.

 

Question is, whats the lower limit for a 6S without stressing it.  Currently set ay 23v or just over 3.8v per cell.

 

Cheers

 

mike

[Frank Skilbeck]

Mike, your problem with that is that while that would be fine for the offload voltage under load the voltage will drop lower, so you'll get the alarm going off shortly after take off every time you open the throttle. Maybe 3.3v per cell would be a better option, i.e. 20v.

[Dad_flyer]

As Frank says, the voltage will drop at full throttle. I have lipo voltage telemetry and with different models it takes a little experience of flying and looking at in flight and post flight voltages to get a good balance. It is usually nearer 3.3V per cell warning to have 3.8V per cell after landing. The alarm will go off early in the flight when I near the top of a large loop, but I can ignore that and know from experience that it is when the alarm goes off with a less stressful manoeuvre that I need to land.

While working it out you can cut the throttle briefly in flight and check the no-load voltage on the Tx, then see what it does at different throttle.

[MikeQ]

Hello

 

thanks for the feedback.  Makes alot of sense.  I’ll follow your advice thanks.

Bit new to electric having flown 46-60 glow sized pattern models for years (I’m stuck in the 90s).

 

One thing i wasn’t prepared for was just how short the flights are ... like 5mins.  Unless I’m doing something wrong.

 

Cheers

 

Mike

 

 

[Frank Skilbeck]

No 5 mins is quite typical on a high power setup, EDFs re even less. But you might be able to extend this with careful throttle management. It's all model specific as well, I have some scale models that will go over 8 mins and some EDFs that must be landed 4 mins.

 

While lipo voltage is good to know, for more comprehensive information something like a Unisens E sensor will give you volts, amps, mah, rpm, height and vario (Not much use if you aren't thermal soaring) in one small package, I have several of them.

[Chris Walby]

I have to disagree, fundamentally measuring volts means nothing to the remaining capacity of the battery. If you measure mAh then you can determine what you have taken out and with the previous charging information then have a really good idea of what's left.

 

The battery voltage is dependent on the load that is being applied even if it is a very high resistance voltage meter or someone trying to extract the maximum C rating from the battery.

 

I have models that have the ESC low voltage cut off set to minimum for launch and then the rest of the flight either the fan or prop unloads or I take it off flat out to readjust the bike clips. You may be able to do something close by prop hanging and then going WOT vertical which will cause the battery voltage to drop and you'll get an alarm, but that won't tell you are running out of battery capacity.

 

IMHO, measure before and after flight, plus a mental note if you are eking it out or giving it some wellie seems to work unless as Frank suggests its worth your while to measure mAh and then you know for sure.

 

PS MikeQ what did you used to do with IC? Did you used to measure how much you had used after each flight?

 

PPS I have an edf that depletes a lipo in 3 m 20s...to well below 20%....now that's a short flight. 

Edited September 12, 2022 by Chris Walby

[Dad_flyer]

I am afraid I disagree with your disagreement 😉. Volts alone means nothing, but you do not have volts alone, you also know your throttle position and the flight profile so far. A volt alarm comes on when the volts are low, but goes off if the voltage recovers. Certainly measure before and after flight, but as you get to know your models, the volt alarm is very handy extra information. The alternative is a time alarm that means even less.

Measuring capacity used needs a current monitor, and a capable tx, which is an extra step in equipment. It also assumes that the battery still has the capacity that you think it had and has not got old. I also fly my Stik on 2200 as well as 3000 batteries, and others on 1300/1000/2000.

[MikeQ]

Hi

 

I still fly IC and there is very little chance of running the Rx pack out of charge in a session.  Sure, with the fuel you ‘just know’ based on your flying style you are good for say 10 mins with a minute or two safety margin.  But with the lipo costing £75 (yes i went for the Hacker branded 4500 6S MTAG 20C TOPFUEL) i’m a bit nervous about damaging  / mistreating the battery.  Im just trying to use the functionality that the Tx/Rx combo provides but I do take your points re using battery voltage alone.

 

Like i say, im no expert around electric models.

 

cheers

 

mike

Edited September 12, 2022 by MikeQ

[MattyB]

3 hours ago, MikeQ said:

One thing i wasn’t prepared for was just how short the flights are ... like 5mins.  Unless I’m doing something wrong.

 

 

What are the resting cell voltages after your typical 5 min flight? On 4500 6S my Sebart Miss Wind does about 7-8 mins from memory, but then I generally do a bit of low throttle pootling about in addition to aerobatics (I'm only a sport flyer). If flying comp schedules I can imagine 5 mins is probably about right on that pack.

[MikeQ]

Hi ... I’ll have to start keeping a track of voltages and resistances and all.

 

Despite 40yrs of flying I would describe myself as an ‘avergae club flyer’ 🤣

 

I can do some pretty complicated aerobatics ... its just not the manoeuvre I’m trying to do or in the part of the sky I want to be.

 

Cheers

 

Mike

[Philip Lewis 3]

With my Sebart Angel 50E I get easily 8 minutes of flight flying aerobatic schedules using 4,000 mah 6 cell packs.

 

I never measure voltage just check with a battery checker what's left in the pack after each flight as that's what is important, I aim for about 30% to be left as conveniently that's not far off storage charge. 

[MattyB]

1 hour ago, Philip Lewis 3 said:

With my Sebart Angel 50E I get easily 8 minutes of flight flying aerobatic schedules using 4,000 mah 6 cell packs.

 

I never measure voltage just check with a battery checker what's left in the pack after each flight as that's what is important, I aim for about 30% to be left as conveniently that's not far off storage charge. 

 

Errrr, you do realise that when you apply your battery checker to get that ~30% reading, all it is measuring is pack voltage, right? Those simple checkers are just relying on the (relatively consistent) characteristics of the lipo discharge curve, i.e. 3.77V/Cell = ~30%.

 

https://blog.ampow.com/lipo-voltage-chart/

 

If the pack is on it's way out though this assumption no longer holds, so I always check mine on a device that gives the individual cell voltages (my Miss wind does carry onboard telemetry with that both transmits this back to the TX and gives a small display for checks on landing). Useful info, but as others have noted an onboard current sensor is a more reliable and effective method of validating the capacity remaining in flight.

 

Edited September 12, 2022 by MattyB

[Frank Skilbeck]

6 hours ago, MikeQ said:

Hi

 But with the lipo costing £75 (yes i went for the Hacker branded 4500 6S MTAG 20C TOPFUEL) i’m a bit nervous about damaging  / mistreating the battery.  Im just trying to use the functionality that the Tx/Rx combo provides but I do take your points re using battery voltage alone.

 

Like i say, im no expert around electric models.

 

cheers

 

mike

 

I think you've just summed up why a 65 Euro (plus tax) Unisens sensor isn't such a bad investment or the Futaba current sensor for £55 here. Although the Unisens gives you a degree of future proofing in that it can easily be reconfigured for several different radio bands, I've set them up on Multiplex, Jeti, Graupner and Futaba.