Low Voltage Alarm Question(s)!

[ZoomNBoom]

thank you for that bit of info.. its good knowing we have a little leeway between the buzzer, and landing..

chris.

Keep in mind this is more true for a multirotor than say, a motorglider. On the latter you may cruise for extended time at low throttle (and very low power draw), possibly even at idle, and then when the buzzer goes off at 3.3, its already 'too late'. Same goes for very high C rated batteries which wont sag as much under load.

But in most cases, it applies, and when in doubt or when I forgot to hook up telemetry, I will often give short bursts of full throttle just to see if it triggers the alarm.

 

[BanditJacksRC]

Keep in mind this is more true for a multirotor than say, a motorglider. On the latter you may cruise for extended time at low throttle (and very low power draw), possibly even at idle, and then when the buzzer goes off at 3.3, its already 'too late'. Same goes for very high C rated batteries which wont sag as much under load.

But in most cases, it applies, and when in doubt or when I forgot to hook up telemetry, I will often give short bursts of full throttle just to see if it triggers the alarm.

That make a lot on sence. I can see a battery sag under a load, so to set the alarm to 3.3 but a difference of .4-.5 volts per cell from throttle punch/idle seems like a lot. If I could program it I think I would still set it higher.
Also would a higher c battery really sag less? if anthing I would think it would sag more because it can deliver a higher draw more effectivly.
Im not an electrical engineer or anything, Im just curious.

 

[Craftydan]

The current demended by the motor will be the current it demands -- set the throttle and it will draw what it wants, whether the pack is High C or low C.

Since a low C pack can't source as large of a current flow, it will sag more under the same current demand. This means your alarm set voltage will need to be set lower for low C, since the sag is greater -- to have a final 3.7v/cell resting, a low C pack will go lower under the same current demand.

so . . .

High-C pack or low current load -> higher alarm voltage since the sag is low (closer to 3.7v resting).
Low-C pack or high current load -> lower alarm voltage since the sag is high (farther from 3.7v resting).

On gliders where I'm only running servos, I'm setting the alarm *at* 3.7v (almost no sag).
On my multirotors, it's 3.3-3.4v depending on the pack I use.
Everything else is somewhere in-between.

 

[ZoomNBoom]

That make a lot on sence. I can see a battery sag under a load, so to set the alarm to 3.3 but a difference of .4-.5 volts per cell from throttle punch/idle seems like a lot.

It would be a lot with a half full pack. .4V sag is not that much when its almost empty, the difference between 3.3 and 3.7v isnt much for a lipo. Test it if you can. On the ground if you must, or if you have a telemetry enabled radio, just put a voltage sensor on it, and log your voltage (and throttle). It will tell you a lot about your batteries.

FWIW, voltage sag under load is so real that for some planes, I take throttle position in to account on my Taranis low battery alarm. I scripted it so that it will trigger at different levels, depending on the position of my throttle. At WOT, it may trigger only around 3.4v, and half throttle around 3.5V and idle at 3.75 or so. Numbers just for illustration, and there is also a time delay built in because it takes a few seconds for the voltage to recover when you reduce the throttle, but you get the idea.

If I could program it I think I would still set it higher.

Yeah, probably. But then I rely on simple timers and voltage telemetry primarily, 'feeling' a loss in power secondly and the lipo buzzer is there as last line of defence, and still useful if you have an unbalanced battery or a dying cell. Or when I forget to disconnect the battery

Also would a higher c battery really sag less? if anthing I would think it would sag more because it can deliver a higher draw more effectivly.

The voltage sags precisely because the battery can not provide the current demanded of it. The voltage sag is actually your best indicator to verify you have a properly C rated battery. If it doesnt sag much or at all, you are flying with an overrated (thus too heavy) battery. The other extreme would lead to low voltage alarms even with a nearly full battery.

 

[ZoomNBoom]

here is log chart I made a while ago, it shows the sagging nicely:

Blue is voltage, that other not so blue blueish the throttle position.

And yeah, that was with a battery that probably had a too low C rating.

 

[BanditJacksRC]

Very cool. I have the DX6i so couldn't really do the telemetry data logging myself. (maybe with the STi module)
but what a great chart, thanks.
This all really explaines a lot, thank you.

 

[BanditJacksRC]

Since a low C pack can't source as large of a current flow, it will sag more under the same current demand.

Yeah this makes more sense than my theroy

 

[jipp]

Very cool. I have the DX6i so couldn't really do the telemetry data logging myself. (maybe with the STi module)
but what a great chart, thanks.
This all really explaines a lot, thank you.

i think we should be sure to be good boys and ask santa for a teranis pluss. heh.

then i will have to learn how to use telemetry.. so much to learn yupp..

i think it safe to say we do not stop learning in ths hobby.

chris.

 

[ZoomNBoom]

"using" telemetry takes no learning really. Out of the box the taranis will talk to you if your receiver voltage is getting low or if RSSI signal strength is getting weak, you dont have to do anything for that. To get lipo voltage you need a $3 sensor and you just configure your desired low and critical thresholds on the radio, and thats about it. Logging the data is also pretty trivial, just assign a switch (or always have it on) and select the interval. The PC software will read those logs and produce those charts. Of course, you can do fancy things like I described above, but thats not exactly only for telemetry .

Anyway, yeah save up and get one, I cant recommend it enough - provided you are willing to do some reading. Not so much for telemetry, but for, well, most things . Taranis is a hobby within the hobby, perfect for when its raining.

 

[RichB]

Am I crazy to be this cautious?

Yes. This hobby is suffering from a terrible case of "telephone game" with respect to what is a safe lipo voltage.

Safe Lipo voltage ranges are 4.2v - 3.0v. Lipos start to take damage as soon as the voltage in the pack drifts away from "storage voltage." Is your pack taking damage every second it spends at 3.0 volts? Yes, on a microscopic level it is. It's also taking damage every second it spends at 4.2 volts. Keep a pack charged for 6 months and see what happens if you don't believe that.

Below 3.0v per cell, available power drops off dramatically and damage accelerates. I believe battery capacity is rated as a discharge from 4.2 to 3.2 volts, for safety. There's a reason that 3.7 is "nominal voltage", it's right in between 4.2 and 3.2.

Do you have any ESCs with a programmable voltage for a power cut? The last one I bought had 3 options: 3.2, 3.0 and 2.8!

Most lipo alarms now a days come with 3.3 as the default. This is a completely safe level. It warns you before you hit 3.2 (the safe bottom end, which is itself just some padding on top of 3.0). The problem comes in when people come to accept 3.3 as the safe limit and they then seek to have alarms at 3.4, then 3.5, etc. I've even seen people say that they never let their packs get below 3.7! That's the nominal voltage of the pack! They say this with pride as if they are super-duper cautious, but in reality they are just killing their packs by forcing them to spend their entire life above 50% charge.

Honestly, reprogramming your charger to cut off at 4.0 volts makes as much sense as using a low-voltage alarm set to 3.5 instead of 3.3.

Anyway. Things are getting out of hand and I'm standing up against it.

 

[ZoomNBoom]

Honestly, reprogramming your charger to cut off at 4.0 volts makes as much sense as using a low-voltage alarm set to 3.5 instead of 3.3.

Anyway. Things are getting out of hand and I'm standing up against it.

And I have to disagree at least slightly. You are correct that storing a lipo at 4.2 volt is not good, but I have done it for 6 months, and it doesnt destroy it. It might have suffered a little bit, but nothing I actually noticed. OTOH, keeping it below 3V for just a small fraction of that time, would kill it, pretty much stone dead since it will discharge further. You can usually revive to some extend a deeply discharged lipo if you do it immediately, but after a few days, its too late and after a few weeks, there would be nothing to revive. So over and under nominal voltage isnt the same thing.

 

[jipp]

ill be the first admit all this voltage is greek to me.. but im learning.. if i can learn the basics to keep my battery's working for a year.. and assuming i have not ruined them from a crash.. ill be quite happy. of course id hope for more than a year on a battery if i paid say a 100.00 but on a 10.00 1300 blah i say i got my moneys wroth if it last a year. lol

thanks for the education everyone.. i think im safe at 3.3v on my default alarm.. and setting my TX to that amount of time.. about as good as im gonna get till i step up to a better TX? and probes that can talk to each other in real time.
chris.

 

[BanditJacksRC]

Anyway, yeah save up and get one, I cant recommend it enough - provided you are willing to do some reading. Not so much for telemetry, but for, well, most things . Taranis is a hobby within the hobby, perfect for when its raining.

Yeah I've been keeping my eye on the X9d plus for a little while now. I'm sure I will be picking one up within the next couple months or so. It seems like an awesome TX

 

[BanditJacksRC]

Yes. This hobby is suffering from a terrible case of "telephone game" with respect to what is a safe lipo voltage.

I kind of figured this was the case to a certain extent, but you just hear about it so much... DANGER DANGER, FIRE FIRE!!

haha thanks for the explination, great info

 

[SteevyT]

Keep a pack charged for 6 months and see what happens if you don't believe that.

I've got one at 6 years that doesn't seem to have suffered too badly from it (ok, 4 from when I sort of stopped with the hobby to when I picked it back up). Unfortunately the plane that used it died about a year ago.