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Multirotor Motors, ESCs, , Batteries, Distribution boards, etc. Multirotor Motors, ESCs, Batteries, Distribution boards, etc.


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Old 07-28-2014, 06:19 PM   #1 (permalink)
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Default LiPo minimum safe voltage??

What is the minimum safe voltage under load ( in flight) before you decide to land? I have telemetry in place but not sure at what point should I land. Unlike 3D style helicopter there are not high spike demands during gentle multirotor flying, as is the case during FPV or videography, so I guess the safe voltage per cell might be slightly lower than what I am used to with helis. 3.3-3.5V?????
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Old 07-28-2014, 06:55 PM   #2 (permalink)
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I get to around 13.8-14.3v under load when my 4S pack is close to reaching 20% remaining.. So right around 3.5v/cell. This should leave a resting voltage of around 3.74v/cell. It'll only dip down below 14v if I give it full throttle when almost depleted, which draws over 40A... This is on my Disco btw.

Just be sure to remember that your voltage while hovering, when depleted, will be a bit higher if your're only hovering or very slow flying for the entire flight. If you're hitting full throttle several times throughout the flight, the depleted voltage when hovering will be lower.... even though both flights, fast/quick and slow/long, bring the cells down to the same resting voltage. Hope that makes sense.

If your telemetry setup doesn't provide mAh consumed, I suggest that you only rely on a timer. Using the voltage as a means to know when to land is very risky, for the reasons I described above. It's too much of an estimation to be relied on for such a critical function. By adding a current sensor, you can fully deplete your LiPo's without having to worry since you always know exactly how many more mAh you have to go before you're running on fumes.
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Old 07-30-2014, 10:51 AM   #3 (permalink)
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Rule of thumb I've always gone by is that a LiPo cell nominal voltage (aka resting, storage, 50%) is 3.7v. Fully charged is 4.2v, "dead" is 3.3v, and critical would be 3.0v, as in, if you get to 3.0v, the pack could be toast, so don't do that.

As Sean posted, it's best to work from a timer unless you can tell during flight how much capacity you've used. When I was testing, my first flight was kept short at 5m to get a baseline and then I went to 7.5 and then 10m under varying conditions, checking how much I put in during each charge session, so I now know that I can safely get a 10 minute flight and have at least 20% left. I can probably get up to 12 minutes depending on what I'm doing and still have a safety margin. My cells haven't been below 3.7v that I'm aware of so that's another indicator I have room to play with.

I have an older v3 EagleTree eLogger and I strapped that on for the first 3 flights as additional confirmation data to make sure all was well. My average constant amp draw is around 8A or 2A per ESC. I was worried for a minute when I saw 45A spikes during my testing in manual mode with hard punches until I remembered that was across all 4 ESCs, not any individual one. Duh.

Related info: I'm using the E300 power system on a F450, Pulse 4S 3300 packs, DX8 radio w/AR8000 RX but no telemetry module as it is on another bird and haven't felt the urge to buy an extra unit.
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Old 07-31-2014, 10:35 PM   #4 (permalink)
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Originally Posted by fsusmithc2 View Post
Rule of thumb I've always gone by is that a LiPo cell nominal voltage (aka resting, storage, 50%) is 3.7v. Fully charged is 4.2v, "dead" is 3.3v, and critical would be 3.0v, as in, if you get to 3.0v, the pack could be toast, so don't do that.

As Sean posted, it's best to work from a timer unless you can tell during flight how much capacity you've used. When I was testing, my first flight was kept short at 5m to get a baseline and then I went to 7.5 and then 10m under varying conditions, checking how much I put in during each charge session, so I now know that I can safely get a 10 minute flight and have at least 20% left. I can probably get up to 12 minutes depending on what I'm doing and still have a safety margin. My cells haven't been below 3.7v that I'm aware of so that's another indicator I have room to play with.

I have an older v3 EagleTree eLogger and I strapped that on for the first 3 flights as additional confirmation data to make sure all was well. My average constant amp draw is around 8A or 2A per ESC. I was worried for a minute when I saw 45A spikes during my testing in manual mode with hard punches until I remembered that was across all 4 ESCs, not any individual one. Duh.

Related info: I'm using the E300 power system on a F450, Pulse 4S 3300 packs, DX8 radio w/AR8000 RX but no telemetry module as it is on another bird and haven't felt the urge to buy an extra unit.
Thanks for your reply. Everything you say is a common knowledge and I have done all that to find the flight time duration which would leave about 20% in the LiPo. But as we all know the packs deteriorate and the under load voltage during flight, I thought could be a good indication of how much is left in the tank, so to speak. I guess the concensus would be that if my OSD shows around 14V on 4s pack, it is a good indication that it is time to prepare for landing with safe reserve to bring the craft home.
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Old 08-01-2014, 04:15 PM   #5 (permalink)
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The important part is to know how much the voltage drops under load with your setup. I know on my QAV400 it's almost exactly 0.5V as I can verify that by doing the voltage calibration procedure in the Naza V2. So if I'm shooting for landing by the time I get to the nominal voltage on a 4S pack (14.8V) I know that I should land by the time the voltage readout in my OSD reaches 14.3V. I also go by the mAh used as well, but the voltage is really what I care about. If I took it down to 14V under load that would mean it would come back up to 14.5V (3.625V per cell) resting. That's not going to kill a pack, but it's going to leave it under the nominal voltage which I try not to do. If I set the absolute LVC down at 3.3V per cell (13.2V) then that leaves it a fair amount of buffer zone to get it back and save the quad in the case I really f**k up and bring it back way too late. I may damage the pack, but most likely I'll get the model back. Now if your voltage drop under load is different than 0.5V then you'd simply need to adjust based on that factor.
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Old 08-04-2014, 11:14 AM   #6 (permalink)
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"dead" is 3.3v, and critical would be 3.0v, as in, if you get to 3.0v, the pack could be toast, so don't do that.
Yeah, well I "did that". Don't forget to charge your pack before flight when the last time you used it brought it down to storage charge. I got a lesson on that this weekend.

Last Tuesday I flew a few times and after the last time on each pack, they were right around 3.7v per cell (nominal voltage) so I left them that way instead of charging them up. When I do fly, it's mostly on the weekends and a week at full charge shouldn't hurt the packs but leaving them at nominal voltage is still best so I've been trying to break myself of my habit of charging at the end of the day.

Fast forward to this past Saturday and I wanted to show my friends my new machine and forgot the packs weren't charged. Hovered for about a minute or so inside (I know, don't say it) and then went outside. Brought it into a hover again and then it came down. Was only about 7-10 feet up and it came down on soft cushy grass and just tumbled a bit with no damage.

Back on the charger (TP 610C) and the cells were between 2.5v and 2.7v. Used recovery mode twice and then it charged up fine from there and all cells are balanced. Haven't flown it again since Saturday but I'm thinking it's going to be ok. I'm just glad I wasn't a hundred feet up or over water, trees, etc. Yikes.

So again, don't do that.
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Old 08-04-2014, 12:40 PM   #7 (permalink)
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Would the safe voltage after flying by 3.7 volts on any size battery?
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Old 08-04-2014, 12:54 PM   #8 (permalink)
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Yes, no matter the size of the LiPo, the voltage after a flight should be no lower than 3.7v/cell. Any lower and you're most likely exceeding the 80% rule, which will effect the LiPo's life expectancy over time.

I've discharged mine down to 0% also, always on accident. While they still work, the internal resistance of the cells has gone up slightly and I believe they've lost a little capacity.
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Old 08-04-2014, 03:03 PM   #9 (permalink)
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Normally I fly mine and after checking the cells they run between 3.7 and 3.9 volts
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Old 08-04-2014, 04:03 PM   #10 (permalink)
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To be honest the voltage can vary a bit, my premium packs keeps giving towards the end of the pack discharge and the voltage stays higher than the "standard" packs, while the voltage is a an indication, flying to time or measuring what you have used from the pack is much better.

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Old 08-04-2014, 07:05 PM   #11 (permalink)
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Quote:
Originally Posted by petedee View Post
To be honest the voltage can vary a bit, my premium packs keeps giving towards the end of the pack discharge and the voltage stays higher than the "standard" packs, while the voltage is a an indication, flying to time or measuring what you have used from the pack is much better.

Pete
Good point. Depending on the quality/health of the cells, the internal resistance will vary... Which will yield different voltages with the same load. So, in theory, a cheap 20C LiPo will experience more voltage sag under load than a 60C LiPo under the same conditions.
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Old 08-05-2014, 01:30 PM   #12 (permalink)
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I've discharged mine down to 0% also, always on accident. While they still work, the internal resistance of the cells has gone up slightly and I believe they've lost a little capacity.
Entirely possible, certainly. I've been logging my pack usage in a spreadsheet and I numbered my packs so at least I'll always know which one had the accident and when. They're Pulse lipos so I'm hoping they're built well and I had a good half dozen flights on each pack before that happened so at least they had a little "break in".

I forgot to mention that when I did fully charge that pack, I put in 3690ma and it's a 3300 pack. Granted, I had to stop the charge near the end to go run an errand and restarted it so minus a couple hundred maybe for margin of error but still not great.

I'll know more on how well that pack responds once I get it in the air again. Need to do that this week. 3.7v per cell after flight is definitely the mark to shoot for. I need a few more flights to confirm but it's looking like 10m is my sweet spot and leaves 20%.
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Old 08-05-2014, 08:56 PM   #13 (permalink)
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Mine did the same when I discharged them down to around 3.0v/cell, but I don't recall just how much over 3300mah it went. I do know that the second time I over discharged the same pack, the charge didn't put back nearly as much as it did the first time I drained it down to 0%.

While the LiPo's may continue to work, their life is shortened every time they're over discharged. After spending hundreds on just four batteries for my Goblin, I read up as much as I could on LiPo care and learned a lot about what I was doing wrong. With enough attention to detail, you can significantly increase the amount of flights you get out of a single LiPo.

Luckily, our multirotors aren't quite as demanding as a 3D heli.. Otherwise, the performance degradation would be much more noticeable.

Here's a video when I lost track of time, flying LOS, and forgot that my timer went off. I was seriously lucky with the conclusion of that flight... The video makes it look rather uneventful... But that was only because I had noticed a drop in power when punching the throttle, which alerted to me to make an immediate emergency landing. If I hadn't noticed that, game over... The voltage dropped like a rock, and within a few seconds of me noticing a loss in power, the motors could not lift the quad.. even at full throttle.

[ame]http://youtu.be/r49u7oOxkT8[/ame]
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Old 08-06-2014, 12:46 PM   #14 (permalink)
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Nice video and nice save! The sad thing is, I think I did notice the power drop and safely landed first, thinking it was perhaps some other issue, and then I tried lifting off again to check. That's when it only got about 7 feet up or so and it sunk in what the issue really was. At least it still had enough power to make the landing relatively soft.

I too don't wish to be replacing expensive packs due to user error. Gotta stay frosty. I'll eventually have FPV gear with the mini iOSD and might also buy an extra TM1000 to use on this (don't want to rip the one on my Aurora off). That should help keep an eye on things but so would simply using my cell checker before flight. If it's not resonably close to 4.2v per cell, it's not fully charged.

I should post the video I captured during that flight to share my shame with the world. It's pretty short.
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