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Old Sep 23, 2013, 06:11 AM
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Lipo Load Voltage Drop

A little FYI: I was reviewing some FTP video with Telemetry data and noticed the battery voltage can drop as much as a full volt under load. This observation was made during hovering so this drop can go much higher during a power climb. Considering most if not all ESC's have a shutdown protection voltage of about 3.0 per cell, this could create a crash condition when using a weak or low C Rated battery.
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Old Sep 24, 2013, 03:01 AM
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For multicopters, it's recomended not to user ESC's self protection for over discharge on Lipo batteries...or change settings in the ESC to think that the batteries are nimh / nicd, so that it doesn't shutdown at 3V...
The protection should come from the FC or simply find ways to know you are at low voltage like lipo buzzer or timer on your remote, telemetry, etc....
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Old Sep 24, 2013, 04:02 AM
Jack
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A healthy battery that has a capacity that is sized to the load will drop from a charged and rested 4.1V per cell to 4.0V or even 3.9V almost immediately when a full or high throttle load is placed on it. And then it will drop slowly as the capacity is consumed. How long it holds out depends on the load, the battery condition, and the battery capacity.

When you throttle back after takeoff or liftoff, the load (input power in Watts or current draw in Amps) will drop, and the amount of the drop it is not linear to the stick travel (half throttle does not reduce the input power by half). It is normal for it to drop as much as 50% or so at 3/4 throttle, and be down as much as 60-75% at half throttle.

Using the 3.3V or even 3.0V cut offs that are the defaults in many ESCs for the LVC is not really a good idea as that takes the battery down to a point where it can get too warm and be damaged. That is especially so if the current draw remains fairly high throughout the flight as happens with a multi.

If you have low battery warning device (a good idea) or you controller has one, and if that is set for 3.5V to 3.7V or so, that is about a low as a battery should be taken under load. When it signals that, end the flight. If you check the pack voltage immediately on landing, it will have already recovered from the warning voltage, it will do that in 15-30 seconds or so and continue to increase for a while. That makes it look like you had not used all your capacity even though you really have.

The temperature of the pack is a good signal as to how hard you are using it, if you wrap your hand around the pack right after landing, it should be no more than slightly warm, like 105F/41C or so. If it is more than that, you are probably taking the pack to too low a voltage and/or discharging it too fast.

If you charger tells you the capacity recharged and you are putting back 80% or so after a flight, you are getting as much as you can reasonably expect out of the pack.
For most of us, rather than relying a LVC signal, timing flights is the best way to get a reliable 75% or so out of a pack.

Jack
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Old Sep 24, 2013, 05:20 AM
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Quote:
Originally Posted by jackerbes View Post
A healthy battery that has a capacity that is sized to the load will drop from a charged and rested 4.1V per cell to 4.0V or even 3.9V almost immediately when a full or high throttle load is placed on it. And then it will drop slowly as the capacity is consumed. How long it holds out depends on the load, the battery condition, and the battery capacity.

When you throttle back after takeoff or liftoff, the load (input power in Watts or current draw in Amps) will drop, and the amount of the drop it is not linear to the stick travel (half throttle does not reduce the input power by half). It is normal for it to drop as much as 50% or so at 3/4 throttle, and be down as much as 60-75% at half throttle.

Using the 3.3V or even 3.0V cut offs that are the defaults in many ESCs for the LVC is not really a good idea as that takes the battery down to a point where it can get too warm and be damaged. That is especially so if the current draw remains fairly high throughout the flight as happens with a multi.

If you have low battery warning device (a good idea) or you controller has one, and if that is set for 3.5V to 3.7V or so, that is about a low as a battery should be taken under load. When it signals that, end the flight. If you check the pack voltage immediately on landing, it will have already recovered from the warning voltage, it will do that in 15-30 seconds or so and continue to increase for a while. That makes it look like you had not used all your capacity even though you really have.

The temperature of the pack is a good signal as to how hard you are using it, if you wrap your hand around the pack right after landing, it should be no more than slightly warm, like 105F/41C or so. If it is more than that, you are probably taking the pack to too low a voltage and/or discharging it too fast.

If you charger tells you the capacity recharged and you are putting back 80% or so after a flight, you are getting as much as you can reasonably expect out of the pack.
For most of us, rather than relying a LVC signal, timing flights is the best way to get a reliable 75% or so out of a pack.

Jack
Very profound information Jack, thank you for sharing. Just a follow-up question. Example, if using a 25C battery, would you suggest upgrading to a 45-50C battery to help with this condition?

Thank you.
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Old Sep 24, 2013, 05:29 AM
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+1 jack
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Old Sep 24, 2013, 06:18 AM
Jack
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If looking at the specs, as I did above, put the motors at full throttle at around 10-11C I would consider having 25C batteries as adequate. But also, I'm pretty new to multi copter power systems. Those use power quite a bit different than airplanes. They use a discharge rate that is sustained at higher levels and there is not any real resting or recovery time for the packs like there would be in a throttle off glide or low powered "cruise" with a plane.

And there are no rules or standards for what sellers say about their batteries. It is rare to to impossible to find any explanations as to the conditions that apply to a stated discharge rate. I would have questions like these:

It the 50C rating continuous or is that a time limited peak rating?

If continuous, is it for the full typical 80% capacity?

Those are things that we generally have to sort out for ourselves in using the packs.

Five or six years ago LiPO packs were a complete mystery to me and I studied them as I got started with them. I also was concerned about the costs and wanted to get good service lives out of the packs I bought and have managed to do that. I still have some 4-5 year old packs that are still serviceable. Those were smaller packs, not used hard, and not abused in use as far as heating or over discharging.

Adding capacity always seemed to be the best way to improve the duration of flights and also, at the same time, to have the battery being used more gently. But multis seem to always be sort of on the weight critical side and adding capacity adds weight so I think it is fair to say that, in general, multi batteries are being used harder.

Simply stated, there is no free lunch on power. If you want more power you have to add more watts, and weight, in motors, and if you want more flight duration you have to add more capacity, and weight again, in the batteries.

The Batteries & Chargers forum is a good place to loiter for a while. I used to just read the thread titles every day, then read a few threads of interest, and learned so much there. I have looked at it a couple of times recently and there is more discussion there about multi-related things I see. Here one thread that goes to your question as an example:

Spend weight on Capacity or C rating? - www.rcgroups.com/forums/showthread.php?t=2002990

One of the biggest differences I see in the discussions over here on the multi forums is that there is much less awareness of and use of watt meters and data loggers, both static and in flight, to let you see more of the details. And there are a lot more of the basic questions that point to people as entering the hobby with the multis.

Since many of the multi guys did not come up through the same learning curve, they don't have the same background and don' t know and understand a lot of the things that the guys that came up through planes to helos and mutis do.

I am most interested in motors and power system details, almost to the point of it being more fun to me than flying. And I am only lightly immersed and knowledgeable compared to a lot of the guys that helped me get smarter on the Aircraft - Electric - General forums for power systems and motors.

There is much more discussion of the same kinds of things that people want to talk about here on some of the other forums. And better answers too to questions too. Eventually, the power systems forum here will get better though.

So, to answer your question based on my background, I would expect to see little gained from from upgrading just the C rating. I would consider going for higher quality batteries (choosing those based on the opinions of the forum's battery experts and users, not sellers) of larger capacity first. And if I do that I am also most likely to also get a higher C rating along with the other qualities that make for the better battery.

I have bought one LiPO battery in the last two years, it is a average quality 3950 mAH 3S pack rated at 30C and weighing 304g and it will be used on a RC Explorer V2.5 tri copter. It is a little on the heavy side, the motors on the tri are bit on the too big size, but I am going to have a potential power to weight ratio that is on the order of 2:1. But power will be used with larger slower turning prop and with a focus on slow and stable flight. I have no interest at all in speed or acrobatics, just in trying something new slowly and carefully.

Jack
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Old Sep 24, 2013, 06:26 AM
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Thank you Jack for your most valuable insight and happy flying!!
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Old Sep 24, 2013, 08:44 AM
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i disagree

imho, an open voltage of 3.7v (maybe 3.6v) at the end of the lipo cycle is ideal.

under load, pushing a lipo to 3.15 or 3.10v per cell is ok (as long as this correlates to an open volatge of ca 3.70v at the end of the flight)

its very easy to push cells under load this low, particulary at the end of the flight time.

im currently testing the worst batteries possible (5-10C cheapo Tx lipos) and have had astoundingly good results with them

yes they have a big internal resistance, and the volatge drop under load is quite marked, but they still fly within the voltage range noted above, and im putting lots of charge cycels through them without any major problems (so far, maybe 30 charge cycles each, 4 packs in total)

certainly worth noting that you dont hit the ESC cutoff as per the OP, but i think driving a lipo to only 3.5v under load is being very generous

PS the 5-10C cells are ridiculously cheap ($14ea) so im not risking much by treating them so 'harshly'

PPS the written 'capacity' on the cell i find entirely unreliable, and the 80% rule varies markedly. i use the open cell voltage method to dertermine correct usage, and im reasonably unconcerned as to what that correlates to claimed capacity
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Old Sep 24, 2013, 08:51 AM
Jack
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"..i disagree..."


OK, but depending on the pack temperature when you take it down to 3.15V, I might disagree with your disagreement.

Were you getting about 80% back in on recharges? And are you still getting that back in after 30 cycles?

It is the heat that is the real signal of the electrolyte suffering unrecoverable damage and that can start happening at in the 110-120F range with that usually depending on the quality of the battery.

Jack
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Old Sep 24, 2013, 08:53 AM
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@Ed_jza80, very interesting perspectives, 3.15-3.10 per cell under load. How long can this level be maintain before issues develops? Is there a potential for a battery fire? What is the temperature of the battery upon landing?

Thank you.
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Old Sep 24, 2013, 10:27 AM
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I would like to add something here.....Full throttle stick on a multirotor is only 75% power on the motor and ESCs.....You need that other 25% to keep the multirotor level or you will have a problem.....If you are at full stick and the motors and ESCs are at full power and your multirotor tilts you will not have the power left to level out the craft......Using a lowc-rated battery is good for a no payload multirotor but if you are lifting a DSLR or any other large payload you will be pulling over 900 watts which is far more than any low c-rated battery can provide.....
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Old Sep 24, 2013, 10:45 AM
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packs are at very worst slightly noticably warm, like really barely luke warm. certainly not 40deg C. if i had to guess probably between 25 and 30deg.

i fly to 3.7v open directly after the flight, (which is about 3.8v open when finally connected back to the charger) and im putting in about 100% of stated capacity back into these cells (2500mAh rated, regualrly put 2490-2560 back in). i can only assume the capacity is a bit underrated.

so far, do degredation in capacity, nor any dead cells.

if you google lipo cell discharge voltage, i think im pretty much on the agreed-upon limit of under load voltage drop, and safe open voltage.

so far so good
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Old Sep 24, 2013, 11:27 AM
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I agree with ed. I use same tx lipos but only take 80% of claimed capacity. MR puts load of 2.5C avetage. Good for 20 cycles and going strong still. End voltage under load is 3.2 to 3.4 volt with above 3.7v open.
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Old Sep 24, 2013, 11:33 AM
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AWE! so in theory, we should be able to drain the battery while in flight to about 9.9-10.2 for a 3S battery with minimum harm.

Interesting and thanks.
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Old Sep 24, 2013, 11:55 AM
Jack
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The throttle percentages as they are often mentioned and discussed here can be very confusing and even misleading. A typical ESC will see a span in the PWM rate of about 500 as the range from zero throttle to full throttle.

And the PWM rate is distributed in a linear manner, 125 at 1/4 throttle, 250 at 1/2, 375 at 3/4 and 500 at full throttle. And if you measure against those increments, the input power and thrust are both very non-linear. The attached image is static testing and shows it very well.

The drop from 1840g at 100% to 1050g at 75% means you lose 43% of the thrust when you drop back to 3/4 throttle from full throttle. At the same time, the input power drops from about 380W to about 190W so you given up 43% of the thrust to cut the input power in half.

I read guys saying here that they fly at 50% throttle or can attain a stable hover at 50% throttle. I assume when they say that they mean 50% of the linear throttle travel (i.e., stick vertical) and not 50% of the input power or 50% of the static thrust. But those comments are never accompanied by any data logging info or any knowledge as to what the thrust was or might be at that 50% hover point.

When I see what happens in static testing, I really want to get an eLogger onto the multi to see what the system is actually drawing in flight as compared to static testing. Results always change when you compare static to inflight but not much is know about the differences for multis.

Jack
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