View Full Version : Several simple questions - 450 related
Aussie_G
06-01-2008, 09:25 PM
I have just a few probly pretty simple questions, answer one or answer all :)
Is there a different canopy that will fit over my batteries better, I have to squish it on, and where it goes over the canopy mounts its starting to open the holes somewhat..
Is it normal for the heli to shake some when the battery is low, right as my ESC starts its soft shutdown, ( I have it set high) the heli shakes - not real bad or anything, Im still in control & can land safely.
Does training gear affect the way the heli flies? if I remove them, should I expect any major changes in response?
I might think of some more questions, but thats it for now..
ChasHeliCop
06-01-2008, 09:42 PM
Is there a different canopy that will fit over my batteries better, I have to squish it on, and where it goes over the canopy mounts its starting to open the holes somewhat..
What kind of pack are you using, the 450 canopy should be snug, but not stressed.
Is it normal for the heli to shake some when the battery is low, right as my ESC starts its soft shutdown, ( I have it set high) the heli shakes - not real bad or anything, Im still in control & can land safely.
You should not run until your ESC kicks in. The low battery and lower head speed will cause the heli to shake. You need to time your flights based on how many mAh you are putting back into your backs after flight. But if your ESC is kicking in, you are flying too long and damaging your packs.
Does training gear affect the way the heli flies? if I remove them, should I expect any major changes in response?
Yes the training gear will make a difference. When you remove the gear, the heli will be quicker and more responsive. Until you can land and take off without any difficulty, or without tilting the heli, basically controlled landings, you need to stick with the gear.
Skarn
06-02-2008, 10:32 AM
What Chas said, but to clarify point 2:
The ESC will give you warning by going into "low voltage shutdown" mode....it will pulsate the tail left/right, left/right until you land it.
I've seen many say not to let it do this, but I personally have done it for every flight since I've started with my TREX back in January. So far, no problems ever. I figure that's what the feature is there for! I have mine set where it starts to pulsate at 3.6v and it does a great job....I've never timed my flights.
But with that said, I can definitely see how it would be "safer" to time them and land just prior to the pulsating.
Skarn
Aussie_G
06-02-2008, 12:00 PM
I have never ever dropped voltage below 3.6 per cell, and havent put more than 2000mAh back into my 2200's
I do agree, I will start timing my packs, just need to figure out the best way to time it.
I dont want to use the stopwatch in the DX7, coz I turn the radio on way before the heli ever leaves the ground, and I dont like messing with the radio once the battery is plugged in.
The biggest problem with the canopy seems to be the wires come out of the end of the battery pack, the pack is only 100mm but with the wire sticking out there it has to do a sharp 90 to head back to the ESC.
this canopy is about done, so I might try and chop on it & modify it to see what I can come up with, I just dont like it squashing the wire on the battery so much & boogering the mount holes so bad.
Would be nice if I could get a bigger battery in there too. like a 2500 or even a 3000 would be sweet.
I dont do 3d, heck I barely do FF, mostly hovering and floating about and Id love a little more flight time.
ChasHeliCop
06-02-2008, 12:28 PM
For a 2200 mAh pack, you shouldn't be putting back any more than 1760 mAh. The timer on the DX7 is very simple. You plug in the pack, and right before you hit the throttle , you hit the timer button on the right of the Tx. The timer begins counting down, and beeps when you reach the time you set. I would recommend starting at 5:30 then measure how many mAh you put back in the pack and adjust your time by 30 seconds each time until you reached "close" to 80% capacity which is 1760 mAh. I don't know what kind of packs you are flying, but you can damage your packs and shorten the life of the pack if you over discharge it.
Aussie_G
06-02-2008, 12:53 PM
Cool, I will check out the timer & button etc.
I have only had the DX7 for a week - still learning it...
Im using TP extreme V2 packs, Im at work so I dont have my numbers here, but I think I have been putting 1850 to 1900 in them.
Ill have to adjust my flight times like you said.
JEB123
06-02-2008, 01:44 PM
For a 2200 mAh pack, you shouldn't be putting back any more than 1760 mAh. The timer on the DX7 is very simple. You plug in the pack, and right before you hit the throttle , you hit the timer button on the right of the Tx. The timer begins counting down, and beeps when you reach the time you set. I would recommend starting at 5:30 then measure how many mAh you put back in the pack and adjust your time by 30 seconds each time until you reached "close" to 80% capacity which is 1760 mAh. I don't know what kind of packs you are flying, but you can damage your packs and shorten the life of the pack if you over discharge it.
What happened to discharging packs to no less than 3.2V/cell? Aren't LiPo alarms based on this? Are you saying LiPo alarms aren't reliable?
What happened to discharging packs to no less than 3.2V/cell? Aren't LiPo alarms based on this? Are you saying LiPo alarms aren't reliable?
If they don't i will.
ChasHeliCop
06-02-2008, 02:14 PM
What happened to discharging packs to no less than 3.2V/cell? Aren't LiPo alarms based on this? Are you saying LiPo alarms aren't reliable?
If they don't i will.
LOL.... HFG, you knew I would, with an explaination. I am not saying Lipo alarms are not reliable, I am saying that people are utilizing Lipo Alarms and ESC cutoffs when they should not be. You should never get down far enough for your ESC low voltage cutout or the Lipo Alarm going off. These features are basically meant to keep you from going beyond the point of no return. It is not wise to rely on either of these sources for the limit on the duration of your flight. Unless of course you like replacing your lipos regularly.
JEB123
06-02-2008, 07:10 PM
Are you saying the voltage level of a battery has nothing to do with the power left in the battery?
There are alot of variables in the voltage equation, capacity left is only one of them.
JEB123
06-02-2008, 07:40 PM
That's correct and internal resistance is one of the most important, no? With all due respect, and please do correct me if I am wrong, but as I recall,(it's been a while) as a battery gets weaker, the internal resistance increases which increases the voltage drop across small r. That in turn means the voltage across the external terminals of a battery under load will get lower as the internal resistance increases and more voltage is dropped across it.. Power still equals E*I right? Why would voltage (under load of course) not be an accurate measurement of remaining power? Do LiPos work differently? They're still chemical cells, no?
Because the load on a motor isn't consistant in a heli (assuming we are past the hovering stage). TIming really is more accurate. Using my timer i get my packs within +-50mah everytime.
JEB123
06-02-2008, 08:07 PM
Okay, so let me get it straight. I want my packs to last too! You charge up to 80% of the batt capacity, and set the timer based on what now? BTW Chas, my LiPo alarm uses a resistive voltage divider that can be changed to set the alarm voltage wherever I want.
YOu charge to 100% then start with the timer low, say 3min. Then keep track of how many mah you put back when you charge. Adjust the time up, if its really low add 2min if its close add 30sec. Over time the pack mah will drop and you will start to feel the power dropping off near the end of the pack lift, just lose like 30sec off the timer.
JEB123
06-02-2008, 08:26 PM
So, you, what, write the flight times on each pack?
They are all around 2100mah so its a common timer.
JEB123
06-02-2008, 08:45 PM
So if you buy a new battery, you charge and discharge it at the same rate as old ones? Still seems to me your method is the same as mine except you're using the current to tell you when to stop discharging and I'm using the voltage. The power is constant. It's always constant. If you can only put back 1760mAh in your 2100 it's because the internal resistance of that battery pack has increased with age and is dropping a specific voltage which is subtracted from the external terminal voltage whether charging or discharging. Am I wrong or was George Simon Ohm wrong?
Capacity is in mah, you want to be putting back 80% of the mah not the voltage.
JEB123
06-02-2008, 10:31 PM
That's a reason, not a technical explanation. I'll grant you that timing may be more accurate, but here's a direct quote from a page the likes of which I attached. This is for lead-acid cells, but they're all the same. C'mon HFG, I don't want to throw out my LiPo alarm, it's easier than timing flights! Note the last sentence:
Every electrochemical cell has internal resistance. As current
moves through the cell, the cell’s voltage changes
because of this internal cell resistance. When the cell is
being recharged, current flow causes the cell’s voltage
to rise. The higher the recharging current the higher the
voltage rise. As the cell is discharged, the discharging
current causes the cell’s voltage to drop. The higher the
discharging current, the greater the battery’s battery
depression. This holds true for all electrochemical cells
regardless of type, size, or environment. While absolute
values vary widely between different acid and alkaline
technologies, the relationship between current flow and
cell voltage remains constant.
http://www.arttec.net/Solar_Mower/4_Electrical/Battery%20Charging.pdf
JEB123
06-02-2008, 10:53 PM
According to this article, I think the real question is does a LiPos internal resistance change with the state of charge? If so, that would make it possible to use voltage measurements to determine SOC. Anyone?
Okey heres my point. First lets look at my FDR over several flights.
http://i79.photobucket.com/albums/j159/hfguy/Latest.jpg?t=1212461064
The purple curves are my current draw. See the first flew flights of just test hovering, the current is nice and steady. The other flights are of me ripping it up, the current is all over the place. If we even assume the internal resistance is fixed (which she aint) then the voltage will be all over the place as we get loses due to the output resistance of the lipo. If you where to pick some voltage to trip the monitor or something along those lines where would you set it ?? If you set it to high it will get triggered the first time you try to pull some serious current, set it to low and you will damage the packs the first time you don;t fly as hard for a pack. Flightpower use to have a nice graph on their site that shows the Voltage vs mah draw for very various C discharge rating, none of the C curves 80% discharge voltage were anywhere near eachother. This means that your pack voltage depends out how quick you discharge it. Using a timer is a good way to average the pack's current draw based on how much you recharge it by. If you take it easy for a pack say just hovering the timer will go off and your pack won't be fully discharged but at least you wont damage it.
JEB123
06-02-2008, 11:24 PM
Still not convinced. Yes, the alarm does trigger when you draw enough I to pull down the E, but it turns off again when you lay off the juice. It only stays on when the E stays below the cutoff threshold meaning the pack can't deliver anymore current for the given load. If you discharge the pack faster, the voltage will drop faster. The voltage is ALWAYS proportional to the current. You will not have a lot of charge left with a low voltage across the cells or vice-versa. GEORGE SIMON OHM.
Alright ill agree to disagree.
JEB123
06-02-2008, 11:48 PM
Look, I'm not trying to prove anything, I just want to know the truth of the matter so I can do things the best way. I can see you're a sharp educated person and I REALLY value your opinion, that's why I grill you for answers, out of respect and no less. I'm a newb to helis but I'm an RC guy and an FCC licensed electronics tech for over 20 years, so I always have to know why. Sorry, don't mean to piss you off. I'm not challenging you in any way. Now why did Chas bail out? Maybe he or someone else has some more input! We should get to the bottom of this issue cause I'm gonna sue the people that sold me that alarm.
Hey fun, learning, friendship and mutual respect. This is what it's all about. Let's get some sleep.