question about lipo cut-off voltage - Page 2 - RC Groups
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Apr 09, 2011, 12:39 PM
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DanT's Avatar
Tip,,,, it cost less to put a battery in the trash !!! By running it down to far .Then having your esc go into cutoff and having your helicopter stop flying and come down like a rock!!
Just use your timer , and check your voltage every now and then... Batteries age and will not keep the same flight times with different types of flying! And it's a lot easer not to have to rebuild your helicopter and buy parts...
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Apr 09, 2011, 03:10 PM
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monkeyhanger's Avatar
>Then having your esc go into cutoff and having your helicopter stop flying and come down like a rock!!

Most ESC's can be programmed to gradually reduce power or pulse and not just shut off all of a sudden.
Apr 10, 2011, 03:00 AM
Registered User
Quote:
Originally Posted by monkeyhanger
>Then having your esc go into cutoff and having your helicopter stop flying and come down like a rock!!

Most ESC's can be programmed to gradually reduce power or pulse and not just shut off all of a sudden.

Agreed, most escs will cut gently, giving you time to land safely. Hopefully you will not need to do a full on autorotation, so if set up properly your bird shouldnt come down like a rock.

after having completely destroyed (well nearly) my trex 450 yesterday I know how annoying it is (and expensive) but this had nothing to do with battery, and much more with me flying it somewhere I shouldnt have been and not having the space to do anything other than dump it into the ground (the other option was dumping it into a colleagues car)
Apr 10, 2011, 08:53 AM
Registered User
You can add this tool to the field box also. http://www.hobbyking.com/hobbyking/s...dProduct=10328.

This is what I and most of the guys at the field do. We break in the battery (easy flight) for about 3 to 4 flights. We then set the timer for 3:30 for 3D. We then check each cell individually and determine how much we pulled out. From there, we adjust flight time based the 80% rule.

So far, for Turnigy (Nano and Non-Zippy) 40 to 50C batteries, we get between 90 to 110 flights before the battery lose it capacity for 3D flying, still good for sport flying, etc. We still can fly 3D but does not have the "pop." For Voltz batteries, approximately the same. Thunder Power, the best of the bunch (but expensive). The Blue and Zippy are the worst.
Apr 10, 2011, 10:40 AM
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RC_Marine's Avatar
something interesting to add. I'm using a castle ICE 50 and I have the cutoff now set to 3.7, was flying yesterday and was in the middle of practicing tic tocks and the ESC started to reduce power because I must have reached less than 3.7v, however is started reducing to much power to quickly and I was barley able to bring it in safely, I was eventually giving it full stick and the heli was dropping fast!!! she bounced off the ground an broke one skid. moral of the story is that relying on the ESC to reduce power is not always a safe method. even on a high end ESC. I'll start using a timer.
Apr 10, 2011, 12:50 PM
Registered User

Getting rid of some myths


I thought it probably best at this point to try and get rid of a few myths with regards to Lipos.

The first thing to remember is that they have a finite life. Think of a lipo like you would do a car. The nicer you treat the car the longer it will last right?.. But it will never last forever, and the more you use it, the more worn out it will be.

If you drive your car fast all the time, always with your foot to the floor, it wont be long before it wears out. Drive it nice and steady and you will see lots of miles on the clock before you hit trouble.

So:

The faster you charge lipos, the less long they will last. There is no need to charge at 1C if you are not in a hurry. There is absolutely no need to charge at 2C or even 5C now with some of the lipos around. A lipo pack may be able to take charge at 5C but that doesnt mean that it is good for it, it just means it will not explode or catch fire (mostly).

Unlike your car where you just fill the tank up with petrol (gas) when it gets close to empty, a lipo has to 'work' when it is being charged. It is not passive in this arrangement, it is actively changing (a chemical rearrangement), so the harder you push it, the more likely it is to say no.


Regardless of the C value of the pack (20C, 30C 35C e.t.c.) the more amps you draw, the harder the pack has to work and the quicker it wears out. It does not necessarily correspond that a 40C pack would last more cycles than a 20C pack if you drew a constant 20C every time you used it. The C value is merely the safe discharge rate in terms of explosion/fire risk e.t.c and not a safe discharge rate in terms of pack longevity. Again thinking of the car, just because you have a bigger engine doesnt mean that thrashing it is going to do it any good.

Doing an unbalanced charge relys on the pack having all its cells reach full voltage on the charge at the same time. This assumes:


1) That each cell in the pack has been discharged by the same amount
2) That each cell will take charge at the same rate
3) That each cell in the pack has the same capacity

All of these are big assumptions. Dangerous ones too for your battery. Unlikely to be dangerous in terms of bang bang my house is on fire dangerous, but certainly can reduce the cycle life.

If the charger is looking for 4.20V on each cell, and you are charging a 3 cell (3S1P) pack, imagine what happens if you have a dud cell that wont charge past 4.15V, or a cell that takes longer to get there...

4.20 (cell1) 4.20 (cell2) 4.15 (cell3)

Cells 1 and 2 are charged, 3 is not, but the charger only sees the total voltage (12.55V) and so assumes there is more charging to do, so you can see where that is going.

Balanced charging is much better because the charger sees not pack voltage but cell voltage, and adjusts accordingly.

I now always do a balanced charge, 0.5C if I am home and charging after a days flying, and 1C if I am at the field and need to charge on the fly.

If you run out of packs at the field charging at 1C then you either need more chargers or more lipos. The economics are self explainatory;

You are likely to get half the cycles at 2C than you will at 0.5C out of a given pack. Your packs will last twice as long, so although the initial outlay is double, the payback is quadruple.
Apr 10, 2011, 01:06 PM
Registered User
[QUOTE=RC_Marine;17931196]something interesting to add. I'm using a castle ICE 50 and I have the cutoff now set to 3.7, was flying yesterday and was in the middle of practicing tic tocks and the ESC started to reduce power because I must have reached less than 3.7v[QUOTE]

The problem here is that (if you are using a 3 cell) then your voltage cut off is way too high.

I use 9V (3.0V per cell) on my telemetry as a cut off point (this is without ESC interference and using the spektrum telemetry). When I reach this point (hopefully I never do, although I have done) the transmitter beeps at me and vibrates to tell me not to be so stupid, so I do not have the issues with an ESC trying to stop me.

3.7 is way too high as a 3 cell lipo will go well below this under load on a normal discharge, and the tic-toc is of course a high current drain move.

If of course you are using 4 or more cells this will not apply in the same way.

The purpose of the ESC cut off is not really to save your lipo, its to save your model. If you look at the discharge curve, when it hits empty the voltage drops off very quickly, and you are likely to brown out your receiver and hence lose all control. A forced, low power and quick landing is of course preferable to this, even if your skids end up bent.
Last edited by jimmyhorns; Apr 10, 2011 at 01:32 PM.
Apr 10, 2011, 05:25 PM
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RC_Marine's Avatar
Last time I had it set for 3.4 it allowed me to drain 2233 on a 2200 pack
Apr 11, 2011, 01:09 AM
Registered User
Quote:
Originally Posted by RC_Marine
Last time I had it set for 3.4 it allowed me to drain 2233 on a 2200 pack
well it allowed you to put back 2233 which isnt quite the same thing, but I agree its not ideal.

The alarm I use is to stop brownouts, I use a timer for flight, for exactly the reasons you mention about having the esc cut power.

Have a look at the curve on here:



See how quickly it drops off? it doesnt really matter where you set your voltage cut off if it on the steep downward curve, if you want to stop brownouts. The difference is a few seconds flight.

The problem is that under load the voltage will drop to close to this downward curve, so if the esc is set too high it will cut in during normal flight. The ESC doesnt see current drain, or total energy used, just voltage, which varies depending on load, as well as the amount you have drained from the pack.

Trying to use it to time your flights is a pointless exercise however as myself and other posters have elluded to
Apr 11, 2011, 05:30 PM
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RC_Marine's Avatar
I think I might set it for 3.5 or 3.6 as a backup and use a timmer
Apr 12, 2011, 09:17 AM
Registered User
Quote:
Originally Posted by RC_Marine
I think I might set it for 3.5 or 3.6 as a backup and use a timmer
I still think that is too high for a 'voltage under load' setting, which is what the esc 'sees'

On a 3 cell 2200mAh pack (and a good quality one at that) I will routinely have a voltage under load of around 10.3V which is 3.4V per cell, and still have 30% pack capacity at the end of a flight. occasionally I get voltages under load of below 10V during 'normal' flight and still have more than 20% left in the pack at the end.

The ESC doesnt know what you are doing when you are hitting the low cut off, it doesnt know if you are flying gently, or using lots of current, all it knows is that the cut off has been reached.

3.5V at 50% throttle is probably an appropriate cut off, but at 80% throttle it probably isnt. The cutoff in the ESC doesnt change with throttle position however so you have to be generous when you set it (i.e. lowest acceptable voltage at 100% throttle) so that it doesnt cut before time.

Hope that makes sense, voltage under load isnt the easiest thing to get your head round


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