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What is happening with my Lipo?

Numpty needs advice

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Erfolg15/10/2011 12:27:58
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I have a Rhino 1550mah 3s Lipo, 25C. After changing the motor in my Sagitta, to a inrunner which pulls 275w, it became apparent that there was an issue. The model would barely climb, or not at all.
 
I decided that the battery needed changing, which affected a cure, now truly vertical climb.
 
Like most modellers, i did not merely throw it (the Lipo) away, I recharged. It charged well going to 4.2v a cell. Measuring the cells with a checker they appear to be essentially balanced. All was well I thought, probably good enough to use on a light small model.
 
Having just completed such a model I decided to use the Lipo on this model. The motor type I have used many times previously a Max CF2822, on a 10*4 which I knew would pull a max of 150W.
 
At the field, the motor seemed to lack power, but I launched the model climbed slowly away (surprisingly retrospectively). But the performance was not awe inspiring, more ahem, need to be careful.
 
Back at home, on the watt meter, I found it was pulling initially 34W, dropping to 28W. I was now surprised the model flew. Checking the individual V, each was apparently about 4.15v. Yet I was not getting the amps. I have not taken any detailed measurements, as I do not know what to look for.
 
So how do you find out what is wrong. Assuming it is a cell issue, how do you detect that the pack will not give the ampage, which cell is failing (if that is the issue). All I know, I appear to have volts but no amps.


Oh, confession time, yet again, testing the set up with another Lipo, I found that the motor is pulling 180W, at 5000revs. not the value assumed or according to the data sheet.. Taking things for granted, is a failing I must get out off. I have previously used 3 of these motors, all as per data sheet. Repeat, must check set up before first flight !

Edited By Erfolg on 15/10/2011 12:29:29

Edited By Erfolg on 15/10/2011 12:33:03

Edited By Erfolg on 15/10/2011 12:34:06

Wingman15/10/2011 13:10:41
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Yep Erfolg -a wattmeter everytime - it really is essential. Found this test data on a 2822 which shows a best prop as 7x4.
John Bunting20/11/2011 13:57:50
105 forum posts
Hi Erfolg; I've only just now seen this.
I wonder if the internal resistance of the cells has gone up. That can happen with NiCad and NiMH cells, as they get older: I'm not sure about LiPos. For a 3s pack, the current would be only about 3 amps at your wattmeter reading of 28 to 34 watts. Did you check the cell voltages while on load?
Tony Fry20/11/2011 14:34:42
42 forum posts
I would have a look at your soldered connections too, big volts drop there if they are dry.
Martin Harris20/11/2011 16:07:48
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Have you considered the battery's temperature? At the field it may have been quite cool at this time of year and it can make a considerable difference to the power that's available - especially as the cells age.
Erfolg21/11/2011 13:15:57
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Martin
 
The battery was goosed.
 
Putting the battery under load caused the amps to drop, though the volts dropped much less.
 
Not being electrical. I am guessing that some aspect of the Lipo was reducing the amps to less than the rated 20C.
Martin Harris21/11/2011 14:40:42
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Every year around this time, we have people expressing surprise at how poorly their LiPos are performing. On Saturday, a clubmate's (admittedly not normally overpowered) Storch failed to climb away and dropped a wing as he tried to gain height and I suspect that the battery temperature was the critical factor.
 
I've noticed this trait in some of my own models in cold weather - even to the point where a bit of vigorous power application to warm the pack will "restore" missing vertical performance after some low throttle activity has allowed the cold to creep into a pack in flight.
 
A friend who takes electric flight far more seriously than me has a purpose built pack heater to ensure full performance from his cells - especially for F5b - but he uses it for his sport models in cold weather. 

Edited By Martin Harris on 21/11/2011 14:49:40

Erfolg21/11/2011 15:41:16
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Martin
 
In my case i do suspect the cold.
 
It is possible, that the battery was damaged earlier in the year.
 
I had a inrunner motor burn out, this probably drew a very high current during the seconds that the motor failed. The current seemed to be high enough to shut down the ESC, for for a few seconds, that lasted possibly about 2 to 5 secs. Enough to invert the model, into vertical dive, for a hundred or so feet. I had given up on the model and it then suddenly took care of itself, from which I managed to land in the field.
 
The model was re-motored, with a 175 w inrunner, which was lac lustre, so I almost immediately re-motored with a 250w motor.
 
Intially the performance was OK, but started to run out of power, ever more quickly, until it could hardly limp to a hundred feet, or so.
 
I replaced the battery, full power again, and a return to a vertical climb.
 
It does seem that resolving these type of issues could be the forte of Telemetry and data download systems.
Steve Hargreaves - Moderator21/11/2011 15:58:21
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Posted by Erfolg on 21/11/2011 13:15:57:
 
Putting the battery under load caused the amps to drop, though the volts dropped much less.
 
Slightly odd Erfolg.....the volts are the "pump" that push the amps around.....if the volts stay reasonably constant then the amps should too....unless the resistance of the circuit has increased in some way.
 
In a given system the volts would need to drop so that fewer amps were "pumped" around the circuit....
 
In very cold weather the chemical reaction of a battery usually slows down which reduces the voltage....this in turn reduces the battery's ability to push amps through the circuit.....his combination results in far less power (watts) since Power equals volts x amps....
Cyclicscooby21/11/2011 16:27:27
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Everyone sounds right, BUT...
 
The mate who 'Warms' his packs, shud not be replicated, unless you are fully aware of what your doing.. !!!
 
Thermal run away is what causes Lipos to catch fire, so be VERY careful..
 
You could try putting it on the cars dash (Just on top, not over a vent), with the heater on for a bit, before flying, BUT keep an eye on it..
 
 
Luv
Chrisie.. xx

Edited By Cyclicscooby on 21/11/2011 16:28:40

Erfolg21/11/2011 16:40:34
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I do not pretend to know much with respect to electricity.
 
I am reasonably sure that I could get the energy out over a period. What I could not get was the energy out quickly.
 
Initially I relegated the Lipo to a low watt application. Initially it was OK, in that i could get prolonged flights. However with time, this has also become unacceptable. So i purchased a new battery for this application also.
 
Perhaps strangely the voltage did not sag that much (circa 10v), but delivered less amps.
 
I do not know or pretend to know, or what to look for. I just know that volts do not seem to be everything, as the watts and amps dropped, yet volts seemed pretty much the same.
 
I know it seems a little selfish, in that at present I do not care, as my new batteries are delivering
 
In the longer term, I would be interested in a testing regime, which enables me to identify a failing Lipo. I would probably be interested in knowing what is going on. Though I am more interested in identifying a failing Lipo.
Steve Hargreaves - Moderator21/11/2011 16:45:37
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Seems odd.....10V is about what you would expect from a 3S under full load but it only seems to be supplying a fraction of the current it used to......the only way this can be possible is if the resistance of the circuit has changed (increased).....
 
Be interesting to see what happens when you fit the new battery.....
Erfolg21/11/2011 17:03:16
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I am not sure that we are not out of phase with our postings
 
With new batteries both models are fine now. My Sagitta has once again resumed its vertical climb. My sport type profile model, flies at full power for a considerable period.
 
Perhaps a bit of useful information, is that the battery did improve a little with running, after perhaps 2-3 minutes. Anyway it has gone to the tip now.
 
It doesseem  a data logging system would have and could provide electric fliers with useful info.

Edited By Erfolg on 21/11/2011 17:03:43

Edited By Erfolg on 21/11/2011 17:04:07

Edited By Erfolg on 21/11/2011 17:04:44

Tim Mackey21/11/2011 17:25:14
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As stated earlier by some, the cold affects them A LOT.
There's nothing wrong with hot lipos, Thunder power for instance reckon they are fine up to around 140 -160 degrees F.
They only start working really well when they have warmed up, either through external means or internal current flowing.
I always warm mine on the journey to the field, and keep them insulated right up to the point I fit them-----and then fly within a couple of minutes of that at most.
 
John Bunting21/11/2011 18:09:35
105 forum posts
If, as Steve suggests, the circuit resistance has changed in some way, how about the ESC? It might have been interesting to monitor the full-throttle load voltage at the motor terminals, as well as at the battery itself.
Steve W-O21/11/2011 18:32:09
2775 forum posts
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Easiest way to get a quick test result is to put one of those multicell led indicators on the balance cable. Probably find one cell is going low under load.
 
Those cheap indicators are quite useful, even when the plane is quite far away you can hear the beeping if the battery is not performing as it should, and land in time before it stops.
Dave Bran21/11/2011 18:35:03
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Imagine shorting a fully charged LiPo with large cables, then just before you disappear in the flames, surely what determines the maximum current in the circuit is the voltage and the internal resistance of the cells?
 
Chemically if you alter the cell contents through deterioration in cyclical use over time or more catastrophically damaging it through overload and mistreatment, you will alter the internal resistance, and so the cell, while it may still show a similar charged voltage to a good cell, will be unable to deliver the same current it once did?
 
Temperature affects the internal resistance, hence current able to be extracted?
 
Most LiPo's I have had "fail" do exactly as described, they seem OK on pack voltage, but no longer deliver the same current they once did.
 
More recently with the newer ultra low internal resistance cells, failures I have had have swung towards a cell "shorting" such that a 3S becomes 2S, and 99.9% of the time it's cell one that "fails".
Erfolg21/11/2011 18:56:46
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With respect to my Lipo, it is all water under the bridge.
 
I now would benefit with a testing regime for the future.

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