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Flight Time Cut in Half!

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Cliff 195914/08/2017 20:26:15
371 forum posts
72 photos

My little KK Pixie uses the running gear from an old Ultrafly Outrage, the motor is a brushless B/06/10, the ESC is the one that came with the model.

I fly it on a 3S 1300 battery, on a mid-throttle setting this gives me around 22 minutes in the air.

Flying yesterday in perfect conditions and cruising around on half throttle I lost motor power and glided safely down, the BEC seemed to cut in, I'd only been in the air seven minutes.

I tried my second battery, which was also fully charged and got a similar result.

Today, having charged the batteries I plugged in my watt meter and got these readings, at half throttle it ran for approx. seven minutes, whilst drawing 2.5A at 26watts, watching the voltage it dropped to 9.0v when the BEC cut in, this is probably correct.

So my question is what could have changed since the last time I flew, why all of a sudden the truncated flight time?

Any ideas?

Biggles' Elder Brother - Moderator14/08/2017 20:43:19
15748 forum posts
1460 photos

Hi Cliff,

2.5A for 7 minutes is only about 300mAh - that's no where near the capacity of your 1300mAh battery! You certainly shouldn't be down to 9v at that point.

The only thing I can think of, off the top of my head - is a big increase in your battery's internal resistance? How old are the batteries and approximately how many cycles do you think they have done?


Frank Skilbeck14/08/2017 21:12:55
4576 forum posts
101 photos

One other thing to check is that you haven't accidentally set your charger to LiFe rather than LiPo, LiFe charge to a lower voltage.

Chris Bott - Moderator14/08/2017 21:32:40
6709 forum posts
1392 photos
1 articles
Being pedantic, just because I'm in that sort of mood. A BEC (Battery Eliminator Circuit) simply reduces the lipo voltage to a voltage suitable for Rx and servos. It never "cuts in".

The ESC also includes a LVC function, (Low Voltage Cutout) which is probably what's bringing your flights to an early end.

Now to be more helpful.
9v is possibly about the right LVC level for a 3S battery so it does sound like the batteries are at fault.
What voltage does the wattmeter read with a fully charged battery, just before and just after applying full throttle?
Simon Chaddock14/08/2017 22:04:18
5579 forum posts
2939 photos

Cltff 1959

Does you charger by any chance record the mAh put in on charge?

If it only shows 300(ish) going in then that is all you will get out which would confirm the batteries are un serviceable.

The other aspect is your figure of 26W from 2.5A (thats just a 2C discharge rate!) suggesting the battery voltage is only 10V. A good 1300mAh battery should be showing a at least 11V at such a low current.. .


Edited By Simon Chaddock on 14/08/2017 22:05:01

MattyB15/08/2017 10:56:09
2002 forum posts
31 photos

Run a discharge on the battery through your charger so you can see the effective capacity and plot the discharge curve (time against voltage). Also (if the charger is capable) measure the IR of the individual cells. These two tests should give you a categoric answer on whether it's a battery issue or not.

Edited By MattyB on 15/08/2017 10:57:04

Ray Dunn15/08/2017 12:52:49
33 forum posts
25 photos

Hi Cliff

Has the battery received a balanced charge lately?

But sudden drop does sound to me more like one cell is dying. If your charger has a display, then it can probably show the individual cell voltages and highlight one cell as having a low voltage at the start of charge.

Incredibly low current, to what I'm used to. it must be an indoor flier I would imagine.


Cliff 195920/08/2017 19:57:01
371 forum posts
72 photos

The batteries are 1300 Zippy's with less than 10 balanced charges each, I do leave them fully charged though and they were both puffed a bit.

I just tried a new battery and all is well again, nice long flight times, I deduce the batteries are knackered, perhaps leaving them fully charged is the reason?

Denis Watkins20/08/2017 20:15:27
4078 forum posts
77 photos

It can be a mystery Cliff

With all the care in the world, batteries can fail prematurely

I suspect sometimes that our " new "packs may already be some years old

Mike Blandford20/08/2017 20:32:13
575 forum posts
22 photos

My understanding is the "fully charged" state of a LiPo depends on temperature. I think it is that at a lower temperature, a cell becomes fully charged "sooner". What this means is that if you have a fully charged cell, then let the cell temperature drop, the cell becomes overcharged, possibly then causing damage to the cell.


PatMc20/08/2017 20:45:41
4297 forum posts
524 photos

Cliff, you mentioned in the OP running at mid throttle for around 22 minutes. Does this mean that you normaly fly until the LVC cuts the power or the power drops & you then land ?

Cliff 195920/08/2017 21:09:50
371 forum posts
72 photos
Posted by PatMc on 20/08/2017 20:45:41:

Cliff, you mentioned in the OP running at mid throttle for around 22 minutes. Does this mean that you normaly fly until the LVC cuts the power or the power drops & you then land ?

Hi PatMc, I can land after 22 minutes to leave about 30% in the battery., the motor at this throttle setting is drawing about 2.5amps if memory serves, it seems as spritely at this end of the flight as at the start.

This morning I flew for nine minutes with 70% left on the new battery.

kc21/08/2017 10:42:27
6215 forum posts
169 photos

If the same thing happened with 2 batteries then I would suspect the battery charger.

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