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Jan 28, 2020, 03:35 PM
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Quote:
Originally Posted by Woga65
4s2p Murata/Sony US18650VTC6 would be 6000 mAh @ 400 g.
But that is the nominal capacity. I have a couple of those 2 years old, 30 cycles. At 10A load they are only good for 2000mAh down tp 3.1V. Maybe in parallel at 5A per cell it gets significally better?
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Jan 28, 2020, 07:35 PM
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The 4s2p VTC6 pack is what I have been using on my small quad for over a year. 6000ma nominal with a 30A constant discharge rating although I have never used that high in normal flight. It has a 2v cutoff that they measured their curve against so you would need to run it down lower than 3.1v to get close their specs. The best liion battery for energy density is 18650GA or MJ1 @10A discharge. I will have to take another look at battery offerings as I think they make a 25A 21700 now.

update: just took a look at a couple of suppliers I have used and they now have the 21700 VTC6A cell rated 4000ma 40A. That should suffice for most small quads when configured as a 4s1p pack and weigh ~300g.
Last edited by pug398; Jan 28, 2020 at 08:41 PM.
Jan 29, 2020, 04:24 AM
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Originally Posted by pug398
...update: just took a look at a couple of suppliers I have used and they now have the 21700 VTC6A cell rated 4000ma 40A. That should suffice for most small quads when configured as a 4s1p pack and weigh ~300g.
ah, good catch. I had been looking for the VTC6A as a 18650 cell. Yes, a good choice among the 21700 LiIon cells, about equal to Samsung 40T.

My problem with the LiIons is for me they do not hold up to what the test data suggest https://lygte-info.dk/review/batteri...ack)%20UK.html Same is true for older VTC6 18650 cells. To be fair, my cells are not fresh anymore, so I might get some new ones and repeat.

I do compare the usable Wh at 10A draw
  1. lipos with 80% rule for usable capacity
  2. LiIons down to 3.1V under load (where typically the voltage cuve under load starts to drop

With all the progress of Graphene in LiPo tech you would expect to see some choice of quality leightweight true 5C/10C lipos, maybe even HV. But to my knowledge there is none?
Jan 29, 2020, 06:55 AM
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Mine always made their rated capacity of 3000 mAh with warning voltage at 3.0 V and cutoff at 2.5 V. My continuous current is usually around 5 A to 6 A.

According to the specs you wrote above a 2p pack should be fine. There are also the VTC5a which are 35 A max without the need of a temperature cutoff. The VTC5a are rated 2500 mAh.
Jan 29, 2020, 07:21 AM
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Quote:
Originally Posted by Woga65
Mine always made their rated capacity of 3000 mAh with warning voltage at 3.0 V and cutoff at 2.5 V. My continuous current is usually around 5 A to 6 A.

According to the specs you wrote above a 2p pack should be fine. There are also the VTC5a which are 35 A max without the need of a temperature cutoff. The VTC5a are rated 2500 mAh.
with VTC6 a 4S2P pack would be 400g - quite over my 300g target.
According to this graph https://lygte-info.dk/pic/Batteries2...pacityTime.png
there is not much time to gain @10A from 3.1V down to 2.8V, but it does make a difference for your 5A load.

You are both right those LiIons do shine with loads in the 5A range.
To me this comes with deminishing returns @10A
Jan 29, 2020, 08:49 AM
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Quote:
Originally Posted by hamburger
with VTC6 a 4S2P pack would be 400g - quite over my 300g target.
According to this graph https://lygte-info.dk/pic/Batteries2...pacityTime.png
there is not much time to gain @10A from 3.1V down to 2.8V, but it does make a difference for your 5A load.

You are both right those LiIons do shine with loads in the 5A range.
To me this comes with deminishing returns @10A
In the very first post of this thread you stated 400 g target mass, thats why the li-ions came to my mind.
Jan 29, 2020, 02:06 PM
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I interpreted from your initial posts that you had tried the VTC6 in a 4s1p configuration. There are very few 18650 cells (at least 3000ma) that would work in that configuration without shortening their life. As you have noted they have a low C rating. Only recently have larger cells become available that have the capacity and the constant discharge capability to be used in a 4s1p configuration. The Samsung 40T is tried and true and meets its discharge specs and also the VTC6A although it has not been tested as much as the 40T. Packs of either type should be closer to your weight spec. Use the non protected discharge current spec and if your max draw falls within that limit then you shouldn't lose too much capacity or shorten cycle life.

Ok just ordered the 21700 VTC6A cells to make up a 4s1p pack. Curious enough to give it a try and it is cold outdoors so need to do something inside my workshop
Last edited by pug398; Jan 29, 2020 at 02:21 PM.
Feb 08, 2020, 12:40 AM
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21700 4200mah 30a are very interesting cells.
Feb 08, 2020, 05:38 AM
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I am waiting for 20700 sanyo 2070c
Feb 08, 2020, 09:15 PM
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Just got the 4s VTC6A pack made up. With power leads and charge tap it weighs 313g. I will get some graphs of the load power shortly. Test quad needs a firmware upgrade with newest PX4 to get rid of "mag inconsistency" nag.
Feb 10, 2020, 08:24 PM
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Preliminary first test show these cells put out some serious current peaking over 60A. It also shows that much load could trigger RTH depending on what cell voltage you have the low voltage limit set at. This one is set at 2.8V so it didn't trigger RTH on me. I knew with the props that were on it a hover would draw ~10A and max throttle would take it up there a ways. Didn't know it would peak above 60A though lol. Next I will check capacity at 10A draw.

adding: test drone 480mm xquad, 1.2kg AUW
Last edited by pug398; Feb 11, 2020 at 10:41 AM.
Feb 12, 2020, 06:03 AM
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Quote:
Originally Posted by pug398
Preliminary first test show these cells put out some serious current peaking over 60A.
Can you take an IR measurement on the cells?
My VC6 cells get really warm under a 15A continuous load. Also, capacity @10A is down from 2600mAh to lowly 2100mAh with LVC=3.1V.
(there is not much to gain from running lower voltage, I use that as safety margin)
Feb 12, 2020, 08:59 AM
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I will get IR measurement for you. I am not sure we are comparing the same cells.
You are correct about lower voltage regarding these cells as they have a 2.5v cutoff. Previous battery pack had 2.0v cutoff and it did make a slight difference in flight time.
Feb 12, 2020, 04:20 PM
Registered User
IR = 19.5 mohms average. Keep in mind these are US21700VTC6A 3.6v 30A continuous 40A w/PCM rated cell. They are not originally released VTC6 cell.
Feb 16, 2020, 03:37 PM
Registered User
Added a 400g dummy payload to the quad to bring the AUW up to 1.6kg. This allowed a 14-15A load while in a 10m hover. Set the battery monitor to 3.1v. This is as close as I could get to your test conditions. The quad hovered for 12 minutes before sounding the voltage warning. Upon landing the battery voltage was back up to 3.4v so this confirms the 0.02 IR since 15A x 0.02R = 0.3v which is same as 3.4v - 3.1v. Battery is charging now to see how much goes back in. Personally I feel 3.1v LVP is too high of setting for my setup and would lower it slightly as long as I can safely land with 2.5v left in each cell. With the quad pulling 15A at the end of 12 minute session pack was only slightly warm and of no concern to me.

update: 3037ma back into pack with low voltage alarm set to 3.1v and 15A current draw.
Last edited by pug398; Feb 16, 2020 at 07:05 PM.


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