Unfortunately, the NCR18650BF won’t supply enough current for the V2450GPS
At 10amps max continuous, the 18650GA could, though.
I’ve played with strapping two stock batteries. At 54g, it flies and I get about 3 minutes more, but it felt like pushing it and not really usable with frequent max throttle towards the end. With 3 batteries at 80g or so the Viper couldn’t really lift off and was just hopping < 6" from the ground.
I suspect the brushed motors just max out pretty fast power wise. At 47g the 18650GA might work ok though.
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Shoulld have added … I am at mile high/1600m altitude, so thinner air. Does make a difference, 10-15% or so less lift compared to sea level… Mileages do vary 
Having big fun with SkyViper. I was lucky to be able to pick up during US trip early this month.
Trying to find better battery option.
-
Standard battery 27g
8-9 minutes flight tine by setting FS_BATT_VOLTAGE to 3.3 or so. default 3.42 cut off too early around 5-6 minutes. -
Tried several 18650 cells. 48g each with connector.
Samsung INR18650-30Q 3000mAh max 20A discharge
Sanyo NCR18650GA 3.6V 3500mAh max 10A
Set FS_BATT_VOLTAGE to lower than 3V such as 2.9. Get similar flight time for 13-14 minutes, consumes 1600 - 1700mAh only.
Sky Viper wants to land at around 3V even if I set FS_BATT_VOLTAGE lower such as 2.9V. Why? Does it have issue with lower voltage than 3V?? -
1300mAh 45C Lipo cell disassembled from used pack. 34g
Can fly 10+ minutes and lighter than 18650, no need to change FS_BATT_VOLTAGE. I prefer this larger lipo cell option. Soldering is a challenge, need special solder for aluminum.
Satoru
I purchased single samples of a few types of cells from IMR and have been experimenting as well. Here is what I see so far.
First, I removed a small amount of weight, perhaps 4.5 grams, from my second SV in the form of the battery holder (See my battery holder post) and the top plastic shroud (no clips, just 4 screws after you remove the bottom cover and the camera board). The prop guards would add about 10 grams if I ever used them so I figure I should have about 15 grams headroom without exceeding stock weight. This thing seems to fly pretty close to it’s weight limit to begin with so every gram counts. I’ve also noticed with the heavier cells, it can’t maintain hover below about 3.1 volts. This is not just because of the internal protection in the stock battery. I haven’t yet checked the logs, but I suspect if you do you will see that the highest output motor in this condition is at almost 100%. I also set FS_BATT_VOLTAGE to 3.0 for my tests. Voltages were measured via wifi/mavlink telemetry. My test hover is performed outside and sometimes includes some wind so results do vary. I also do a short rapid climb every minute or so to get a feel for how much headroom we have on motor power and to make times more realistic than a static hover. The extra current draw from the climb is sometimes enough to drop the voltage/power enough to cause the end of the hover. Reported weight includes JST connector and wire. I solder directly to positive terminal and side of battery case (as quickly as I can to avoid heat spread). I suspect such abuse is frowned on but I have no way to weld it and I bought cells without tabs. My conclusions and measurements are below:
Stock battery, rated 1200 mAh: 27g, Based on testing I also find that about 9 minutes is maximum safe flight time, flys very well with fairly comfortable power:
Reduced weight SV flew for 10:12 before test hover could no longer be maintained. The voltage was about 3.0v immediately before landing but bounced back to about 3.4v just a few minutes after landing. I put 1123 mAh back into it afterwards. No significant swelling of the cell, but I suspect I’ve reduced it’s working life. No cell should be expected to run this close to it’s rated capacity for many cycles. I won’t be repeating this test on a stock cell.
Sanyo NCR18650GA 3500mAh 10A: 48g, No significant gain in flight time over stock. Flew OK but feels sluggish and climbs a bit slowly:
Voltage drop due low current rating is limiting factor.
LG HD2 18650 2000mAh 25A Battery: 45g, Provides 12-13 minute flight times. Flew fairly well, perhaps a bit sluggish:
Hovered until 3.0v, 13:45 flight time required 1515 mAh charge. Second test hovered 12:53, required 1480 mAh. Capacity/voltage drop were limiting factors.
Sony VTC6 18650 3000mAh 15A Battery: 49g, Provides about 15 min flight. Feels sluggish, climbs slowly:
Hovered until 3.1v, 15:45 flight time, required 1770 mah. Current draw/voltage drop/weight were all limiting factors.
EFAN 20700 4300mAh Battery: 63g, Rated 15A. Could barely get off the ground and flew for only a few seconds:
Just too heavy and not enough current!
Sanyo NCR20650A 3100mAh 30A Battery: 53g, Flew for perhaps 3 minutes. Didn’t bother recording the details:
Too heavy.
It looks like 45g might be about the sweet spot. At this weight, we can lighten things so we are only about 3 grams over the weight the stock drone flys indoors with the prop guards without removing parts.
At this weight, it looks like we need a cell that can supply 8 amps all the way down to 3 volts. The power which can be supplied at 7-8 amps before we reach this voltage will determine the maximum flight time.
I also have a 30c 2000 mAh 2S lipo pack on order which should come in at about 45g after I separate the cells. (Why is it so difficult to find high current 1s packs?) I’m hopeful this may provide a reliable 15 minute flight with some headroom.
I may try some more Li-Ion cells as well if I can find some more on the light side with high enough current. I’m guessing they may be less prone to damage through high drain/low voltage.
–Brad
@brad112358 and @Satoru_Sasaki, great info. Thanks for taking the time to test and post the info.
~16min on the VTC6 is actually pretty impressive.
According to Viper Boards Pictures there are 18 screws holding the SV together. Any idea how much those weigh? I’m wondering if swapping screws for some tape / glue would gain anything (other than mid-air disintegration, that is 
)
Ditching the LED’s would be an option too.
IOW the stock battery that fit’s in the quad nicely and gives 9min of flight time is about right? I concluded this after trying the Sanyo NCR18650B battery.
Good stuff and info!
Satoru_Sasaki, (and brad112358) strange you can only consume about 1/2 capacity. According to Panasonic specs for a LI-ION 18650 GA, for instance, voltage at 1/2 consumed capacity should be in the 3.3v range (at 10amps discharge). Notably lower than comparable LIPO at same discharge btw. I know those curves are to be taken with a grain of salt, but still, you should be able to get a solid 3000mAH out of a 18650GA,
I wonder if hardware battery cut-off (Tridge mentioned there was such a mechanism to prevent over discharge?) can explain some of this? And answer your question as to why Viper is landing early irrespective of what you set voltage failsafe at?
But then again you mention landing at 3V and not 3.3V or higher, so this would not fully explain it.
Hmmm …
The other culprit has got to be the added weight, shooting up current consumption and thus lowering that red voltage curve way too much.
Thank you @brad112358 for sharing your experience, Great effort to reduce weight! Geared brushed motor is efficient, as you might know there are great 52 min flight record with 98g AUW mini quad. Regret I should picked up more SV for mod…
Yes, Sanyo NCR18650GA discharging voltage was lower than Samsung INR18650-30Q. Sony VTC result is great. I am not familiar with these LiIon cells, should learn more…
@OlivierB, yes, that is a mystery why SV is cutting off at 3V, consume only half of the capacity. I checked log again, no error message but found ESC output C2, C3 is becoming max for this flight. Probably battery location is not balanced well for this flight. I need to check other logs more carefully before concluding.
Satoru
I don’t think failing to maintain altitude at 3 volts is any mystery. As your graph shows (and as I had guessed in an earlier post), one of the motor outputs reaches 100% in hover at this voltage indicating power is insufficient to maintain hover at this voltage/weight. Note that available power is roughly proportional to the square of the voltage so it drops more quickly than you might think between 4 and 3 volts.
BTW, I did receive the 2000 mAh LiPo pack I ordered last night and got one flight in this morning, so I have one more data point to add to my previous tests:
Venom Fly 30C 2S 2000mAh (reworked cell): 41.5g, Provides about 14 minutes flight, Flies pretty well:
In windy conditions, I lost hover after a quick climb at 3.0 volts, 14:25 flight time. It required 1680 mAh to charge. The limiting factor here seems to be internal resistance. I don’t believe the capacity or 30c claims the manufacture makes, but the result here is still pretty good.
Note, dividing a commercial multi-cell pack into 1S configuration is not a task to be undertaken lightly. Much care must be taken not to puncture the wrap, or short or overheat the tabs while separating and then soldering new leads. Use a heat-sink on the cell end of the tabs when soldering. Also, take advantage of the existing solder because soldering on bare aluminum tabs requires special procedures/materials. I made the mistake of separating the tabs joining the two cells at the edge of the soldered area and had a very difficult time getting solder to stick to the tab that was left bare. I think it would be much better to cut the soldered region in half, leaving some soldered area on the tabs of both cells and making soldering the new JST battery lead much easier.
Overall, I think this is my best outcome so far as the flight time is almost as long and the SV flies much better at this weight than it did with the heavier, 49g Sony VTC6 even though the Sony flew longer. All-up weight after removing the plastic I mentioned earlier is about the same as flying stock with the prop guards installed.
I do have a few more samples of 18650 cells on order, all at 45g or less this time. I will post again when I have more test results to report.
–Brad
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Something I should have mentioned earlier:
One big caution for anyone else tempted to experiment with these Li-Ion cells. The metal can the cell is sealed in is ferromagnetic which means, as opposed to LiPo cells, even when no current is being drawn, they can interfere with the operation of the compass which is critical to correct operation in loiter mode. Some cells are worse than others. I have been using a powerful degausser designed for bulk erasure of magnetic tape with mixed results. Be sure to check for accurate compass operation before using GPS based modes.
–Brad
Brad,
So the 14min is on the SV with your weight reductions, correct? Any sense of what sort of flight time we might get with an unmodified SV with this battery config?
I did some more testing today:
I added 9g of mass in the form of the prop guards and flew outside. This brings the total weight to 4 grams more than the stock drone would be with out the prop guards and without my weight reductions. There was less wind and the 41.5 g 2000mAh LiPo flew for 14:20 before it could no longer hover. 1677 mAh were required to charge.
Then I flew with just two of the prop guards. This simulates stock weight. The 2000mAh LiPo flew for 15:01 and required 1690 mAh to charge.
Then I flew with no prop guards. This is 5g less than stock weight, the same as my earlier post. It flew for 15:20 and required 1730 mAh to charge. Note, this was better than my first test on Thursday, perhaps because of the lighter wind.
The voltage drops to 3.0 volts just before it looses the ability to hover, but it rebounds to 3.6 volts after resting a few minutes. It seems cell damage is not likely even when this battery is flown in the SV until it can’t maintain altitude. I think setting the fail-safe voltage at about 3.2 volts should work well for this configuration.
So, to answer your question, the 5 gram weight reduction I did does help, but you can expect about 14 minutes of flight time with this battery, even with the stock weight (without prop guards).
I also flew again with the Sony VTC6 with 2 prop guards (to bring it up to stock weight), but I got significantly less flight time than I saw the first day I tested this cell. I only got 12:30 flight time and the cell only accepted 1522 mAh charge afterwards. It seems the extra weight caused the voltage to drop much faster or perhaps the internal resistance of the cell has increased due to wear, or something else went wrong. In general, this cell is too heavy for decent performance anyway so I will not be using it for further testing.
EDIT: A later flight with the Sony cell seemed to land itself at about 7.5 minutes. But I noticed the voltage was still high enough that it should have been able to fly. I took off again and it flew for another 7-8 minutes. It seems the “something else went wrong” hypothesis above was the correct one. I’m not sure why it descended. Possibly GPS issues I was having caused the altitude estimate to be off. I still think this cell is too heavy to fly well so I don’t recommend it.
Still waiting for more Li-Ion samples to arrive.
–Brad
I have a few 3.7v 700 mAh batteries. Would it be possible to wire them in parallel to get 3.4v 1400 mAh for extended flights
That would definitely work.
You might get more flight time, depends on weight, but it’s tricky. There a sweet spot somewhere, above which more mah doesn’t give more flight time because of weight and motor properties. 1400mah instead of 1200 will be below this though assuming same specific energy, although you might only get a minute more or so. Three of them may also work, the Viper can easily fly with 1.5 times the weight of the stock battery.
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Not really understanding batterys that well , i do know its the mah that more or less dictates to flight time. So what about using two stock 3.7v 1200 mah batteries and wire them in parallel, Would this not up the flight time?
The increased mah has to be offset with the increased weight. The relationship is not linear, and there is a peak after which adding batteries decreases flight time.
There’s actually a paper that was written a few years ago showing that theoretically you cannot increase flight time when weight of battery exceeds twice the weight of copter without battery. (Using plain physics, potential energy, etc …). In practice optimal flight time deviates from theoretical of course, sometimes widely, depends on motors, props, etc … bit in some cases (not the Viper) I’ve seen it as a pretty good rule of thumb.
In the case of the Viper doubling the batteries will get you more flight time, just not much, 2-3 more minutes I think, and control won’t be as good.
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Thanks, that makes things a little easier to understand
Blue skies, 75deg, 12mph wind. I decided to try some battery options. I rigged up a parrallel harness and combined 3 750mAh batteries. One battery in the batt holder and the other two zip tie4d on below. The combined voltage was at 4.08v. I hand launched because it would not sit on the pad level. It seemed to fly well but I am not into aggressive moves. Slow and stable is fine with me. I realized that I had forgotten to start the timer but it was too late now so I don’t have any comparrative numbers. I fooled around for a while, taking it up to 170’ in POS_HOLD mode and it held position although it had to pitch hard to windward.
Next up was a pair of 1200mAh batteries combined voltage 4.14v. and this time I was prepared with the timer. Hand launch again because of the belly load but it flew normally, very steady and I had no problem with handling and manueverability. I kept it in a low hover close by as I was going to try to get a photo of the belly mounted batttery and I was most interrested in flight time.
Suddenly it began to descend. I could not stop it with the throttle. It hit the ground at 1min 12sec of flight… I could get it to re-arm but it did not have the will to fly. Battery voltage was 4.02 by the time I got in the shop. I had no more time to work with it so I will get at it again tomorrow. The combined weight of the batteries was within a few grams of the 3 750mAh batts so I expected similar results. I did not expect it to give up after 1min or so.
Any thoughts?