Hello, I am completely new to this and just finished a hexacopter build. Let me first explain my issue. After calibrating my ESC's and arming, I was able to get my hexa in the air in Stable Flight Mode. Now the issue is that after in the air for only a few seconds the hexacopter seems to loose power and slowly looses altitude. Now let me tell you about the battery I am using. I went with a LiFEPO4, and after doing a lot of research I ordered a 12.8V 6.4Ah, 81Whr with a max continuous discharge rate of 24A. I have 6, 850Kv motors with 20A ESC's using APM 2.0. I have searched so many forums to try and find a solution, but to no avail. This battery is not a common type to use, it is a Lithium Iron Phosphate and has a lot of pro's and con's.
I have heard of people setting the battery type to NIMH, in some setting somewhere, but I have no idea how to do this. The copter is completely stable in the air and I have even changed the PID settings to make the throttle more responsive but after a closer observation, I can clearly hear and see the props slowing down after a 10 second hover causing it to land hard. It seems as though I am losing power after a short discharge. Another strange thing is that once I land and I power down, I can throttle up right back into the air with no problem at all and hover for a few seconds, and I can keep doing this for at least 15 mins before my battery starts dying. My questions are: do I have to calibrate my ESC to the LiFEPO4 battery I am using? Is the battery I have powerful enough for a prolonged flight? I spent a lot of money on these batteries trying to get a long flight time, I did a lot of research and made sure to get a battery that was compatible with my demands. PLEASE HELP, I am completely stuck and it sucks only being able to fly for a few seconds at a time. I am so close, I just need a little help. I literally signed up on this forum because I am lost at this point.
Replies
Hi Seth,
Depending on the weight of your setup, the motors will draw much more than the 24A .
I've been working on flight times, etc, as well and have been doing some serious math. I initially built a jDrones hexacopter, then an Octacopter and now a OctaQuad x8. I'm using Lipo, not LiFEPO4. However, the basics remain the same.
Only recently I purchased a Attopilot 90A board, so I could also measure Amps going thru the batteries. The good thing is that my math matched the results thru the Attopilot board. Approx 36A in hover on an OctaQuad. I suspect your hexa may be close or well over the 24A.
I strongly suggest to spent money on a Lipo that is at least 35C and see what happens.
I have used a couple of HobbyKings NanoTech turnigy 4s, 35-70C, 5000mAh and they puffed! I have also used their Zippy 4s, 40C, 5000mAh, with great results!
Thanks
Alexander
Sounds like your batteries just can't hold their voltage under the load. LiFe drop their voltage dramatically at the beginning of the discharge, then hold it quite steady and then drop quickly at the end.
The best LiFe batteries for RC are the branded A123 type, when they first came out the easiest way to get hold of them was to buy a DeWalt Tools spare battery pack, cut it open and solder up your own packs. I had 4 x 4 cell (equiv to 3 cell LiPo). I had more than 400 flights each before capacity started dropping! BUT, they are VERY heavy for their capacity so not suited for 'copters and are expensive per mAh. Newer LiPo's are much more advantageous. I'm now trying Zippy Compact's from HobbyKing, had Turnigy's before which gave me 170 flights before the capacity was no longer useful.
Flight time is fairly simple - how many amps does your 'copter draw? 30 amps constant from a 5000mAh battery will give you 8 minutes hover (fly to max 80% of capacity to preserve battery - ie 4000mAh).
If you don't know the amps then you can extrapolate from the actual flight time. If you have an Android phone check out my app: here
Max 24Amp sounds a bit too low for your hexacopter. Even our really light hexas will take that and a bit more to run hover. So it looks like you are running at maximum or even a bit over maximum for your battery. Depending of the weight of your hexa you should have battery that at least can provide minimum of 40 Amp continuously.
I have seen current draws like 60 Amp on hexas really easily if you give some throttle. And if your battery cannot provide that, ESCs will go and slowly shut them down as a safety measure. So get a better/bigger battery (but don't buy Turnigys)
If you are dead set on LIFEPO4 packs This may do the trick for you. http://www.hobbyking.com/hobbyking/store/__14070__ZIPPY_Flightmax_4...
Get 2 run them in parallel and you will have about 8 amps for 1kg of weight
If you want to check calculated flight times use the following calculator. http://www.ecalc.ch/xcoptercalc_e.htm
Also be aware that your batteries cell voltage is lower than a LiPo, so if your ESC is set to LiPo then it will be shutting down thinking the battery is flat, especially if the battery is straining to maintain voltage at your given current rating.
Set your ESC battery type to NiMh and try it again to see what happens.
A battery that can supply only 24 amps continuously is not enough to power any hexacopter that I know of. If you're using the purple motors from the store you'd need at least three of the batteries you have in parallel to provide sufficient amperage for reliable flight.
Based on what you've said, your battery cannot deliver the current your hex requires. Once you get up to hover speed, the current drain causes your battery voltage to collapse. Be careful as you may be damaging the battery now, IDK, I've never used this chemistry.
We rate RC batteries in "Cs", with C being the continuous current when discharging the full battery (as rated) in 1 hour. Thus in your case a 1C battery could deliver 6.4A for 1 hour. Your battery seems to be ~4C, which means it could deliver ~24A for 15 minutes.
We typically prefer a much higher C rate for multicopters, and for brushless motors in general. 20-30C is a common rating, but what you need depends on your total max current requirements. Since you have 20A ESCs, one might say you shouldn't use more than 20A/motor continuous, so say 120A max continuous total. Thus for a 6400mAh battery, one would need 120A/6.4A ≈ 20C battery plus any additional margin you'd like for whatever. If you have the standard 850kv motors, 15C would probably be fine as they don't take 20A. Notice that the C rating you require changes with pack size, with smaller packs requiring a larger C to get the same max continuous current.
Low-C batteries are great for other RC use, like powering accessory electronics, servos, Rx, Tx, etc. What they aren't great for is our motors...sorry about that.