I've been doing test flights with my quad for a couple weeks now using 3S 2200mAh 20C batteries. Admittedly the first few flights I was running the battery down to the point it wouldn't lift off the ground anymore (about 10 minutes of messing around) and then using a balancing charger to charge them back up, but now I've been reading about the do's and don'ts of LiPo packs.
I think my quad is on the heavy side, I haven't weighed it, but it's arms are made of slightly thicker aluminum than the 3DR ones and my legs are garolite, but slightly bigger also. The rest of it is mostly 3DR stuff, the motors, ESCs, APM 2.5, and so on. So after reading this article I decided to solder the voltage leads of my Spektrum telemetry module to the battery input wires of my power dist. board and watch what my battery was doing, I set the alarm on my DX8 for 11.2 volts (based on that article I linked to) and much to my surprise it only took 3 minutes of flying for the alarm to go off.
In 3 minutes I was going from 12.6 to 11.2 volts under load in the air and it would creep back up around 11.8 if I landed and let it sit a minute but not 30 seconds later after going back up in the air it would dip below 11.2 again.
So here's my question, if LiPo batteries aren't supposed to be run down past 3.7v per cell regularly, why do all the fixed battery alarms not go off until 3.3v? Also my DX8 default alarm parameters was something like 9.8v when I selected 3S. When I do run my batteries down to the 3.3v/cell range, they take 60-65 minutes to charge and balance at 1C (2200mAh) and when I stop at a load voltage of 11.2 and balance one, it only takes about 20 minutes to charge and balance because it's starting out around 3.8v/cell unloaded. Do I need to disregard the 3.7v/cell thing and run them down to 3.3 and get my 10 minutes of flight and 1 hour charge time, or do I deal with 3 minutes run times until I lose weight or get bigger batteries?
Just to put some actual limit figures up.
LiPo voltage range is 3V flat, 4.2V charged.
2.5V is the kill voltage, below this you won't recover the battery (or cell), although there are exceptions (On higher quality/higher Lithium content batteries we have recovered from 0V, but I wouldn't make it a habit).
There is no useful current left in the battery when it reaches 3V, this is when you get the sags, but the battery is not yet in danger.
You will need time to land after triggering an alarm, so take this time into account when setting alarm voltages.
Having skimmed the article the voltages quoted are all 'no load' which not an indicative voltage, use your 'under load' voltages as indicators.
Different brands of battery recover voltage differently, remember "Lipo's ain't Lipo's", you get what you pay for (in most cases).
And just cause it's written on the net don't make it true
I guess I'll set my radio warning around 10v and see what kind of run times I get.
So what is everyone here running on their quad? I assume most people are using 3S batteries but what's mAh and C ratings?
My charger doesn't count the mAh (I don't think, it's a Accucel 6) but it does show the mA it's charging at and times the charge, as well as showing total and each cell voltage. I've been seeing one hour charge times with it on balance mode where it gives it 2200 mA right up until the cells are at 4.18 or 4.19 and then it ramps down while finishing the balance. So that right there tells me it's putting about 2200 mAh back into it, hmmm.
You may need to upgrade a bit battery-wise - these days a LiPo with only a 20C rating is possibly on the sketchy side in terms of actual performance quality. Also brutalizing the batteries for the "first few flights" is not at all a good thing. If you ran those batteries down really hard they may well be compromised.
I've been flying helis (up to 600 class) and planes and running big fast R/C cars (presently the big ERevo...) on LiPos for over five years, and get excellent performance and longevity using the following guidelines:
1) Don't buy the very cheapest batteries. You don't need anything like 100c monsters unless you're flying super aggressive 3D etc., but medium- C rated batteries will last longer and be more reliable and they don't cost all that much more if you stay away from the big old brand names. These days I buy the Turnigy "Nano" series in all sizes and am real happy with them.
2) Use a charger that tells you what you've put back into a battery in amp/hours. This is essential.
3) Break your new battery in conservatively in terms of total time and high burst loads. Log your flight time. Now when you charge you can see exactly what you're using per minute of flight. So if you flew a 2200 mah battery for 5 minutes and put 1000 mah back in, you're using 200 mah per minute. (Higher math!)
4) Use the above per-minute figure to set your timer alarm so that you will not exceed 80% of rated battery capacity. Here that would be 2200 X .8 = 1760 / 200 = about 9 minutes. But remember that how you fly from one time to the next and ambient temperature will vary and will affect actual battery draw. So be conservative - here I would set the timer at 8 minutes.
5) Always balance when charging. Always.
6. I charge at 1C regardless of any higher rate allowed by the battery manufacturer. But that may be excessive caution just on my part.
7. Feel your battery after flying. I like them to be barely warm, if at all. In time you'll know by feel if something is starting to go wrong, either with the battery or elsewhere.
8. I sometimes use a cheap dedicated voltmeter, the kind that connects to the balance plug, to determine the following in the field:
a) Is the battery I'm about to use fully charged? (should show 4+ no-load volts per cell. )
b) Is there a problem with an individual cell? (significantly different voltage than the rest)
c) Is there roughly enough juice left to make another flight? (if I forgot to start or stop the timer)
Other than these things I don't routinely pay attention to voltage.
Hope this helps.
4)Now when you charge that battery you can see exactly what's going on.
The onboard alarms for 3.3 volts per cell let you know the packs are basically empty and you need to land immediately. If you set the alarm on the transmitter for 3.7 volts it gives you a heads up but you don't have to land right away. I have my transmitter set to warn me at 10.5 volts for a 3S pack which is in the middle of the "heads up" and "oh crap" numbers.
The article states 3.5V as the low, that's a warning of 10.5 low not 11.1 as you have set. This may extend your battery life
(PS: I don't like how the article suggests throwing LiPo is the trash when fully discharged, does anybody do this?)
I have the same amount of time (3-4) min if I setup an alarm for 11.2v, for 1 x 3S 2200.
Usually I got 7-8min, but I go around 10v (what displayed when flying). When I go to the ground (without power), the voltage is around 3.7-3.8 per cell.
I did try also using in parallel 2 x 3S 2200, time is longer, like 10 min, but never double (which is normal)