Cold weather/LIPO issues

I was flying my S800 EVO in Virginia over New Years and experienced the same issue. It was just around 32 degrees Fahrenheit, and my iOSD showed voltage drop right away to 20-22 under load. I was cautious with my flight times, normally I'm getting around 12-14 minutes but I played it safe and kept everything around 8 minutes and had no problems. Use a voltage meter on the battery and set to 3.3, then keep batteries warm in a cooler with some hand warmers. But hey at least the air is dense in cold temps, haha!

 

When i flew in mongolia last year in about -30C the batteries would instantly sag to almost cut off point but after a couple of minutes of flying the voltage would rise back up again as the batteries generated heat. The mistake i made with insulation was to make a jacket for the whole hub which covered battereis, FC ESCs etc. This insulation made it over heat and i melted my EC5 connectors!.. even in freezing temperatures! so If your insulating batteries, don't insulate the connectors.

 

HI Justin

Are you running MK electronics? I understand your heistation but I found that that power consumed during flight is a much more accurate way to determine when you need to land and I have found that my batteries operate between 21.9 to 21.2 much longer than typical. Let me explain.

Last summer I got to a point where my battery voltage was also dropping very fast and thought that all my batteries just were starting to exceed their life cycle. But I was able to determine that this was not the case.

When this started happening to me I noticed it was starting to happen to all my batteries at the same time. This was occuring on 7 diffeerent pairs of batteries (QC 6S 10000). Most pairs had about 40 to 50 flights on them at this time. So I did a test to see if the batteries were indeed starting to fail and their performance starting to decrease.

The first thing I did was take a set of batteries that I knew were 70% exhausted from a flight. I then took those batteries straight to the chargers and charged them fully. Once they became fully charged I wrote down how much energy the chargers put into both batteries to get it back to 100% charged. I then took this number from both batteries and added them together.

The next thing I did was look at the GPX log from that flight that batteries were used and got the total energy used during the flight. I then compared that number to the total amount the chargers put back into the batteries and found it to be very close about 2% off.

Then I rigged up 10 pounds of dummy weight to my copter and did another flight with a fresh set of charged batteries. I made a flight in my local park and hovered the copter about 10 feet off the ground over soft grass. This way if the copter ran out of battery power it would minimize damage. I figured what was worst, possibly crashing my copter with no gimbal or payload or replacing 14 batteries? I chose to do the test and see what happened.

So I did a flight where I was just hovered the copter and watched the data on my radio for battery voltage and energy consumed during flight. What I found was very interesting.

I found that after just 1.5 minutes in flight the battery voltage already dropped to 21.9 but had only exhausted about 2500 mah of power. The copter stayed in the 21.9 to 21.6 range for about another 6 minutes where total energy consumed showed about 8,000 mah. The copter then stayed in the range of 21.5 to 21.2 for another 8 minutes and had that time total energy consumed showed about 16,500 mah. It then stayed at 21.1 to 20.9 for another 3 minutes before power consumed showed was 19,400 and at that point I landed very quickly. So as you can see my copter was operating in the 21.9 to 21.2 range for 15.5 minutes but if I landed when it showed me 21.6 I would have only had 7.5 minutes of flight time. The voltage drop was prety consistent until it reached 21.0 where it normally had spent about 90% of both batteries. I have found what works well for me is to start coming home at the 16,000 mah spent mark which is normally about 16 minutes in flight, and normally at 21.2 volts instead of the normal conventional 21.6 volts. I do monitor all 3 forms of data closely and if I see anything different about any of the 3 parameters I would land sooner if needed.

Why this changed and why this occured is still unknown because I was continuing to have great flight performance out of the batteries and nothing really changed except the voltage sag was much quicker to the 21.6 to 21.9 mark. Now I know that your situation is a little different because you will get less flight time when its colder but some of the voltage sag can just be contributed to other factors and that doesn't necesssarily mean your batteries wont work inthe cold. I was still getting about 15 minutes before I reached 21.2 mark when I was testing my new wookong setup last month so I didn't notice much change between 21 degrees and a typical 80 degree day, probably only about a 1.5 minute drop off in flight time. I'm sure this does drop off sharper when getting closer to 0.

One thing that I want to stress to anyone reading this is that for anyone that wants to do this kind of test you have to make sure that the BL's are reporting energy spent very accurately compared to the amount of actual energy spent. If you find that these 2 numbers are more than 2-3% off you need to change the offset number in the BL menu inside MK tools until you get about 2-3% off. I try to shoot for the BL's to overreport energy spent as to being a little less than the actual amount for a little safety cushion.

Ever since I did this test I have great confidence in how much energy is in the batteries at any given moment. I still monitor battery voltage and if I see if dropping any faster I would land faster but I have yet to see any changes to my copters flight behavior. I have flown about 200 flights since and everything has been great. I just have no explaination why the voltage started sagging all of of suddent on 7 different pairs of batteries.

 

Current draw with a 10 pound payload differs a little bit depending upon the motor I was running however I did notice that it would primarily be in the 80 to 85 amps range. So I guess Ohms law would dictate the internal resistence of the batteries would be lower, at least with a fully charged battery compared to lesser charges. Not sure if the internal resistence of a battery would change at different levels of charge on a Lipo.