Flight time and battery temperature

Maviac · May 13, 2019

Hey folks. Since I've been reading these boards regularly I've been extremely diligent about my batteries and keeping them in the recommended storage range when not flying. I've also been watching my temperature during flights, which leads to this question:

There is a general consensus to land with at least 30% charge, which is a very good idea. But much of my flying is very local - meaning, the aircraft is always within 100 feet horizontally, or at least a few hundred feet during the flight. There's no real worry about having enough juice to return home. So, I have felt pretty comfortable flying down to 15-20% knowing I can land in 20-30 seconds at any given time.

But since watching the temperatures, I have noted that battery temps go up substantially (this is a Mavic Air) during the last parts of a flight, particularly below 30%. The other day, I flew 2 batteries, one charged to 100%, one to 77% (I virtually never fly without a complete charge, but I had done a quick flight earlier in the day and had a quick specific shot I wanted to get and was never more than 50 feet horizontally so felt ok with it). In both cases, the batteries were right at 50 degrees or below until they hit around 30%, and then started quickly rising, both reaching more than 53 before landing at around 18-20% (the 100% battery was a 15 minute flight, the 77% one, 6 minutes).

There's probably some chemical/physics reason for the higher temps at low charges (it sort of makes sense) but I'd never really noticed it before. Unless my situation is a one-off, this is another good reason to always land with 30%, range issues aside.

Former Member · May 13, 2019

How many cycles on your batteries? 53°c is on the high side for a Lipo.

Maviac · May 13, 2019

Thunderdrones said:
How many cycles on your batteries? 53°c is on the high side for a Lipo.

I started to send my original message as a direct message to you but I've bugged you enough so I didn't want to annoy you. One is, I think, around 15 and the other is around 7, maybe 8 (I meant to check this past weekend but I think that's a good estimate).

This is why I was asking about MA vs. MP/M2 cooling a while back, because my temps seemed to be a lot higher than what you were talking about as far as safe ranges for LiPOs (when I posted that, I wasn't checking that regularly, but I remembered the temps).

Do the logs track temps? I'll download them if they do.

Former Member · May 13, 2019

Maviac said:
I started to send my original message as a direct message to you but I've bugged you enough so I didn't want to annoy you. One is, I think, around 15 and the other is around 7, maybe 8 (I meant to check this past weekend but I think that's a good estimate).

This is why I was asking about MA vs. MP/M2 cooling a while back, because my temps seemed to be a lot higher than what you were talking about as far as safe ranges for LiPOs (when I posted that, I wasn't checking that regularly, but I remembered the temps).

Do the logs track temps? I'll download them if they do.

It's not a problem. Do you have the airdata app/software? The free version is great but for a little more money per month you get some pretty extensive flight and aircraft data. And you don't have to download flight logs to see what happened.

Maviac · May 13, 2019

Thunderdrones said:
It's not a problem. Do you have the airdata app/software? The free version is great but for a little more money per month you get some pretty extensive flight and aircraft data. And you don't have to download flight logs to see what happened.

No. I'll download it and track going forward, though. So 53 is high for a MA? Again, it never gets above 50 until I get down to 30% or so.

Maviac · May 13, 2019

Thunderdrones said:
How many cycles on your batteries? 53°c is on the high side for a Lipo.

And yeah, I thought so too. That's why I'm consulting the expert.

To expand on the topic a bit, I asked months ago about "shock cooling" batteries (which isn't really what I was doing but I used the term loosely). Is there any harm, or benefit, to trying to quickly cool a just-used LiPO battery? I did a lot of flying this past weekend and was near my car, and stuck my depleted batteries in front of my AC vents after use (while I was on the next battery) to cool them down more quickly. Any chance of harm from cooling too quickly?

old man mavic · May 13, 2019

the temperature of the battery is a direct cause of the chemical reaction going on in the cells, as you know the overall voltage drops as the cells become depleted and then the battery has to work harder to keep the motors working,,which in turn produces more heat,the cells need to cool down so that the chemical reaction can then work in reverse to allow the cell to recharge. the voltage operating range of the electrical components in the mav allow for the changing voltages during use,to allow your batteries to last longer it is better not to discharge them to very low levels just invest in a couple more so you can fly longer i would not try shock cooling,

Maviac · May 13, 2019

old man mavic said:
the temperature of the battery is a direct cause of the chemical reaction going on in the cells, as you know the overall voltage drops as the cells become depleted and then the battery has to work harder to keep the motors working,,which in turn produces more heat,the cells need to cool down so that the chemical reaction can then work in reverse to allow the cell to recharge. the voltage operating range of the electrical components in the mav allow for the changing voltages during use,to allow your batteries to last longer it is better not to discharge them to very low levels just invest in a couple more so you can fly longer i would not try shock cooling,

I understand and agree with all of that, except perhaps for the last part. Why would you not recommend cooling the battery quickly? Since hot temperatures are very bad for lithium batteries, if we could cool them to something close to room temperature quickly, why is that bad (it may well be, but I'm curious to hear from anyone who knows for sure)? In the absence of info otherwise, it seems better than letting them naturally cool off where they stay hot for a while.

Former Member · May 13, 2019

To cool batteries faster, you can let a fan blow over them after a flight. Im not sure that putting them in a fridge or cooler would damage them, but Ive never been in that much of a rush to get a battery cool. Usually within 15-20 minutes after a flight, a battery is ready to charge again on its own.

Batteries usually wont get hotter than 50°c. If they do, its time to land.

old man mavic · May 13, 2019

i think that maybe the term you used ie shock cooling i perhaps,misunderstood to me that term would mean a very rapid reduction in temperature by placing the battery in say a cooler with ice packs in it ,on the other hand,putting the battery in the air from a fan out of the sun would assist the cooling process.

there is another factor to consider as well and that is the ambient temperature of the air ,there is a vast difference between in the shade outside to in a hot vehicle in the shade,and unless you have a genuine need to charge any battery whether at home or in the field straight after it has been used,then the issue does not really arise, one other point to remember is ,because of the position of the battery in the Mavics there is going to be considerable heat transference,from the internal heat of the Mavs as they are operating,of course that can be a good thing in cold temperatures as the battery does not work as well in the cold

Maviac · May 14, 2019

old man mavic said:
i think that maybe the term you used ie shock cooling i perhaps,misunderstood to me that term would mean a very rapid reduction in temperature by placing the battery in say a cooler with ice packs in it ,on the other hand,putting the battery in the air from a fan out of the sun would assist the cooling process.

there is another factor to consider as well and that is the ambient temperature of the air ,there is a vast difference between in the shade outside to in a hot vehicle in the shade,and unless you have a genuine need to charge any battery whether at home or in the field straight after it has been used,then the issue does not really arise, one other point to remember is ,because of the position of the battery in the Mavics there is going to be considerable heat transference,from the internal heat of the Mavs as they are operating,of course that can be a good thing in cold temperatures as the battery does not work as well in the cold

I know. I used the term sort of hyperbolically (and metaphorically) in a prior thread (in my early droning days). Shock cooling refers to something specific when it comes to airplanes; I was asking back then if there was any harm in cooling batteries quickly to allow them to be recharged. At this point, I have enough extra batteries that I’m less concerned about that, but just wondering if there is any benefit to cooling them extra fast to get the temps down.

Thunderdrones mentioned putting them in front of a fan wouldn’t be a problem; I’m wondering if there’s any harm in putting them in front of my AC vents in my car blowing cold air. I assume not, but I don’t pretend to understand the chemical properties of lithium.

WithTheBirds · May 14, 2019

Anywhere between 10 and 60deg C is fine. Below 10C Anode playing can be an issue with above 60C carrying an accelerated rate of chemical breakdown. LiPO. LiPO deliver higher current at increased operating temps.

It’s no mystery why temp increase is elevated at lower charge levels. It’s simply IR losses. As voltage decreases current drain increases for any given power level (battery IR will provide for higher dissipation).

ToddCarey · May 14, 2019

Something else that is happening as the battery discharges, the voltage drops. I order for the motors to do the same work with a lower voltage, the amperage goes up. Remember power = volts times amps. Higher amperage will always cause higher temperatures.

Maviac · May 14, 2019

Thanks everybody. This has been a helpful and informative thread.

WithTheBirds · May 14, 2019

ToddCarey said:
Something else that is happening as the battery discharges, the voltage drops. I order for the motors to do the same work with a lower voltage, the amperage goes up. Remember power = volts times amps. Higher amperage will always cause higher temperatures.

The extent to which increased current might cause a significant temp increases relates directly to the IR of the battery.

Maviac · May 14, 2019

WithTheBirds said:
The extent to which increased current might cause a significant temp increases relates directly to the IR of the battery.

IR = Internal resistance, I assume.

Former Member · May 14, 2019

WithTheBirds said:
The extent to which increased current might cause a significant temp increases relates directly to the IR of the battery.

And I believe that internal resistance increases as the battery gets older. That's why I ask how old the battery was. Good point.

WithTheBirds · May 15, 2019

Thunderdrones said:
And I believe that internal resistance increases as the battery gets older. That's why I ask how old the battery was. Good point.

Yes- IR does increase with age. Interestingly the BMS in our recent DJI batteries measures IR and it is a major factor in the algorithms employed to report battery health and keep the remaining % and time to empty honest. Higher IR = more significant voltage sag under load = higher current = greater dissipation in the battery cells = more heat....