I thought I had been doing a good job keeping my batteries from puffing. I typically fly with 4 or 6 battery packs, and launch from 1600 ft elevation. I’ve even run them to 3.0V per cell without puffing and have flown at full throttle the entire flight. A few months back I spent a week flying at 6000 ft and only flew with 6 batteries. This week I’m at 6000 again and have been flying with 4 batteries, and they are all starting to puff. The puffing is not significant to cause a safety concern, but I am concerned about shortening the life of my batteries.
I believe the puffing is heat related, as the batteries are hotter than normal launching at 6000 feet.
The question I’m asking, is the thinner air at 6000 generating more heat for the batteries when flying (I’m sure I need more power to fly)? Would changing the pitch of the props help at higher altitudes? Would switching to 7 cell packs make a difference? Any other ideas?
I think every time you charge and discharge your batteries you are adding a level of damage. When they get hot they damage faster. Charging to 4.2V is worse than charging to 4.15V. You shouldn’t ever drop below 3.2V even though they may charge fine after. I don’t drop below 3.5V including voltage sag… in other words, it’s not the resting voltage that matters but the voltage during amp draw. At the end of my flights I ease up on the throttle so the voltage doesn’t sag below 42v (3.5 per cell).
My understanding about altitude is that with thinner air you need to go faster to get the same lift but it’s also easier to go faster because the air is thinner and therefore takes the same amount of power. The props spin easier through thinner air so a higher pitch prop is needed to get the same amp draw. However, your claim that you need more power at high elevation goes against that theory. That might hold true with a plane more so than with a paraglider.
7 cells wouldn’t change anything unless you are adding capacity while doing so. The damage comes from the amp draw per cell. 14s would give higher volts with lower amps to the ESC and motors but your concern is with the battery cells and the voltage per cell is the same regardless of the pack voltage.
I wonder if the lower air pressure allows the packs to swell more? The more they swell the more the resistance goes up. The more the resistance goes up the more heat they generate. When you get back home I wonder if there will be a noticeable difference in how puffy they are?
Last winter I killed two cells by flying with cold batteries. Cold batteries have a higher resistance so the voltage sag is worse and I allowed the voltage to sag too low while taking off even though my batteries were fully charged. So, while heat is bad on the batteries they actually perform better warm.
Yesterday I flew with batteries that were halfway discharged but were at ambient temperature. The voltage sagged down to 42V on takeoff but after they warmed up a bit I was able to draw more amps without hitting 42V. Usually when a battery is half discharged its already warm and doesn’t sag as much.
Good stuff to keep in mind GliderPilot. Thank you. I’m thinking a battery condition/temp should be part of the preflight so I fly with the battery condition in mind. At least as long as the budget for batteries is an issue. If you’re OK with getting 100 cycles or less out of a $1500 pack then go for it!