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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Dennis Toth on October 29, 2008, 10:29:27 AM
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Guys,
The first cold front has dropped down to FL sending temps into the 40's. This sometimes happens during the King Orange meet held in January. My question is how does lower temperatures affect our battery performance? My understanding is that the LiPoly's like it warm. As anyone operated ECL in low temps and how did you maintain full flight performance (battery warmer, hot box etc)?
Best, Dennis
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I kept mine inside my jacket. Actually, kept them in the car on the dash, under the winsheild in the sun,that stayed warm for a bit, then Put them inside my jacket before my flights.seemed to work on, but i only tried it like twice so I really dont have a ton of experience. are you pushing your batteries hard enough that you need 100% efficiency?
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Yikes, ~^ 40 degrees in Flouridough!
I do as Mark says. Don't leave the batteries in a cold car, keep them inside until you get ready to go to the field. Then I will keep them in my jacket. Put into the plane just before flying.
What I understand is that the battery resistance goes up as they get colder. What this will do is warm the batteries up at the beginning of the flight wasting energy, because your ESC will increase amp flow due the the larger-than-normal voltage drop out of the battery--just keeps watts constant if you are running a governor. Of course once the batteries warm, the resistance will begin to drop, so to some extent, the this fixes the problem. But I will guess you will use a bit more than normal out of the pack, first since they will be slightly colder, and secondly if you keep the motor rpm the same as in summer, the colder and thicker air will make the motor work harder. It also produces more thrust, but then the airframe drag is also worse. Not sure how it all plays out! For a glow or IC engine, cold air in combustion gives more power out to handle this (so cold air=good for plane), but we don't have this effect in electric. Lift will be better.
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I have also wondered on this topic, about the possibility of leaving your packs say 10 or 15% discharged, and then prior to flying put them on the charger to top off, and warm them up at the same time, thoughts?
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Even though I do charge at 2C with my FMA 4s charger (max rate I can charge a 2100 pack with the 4A limit of this charger), the batteries never have felt particularly warm to the touch when I unplug them. So I am not sure if charging would do much.
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Guys,
On the battery use I use about 70% out of the 3850mah pack and the voltage has not dropped below 15.1v at the end of the flight. To store the batteries, I have taken a small size ammo box and folded some aluminum flashing to form pockets to hold the batteries.
The box will hold 6 batteries standing up. Since I formed the pockets from one piece of 6" wide sheet between the pockets there is about 1/4", this allows a little give in the pockets but could be wide enough to slip in some of those thin hand warmer packs. I think a couple of those and putting the box into a foam cooler (ah, warmer) might keep things from getting to cold.
Has anyone use this type of hand warmers? How warm do they get?
Best, Dennis
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Dennis,
I think if you put warm packs (from the house) into a styrofoam cooler for insulation, then they will stay warm enough. I guess it depends on how long you are at the field.
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Hi Gang,
I have even built a small styrofoam cooler (warmer?) box with a tiny slit for the wires to poke through.
When it's cold, the batteries leave the shop in the box and they get put right back into the box immediately after a flight. Charging happens in the box as well.
In really cold weather, it gets important to get the battery up to body temp, so I'll even run the plane for 1 minute, put the pack in to the box and charge. That gets them into the plane with the insides up to a reasonable running temp.
If the packs come out of the plane at less than 95 degrees, then partially choking off the cooling air with a chunk of tape may be the ticket.
later,
Dean P.
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Hmmm, is there a need to CHARGE cold packs any differently?
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FWIW, I only charge a pack if it's near "room" temperature (~70's). I think my FMA charger actually goes into slow mode (=safety) if it senses the pack is cold---don't ask me how it knows it is cold except by sensing how resistive the pack is.
I am pretty amazed about how these chargers figure out what the pack capacity is. Occasionally the charger does get confused, maybe about once in 10 or 15 charges. In that case it goes to safety charge, which is an incredibly slow charge rate. Usually when I notice it (like why is that charge taking so long???), simply unplugging and replugging the battery back in gets it back to normal mode.
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Good question Dennis.
That's why I'd take the packs to the field after charging at home, run them just long enough to warm and then topp-off charge in the hot-box.
Charging very cold cells is not a good idea, and even the best charger does not have the ability to measure the pack temp and compensate the endpoint voltage.
The newer FMA designs do use very cold ambient temps to make an adjustment, but nobody actually puts thermistors in the pack like the high-buck cell phone manufacturers do.
Larry Sribnik at SR tried to make thermistors a standard for NiCd packs a long time ago, and it didn't happen there either.
later,
Dean
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Guys,
At what ambient temperature do the packs start to drop off in power output?
Best, Dennis
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Dennis,
I don't think they actually lose power or energy, but what happens is that the internal resistance goes up. So now when you try to draw your 300 watts, the IR voltage drop in the battery eats some of the energy. So your ESC compensates by opening the throttle more, letting more current flow. The net energy into the ESC is the same (lower volts, higher amps), but the battery is absorbing more energy by heating up faster than it normally would. Of course when it warms up, its resistance goes down, so eventually you tend to get back. But the initial heating is lost to the ESC/motor combo.
Here is a spec sheet for my 2100 mAHr cell.
Notice the discharge is at 0.5C, much lower than we do. I am guessing this is so they can keep the battery at the specified temperature. In our case, the battery temperature is changing.