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

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.

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.

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.

Hmmm, is there a need to CHARGE cold packs any differently?

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

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.