Electric R/C Flight

By Heather Mardon

Part 7: Batteries

Batteries are the equivalent of fuel for our electric planes. So just as there are several types of IC engine fuels (Glow, Petrol, Diesel mix) and expertise on knowing the right mixtures etc, there are several types of rechargeable batteries and much to learn of there use and care.

Introduction

Electric flight places very high demands on battery cells, drawing high currents and causing high temperatures to build up in our packs. In most flight applications the cells are being used beyond the manufacturers recommendations. This is because there are no batteries specifically made for electric flight, we end up using cells designed for applications such as power tools and cordless appliances.

Electric flight requires cells that can provide high amounts of current while maintaining a voltage close to the nominal value of 1.2v. A battery has several characteristics that can tell us if it suitable for use.

They are:

By taking the capacity and dividing it by the weight you will end up with an energy density figure eg: A Sanyo KR600AE has a capacity of 600 milliamps/hour (This means if you discharge the cell at a rate of 0.6A it will last for 1 hour, sometimes referred to as a 1C rate) and a weight of 19 grams. Divide the two together and you get 31 mAh per gram. The higher the value the better, but this will need to be weighed up against the cells ability to supply the current you need.

The other parameter is the cells internal resistance. The higher the resistance the lower the voltage that will end up at your motor. Also the more heat that will build up in the pack as that energy has to go somewhere.

There are other factors but they are harder to quantify and have mainly to do with the cells chemistry. For example the Sanyo 1100AAU has a higher internal resistance than the smaller 600AE but will out perform it in voltage at the same current load.

NiCad’s

Nickel Cadmium cells are the most abundant and excel at providing high currents at high voltages. Most of the NiCad’s we use can be fast charged by automatic peak detect chargers. The following is a list of some typical cells and there suggested maximum charge currents and there maximum continuous current draws.

Model Max Charge Max Discharge

Sanyo KR600AE 2A 12A

Sanyo CP1300SCR 3A 25A

Sanyo CP1700SCR 5A 40A

Sanyo 1700SCR 5A 40A

The discharge curve of a NiCad is very flat meaning that it holds its power well until near the end of the curve where it rapidly declines. Cells will loose some of there charge if left unused for several hours. It is best to repeak your packs before flying them if you want to get the maximum capacity out.

NiMh

Nickel Metal Hydrides are a more environmentally friendly due to there lack of cadmium. They also have a higher energy density than NiCads. The down side is that they have a slightly lower voltage than a Nicad and they cannot handle as much current as the same weight NiCad. In the high performance cells the NiMh are also more expensive. NiMh cells should also be charged at a slower rate than there equivalent sized NiCad.

In use a NiMh is best used hot off the charger as the cell voltage will decay quite rapidly. Make sure your charger is compatible with NiMh cells before trying to charge them as the peak is not as pronounced as NiCad’s.

Here are some discharge curves for some typical cells

NiCad folklore

There's a tremendous amount of folklore about NiCads, most of it at least somewhat misguided, much of it just plain wrong. The worst advice ever given to a NiCad battery owner: discharge it completely. Discharging a NiCad cell is fine, and may even be useful if the cell was previously overcharged. Completely discharging a NiCad Pack, however, is never a good idea. Because no two cells are identical you run the risk of getting ‘cell reversal’ if a pack is discharged completely. Cell reversal usually results in permanent damage.

The NiCad ‘Memory’ effect is actually called ‘Voltage Depression’ and occurs when a cell is Overcharged. This overcharging of the active material in the electrode reduces the cells voltage. The effect is erased by discharging and recharging that portion of the active material which has experienced the extensive overcharge.

For the best performance in your NiCads you should peak charge your cells and throw away your trickle charger.

The bottom line - yes there is a memory effect, but it is very small and not worth worrying about