Electric Flight in Australia

 

What are nickel metal hydride cells?

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Nickel-metal Hydride battery cells provide more capacity in equivalent sized packages than nickel-cadmium cells, while also eliminating some of the concerns over the use of heavy metals in the cells. They are becoming popular in applications where extended battery run time is valued.

Nickel-metal hydride cells are essentially an extension of the well-proven sealed nickel-cadmium cell technology, with the substitution of a hydrogen-absorbing negative electrode for the cadmium based electrode. This substitution increases the cell electrical capacity by up to 40% for a given weight and volume, and dispenses with the toxicity concerns. The remainder of the nickel-metal hydride cell is quite similar to that of the familiar nicad.

NiMH cells are more sensitive to charging conditions than nicads, but the charging process is essentially similar: fast charge with constant current and terminate when overcharged. However, the detection of the overcharge with NiMH is more subtle and it is better to have temperature sensing as well as voltage peaking. NiMH cells are actually exothermic (whereas nicads are endothermic), which means that they generate heat when being charged. When the cell reaches overcharge, where the bulk of the electrical energy input to the cell is converted to heat, the cell temperature rises dramatically. That’s why it’s best to have temperature sensing as well as peak-voltage detection. Internal cell pressure also rise dramatically in overcharge, hence the need for a safety vent which remains free of obstruction. Without a safety vent, the cell could be physically damaged.

The only other variation is that NiMH cells go flat on the shelf faster than Nicads – we say that they have a slightly higher self-discharge rate.

Well, what does all this mean for the electric flier? While we welcome the increase in capacity for a given size, NiMH cells remain more difficult to manage. It is harder to charge them correctly (than it is for nicads), so they are subject to failure more easily. They also will not deliver the high amps we require for competition flying, so the nicad remains the cell of choice for that forum.

NiMH does have its place, though. If you want to fly a sports plane, or scale project, or a prolonged glider event, and you would like a long motor run at low amps, the NiMH cells are unparalleled. Use them for this application up to 30A, and charge them carefully – particularly watching out for the heat build up as they reach their peak.

One last comment. NiMH cells were very touchy when they first became available. They would fail without warning, sometimes after a very short life. They have improved dramatically, and we owe that technological advancement to their use in mobile phones and laptop computers. They will continue to improve, so keep your eye on them for future electric flight use!

Post Script: I am now using PUNCH 3000 NiMH cells from Electronic Model in France for high amp applications. These pushed and matched cells will certainly deliver the current - up to 125 Amps and more - so the above comments are already obsolete. Watch these cells - they are more environmentally friendly, and will take over from Nicads.