A series of articles on electric flight by Rob Ireland
If you have read some modelling magazines recently or are generally aware of battery technology, you may appreciate how many different cells are available to us.
The different technologies encompass nickel-cadmium (NiCad), nickel-metal hydride (NiMH), lithium ion (and lithium poly in different chemistries). The resulting range of suitable cells is bewildering, and runs into hundreds. They are constantly being improved with older styles being discontinued.
Electric model fliers are benefiting from the vast research resources allocated in the quest for better cordless power tools, and mobile IT equipment such as portable computers and mobile phones.
The requirements are the same in these cases, and can be broadly stated as;
a) rechargeable
b) high power density
c) high current discharge capability
d) fast charge capability
e) affordable
In this article I will illustrate some of the cells and battery packs I use, which meet the requirements above. All of these are predominantly NiCads, with a couple of NiMH types. These cells can be broadly considered as supplying 1 Volt under load.
Cells are governed by various “numbers” and I will describe these here;
Diameter & height - (obviously enough) to allow us to build them into a particular model battery tray, or to design a suitable one.
Weight – essential to check on flying weight and centre of gravity.
Internal resistance – only useful if you are using a computer program. Basically, the higher the resistance the less able the cell will be to deliver current without overheating. Same for charging.
Capacity – usually stated in milliamp hours (mAh).
Discharge rate – the maximum current which can safely be discharged. This is referred to in fractions of “C”, where C is the capacity of a cell in Amp hours (Ah). For example a 2000mAh cell with a discharge rate of 20C will be capable of supplying 40A current.
Charge rate – as for discharge this is quoted in “C”s. Charge rates are usually much lower than discharge rates. For example, the same 2000mAh cell might quote a charge rate of C/2, in which case charge current must be limited to 1A. A charge rate of C can be charged at 2A, and at 2C a 4A current could be used. The charge times for each case will be in excess of 2 hours, 1 hour, and ½ hour.
That’s the basics – some useful cells are shown below;
From left to right;- 1. The often referred to subC NiCad cell. This is an industry designation of the cell dimensions being 23mm diam x 43mm high. Weight is 2oz and the capacity is 1700mAh in this case. It is a fast charge and high discharge type with the internal resistance being approximately 4m?. When I took up electric flying (c1995) this was a new cell – the previous capacity in this size being 1400mAh, and before that it was 1200mAh.
2. Another subC NiCad. This one was reclaimed from a batch of cordless power tools and had an unmarked paper sleeve fitted. Its capacity is 2000mAh and other specifications are the same as the 1700 above.
3. Slightly smaller than subC, this is a NiMH cell manufactured by Panasonic. Still OK for fast charge/discharge it has a resistance of 5.5ohms and weighs 1.48oz. Capacity is a nominal 2000mAh and dimensions are 23mm x 34mm. I use 8 of these built into the cowl of my Sopwith Pup where the main criteria was for a size of cell which would get the balance about right.
4. An AA size NiCad “pen cell”. This is a Sanyo of 1100mAh capacity. Dimensions are 14mm x 50mm. Internal resistance is approximately 19ohms and these cannot be fast charged/discharged. Discharge is limited to 1C (under 11A). I charge these at 2A maximum (<2C).
Some battery packs made by connecting cells in SERIES ONLY;-
An 8-cell pack of NiMH cells. These are “Puffin Hi-Flow” cells of 1050mAh capacity. Battery weight is approximately 6.5oz. Discharge current is quoted as being limited to 13A. The pack should charge OK at 2C, unfortunately the software in my charger will not correctly charge NiMH’s at this rate and I am limited to 1C, which means it takes an hour! A Velcro strip is attached for mounting into a Junkers EF128. The pack is ideal for the Wattage Mirage, Parky & F22 Raptor. It should also operate my MiG 15 but hasn’t been tried yet.
Also suitable for; Pico Cub, Coyote, Impress, Ezette & Ironic Mk2
A 12-cell pack of NiCads, at 2000mAh capacity. (Still to be fully insulated). This powers “Ampmaster”. At a weight of 24oz, the maximum discharge is about 35A.
Another good NiCad from Sanyo, the RC-2/3SC. 1200mAh capacity, measuring Ø23mm x 26mm. The 8-cell pack here weighs about 10oz. Discharges at up to 30A. Charges at 3.5A approximately. Assembled with end-to-end soldered cells, this pack was designed for the MiG 15 but has been used extensively in my Grumman Wildcat. Also now destined for use in Spitfire Mk2A and Ripmax “Easy Street”.
Also suitable for improved power to weight ratio in; Twin Star, Nebula 2000E, Aerojet, Filip 600, Rascal, Sorrento & Zephyr
Sanyo 2000SCR 7-cell pack of 2000mAh capacity. Capable of fast charge/discharge. Pack weighs 14oz. used as standard in my Electrafly.
Also suitable for; Twin Star, Nebula 2000E, Aerojet, Filip 600, Rasca, Sorrento & Zephyr
The highest density Sanyo subC NiCad, the CP2400SCR at 2400mAh capacity. Here assembled into an 8-cell pack weighing 16oz. This pack can also be utilised in the following models which can cope with slightly more weight, for example; Rascal & Zephyr
A good source of cell data used in the Electricalc and Motocalc programmes is available on the web at www.members.aol.com/kmyersefo/page17.htm
Suppliers:
The following suppliers are good sources for many cells including those discussed above;
1. The Electric Aeroplane Company, (01270 588921) www.modellhaus.co.uk
2. West London Models, (020 8890 2326)
3. Overlander, (01524 793328) www.overlander.co.uk
4. Puffin Models, (01454 228184) www.puffinmodels.com
Next time……Motors