ndrew Gibbs publishes a free newsletter and some extremely good guides on electric power. These guides can be purchased as e-books through his website at
http://www.gibbsguides.com. A couple of years ago these were available in paper form, but now they are e-books only—well, really PDFs that can be downloaded from his website. I highly recommend Gibbs Guide #1 Mastering Motors if you are mys- tified by electric power. This does not specif- ically cover the smaller pager motor sys- tems, but the principles still apply. He will be coming out with another guide expressly covering these smaller systems. His guide to Lithium Polymer Batteries is a must read if you use them and we all do. He explains in plain English how you need to treat them and why.
ElectricFlight A
Another excellent treatise, “LiPo Batter- ies: Internal Resistance, Max. Current Draw, True C-Rating” by Edward Sadok is on the Whitby Aero Modellers site: www.
whitbyaeromodellers.com/index.php under user generated content.
Li-Po batteries are the lightest power source readily available to us, but do present some operational and safety problems. It’s well known that a single Li-Po cell has a nominal voltage of 3.7 volts. This is what a cell reads at about 80% of its maximum charge of 4.2 volts. The absolute minimum a cell can be taken to is 3.0 volts. Below this voltage the cell deteriorates and traces of copper may internally short the cell. Cells can also be damaged by excessive heating. Above 4.2 volts the cell breaks down and overheats, releasing oxygen, and starts to swell the cell producing the dread- ed “Silver Sausage”. The main things that shorten Li-Po battery life are: heat, over charging (voltage and current), and over dis- charging (voltage and current). Using a Li-Po charger with a balancing connector at the correct charge current takes care of the over-charging problem. Of course, a single cell doesn’t present the bal-
by stew meyers You can reach Stew Meyers at 8304 Whitman Drive, Bethesda, Maryland 20817, or via e-mail at
stew.meyers@
verizon.net
PHOTOS: COURTESY ANDREW GIBBS
ance problem. The conservative charge cur- rent is the “C” rating. Coping with the over- discharge problem is up to you and it is the number one killer of Li-Pos! The current drawn should be at no more than 2 times the “C” rating, and the battery should not be dis- charged below 3.0 volts per cell. A more con- servative approach is to only go to 80% of these: 1.5 times the “C” rating and 3.2 volts per cell.
The voltage rating of a Li-Po is 3.7 volts per cell. The capacity of the cell is stated in amp hours or milliamp hours for small cells. These are pretty obvious. But what is the “C” rating? The C-rating is defined as a mul- tiple (or multiplier) of the amp hour capacity of the battery that results in a current that the battery is capable of discharging contin- uously over a full discharge cycle, in 1/Cth of an hour.
It’s the multiple of the capacity that keeps
the temperature generated inside the Li-Po battery low enough that the cell will not be damaged due to heat. A cell’s thermal inter- nal heat dissipation capacity is nominally about 6-watts/Amp-Hr. (This is a rule of thumb not an absolute value.)
There are obviously some bogus “C” rat- ings out there. A “C” rating of 60 would al- low one to completely discharge the battery in one minute without it destroying itself. A “C” rating of 10 would discharge the cell in 6 minutes. My suspicion is that some battery sellers develop their “C” ratings by seeing how much current they can pull out of cell before it ignites rather than the rate at which the temperature does not rise above 50 degrees Celsius—the temperature at which the cell degrades.
In addition to the high temperature prob- lem, there is a low temperature problem for charging. Crystals can form in the elec-
PHOTOGRAPHY: STEW MEYERS
Manual test setup with two VOMs and a light bulb load (above left) to the West Mountain Radio Computer Battery Analyzer (above center) is available from Radical R/C, and will show you the discharge curve for a battery. It does not compute the IR measure. The Medusa Power Analyzer (above right) set up
40
to measure IR in a manual mode. The external battery is used because the voltage of a single cell is too low for it to function. The Medusa is no longer made; however, Astro and Watt’s Up watt meters will work but don’t have the option of computer output.
JANUARY 2014
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