Understanding and Measuring Cell Performance

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THE RAGONE CHART IS THE TRULY EFFECTIVE WAY TO EVALUATE CELL PERFORMANCE.

Study the attached chart carefully. It is the most significant tool for understanding battery performance that you will find anywhere. This particular method lets one compare any form of energy from batteries to jet engines. For now, we will stick to batteries including Li Pos and Ni MH since Ni MH is next best to Li Po for specific energy. There are several powerful observations about this chart. To date, there have been all kinds of claims made for Li Po cells, most with no substantiation; just claims about how great the cells are! The Ragone plot sorts it out in a way that lets you judge. The Ragone plot is a useful tool for many things. We haven’t time to go into all the mechanics of it here. However, you are invited to review a 2-part article that Matt Keenon wrote about it in Microflight magazine Sept//Oct 2004 that presents it fully. The Ragone plot is developed from discharge curves. Calculations are for V avg X A= watts and watts X time with both divided by cell weight.
2. The left ordinate shows what the battery can do as the wattage demanded from it increases. If some claims heard for certain chemistries are to be believed, those cells could deliver 150 amps/cell with no loss of capacity. For a realistic cell, as we increase current drain, the cell loses some capacity and voltage is depressed. If not, it would mean that the cell had zero internal resistance! This chart shows reality. The bottom scale is watts and that is the product of average voltage and average current. If, e.g., a cell shows a lot of voltage depression, then the wattage falls. If the cell can handle only, say, 5C current drain, Watt hours and watts are depressed and the only way to get higher and to the right is by paralleling packs. Note the statement at the top , right. This is why we see some packs marketed that have four packs in parallel in order to handle 40 or so amps. Those packs are very large and very heavy.
3.The red tick marks show how to get more W-H out of a given pack. The cell shown is the very popular KOK 1500, the staple of park flyers. If you try to demand > 1600 watts/kg, you won’t have much W-H and it takes W-H to get flying time. By paralleling four packs and demanding 400 watts/kg per parallel pack, you could fly for more than 20 minutes. You could fly a 4 lb model at 100 watts per pound of model and have an aggressive aerobatic model at this design point. LIPOCALC II at our web site, will let you optimize the design as it walks you through this process automatically. You input the volts you want and amps and it does the rest.
4. Flying time is the dotted red line that is derived by dividing W-H/Kg by the Watts/Kg demanded. In the example, flying time is increased from almost nothing for a 1P pack to over 20 minutes for a 4P pack.
5. As you add more cells, pack weight goes up and watt-hours goes up. Going from 1P to 2P doubles W-H, but so does weight( and obviously, cost). The key parameter is how much energy you can get from a given weight of cell as current drain is increased. This can only be done by cell selection.
6. If you divide the specific energy on the left by specific power demanded at the bottom, the run time is the product. For example, the cell shown can deliver 1250 W/kg at 120W-H/kg and 120/1250=0.096 hrs=5.76 minutes; the 2P tick mark. Since the cell weighs 0.032 Kg, the W-H= 3.84 and watts=40.

This post is to ullustrate the use and development of the Ragone plot. Posts will be added to present data for currrent cells available.
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