Using DeWalt 36V Packs (M1)

DISCLAIMER: These articles are the findings and opinions of one person. Long-term durability of these cells is not known for our application/construction techniques. Any cell technology that we use is potentially dangerous. Do not work with these cells if you do not have a healthy respect for them, or if you lack the requisite skills.

This article takes a quick look at using the new DeWalt 36V cells in actual applications. It compares them with LiPo and NiMH cells in the same airplane. To learn more about these cells, you can access other articles by clicking at the bottom of this page. We can compare the new M1 cells with both LiPo and NiMH cells. Each has pros and cons. M1 is heavier than LiPo but lighter than NiMH. M1 is cheaper than LiPo but more expensive than NiMH, etc., etc. We tend to use rules-of-thumb in designing our models, e.g. watts-per-pound. Another useful one is that one LiPo cell is equivalent to 3 NiMH cells. This thumb rule could also have the corollary that one M1 cell is equivalent to 2-1/2 NiMH cells. The real world is often more complex. To look at a real-world example, I chose my Kyosho "40-size" Pitts biplane. I am happy with the way it flies. It has a 650-sq.in. wing area and weighs 6 pounds even. It is powered by one of dem new-fangled rotating-case jobbies, specifically an AXI 41/20/18 with a 13x6.5 APC-e prop. I use two Kokam 3Sx3200 packs in series. I consider these the gold standard in cells. They have more capacity than I need, but I'm conservative about pushing "C" ratings. ElectriCalc says it will climb 90 degrees. It does and I like that. My approach was to substitute cells in ElectriCalc, adjusting the number of cells to get the desired 90-degree climb. The four cells used were Kokam 3200, PolyQuest 2100, M1, and GP3300, a good NiMH cell. Some of the results are extracted below from ElectriCalc's Print screen, which lets you compare setups side-by-side. | Cell Type |KOK3200-20C |PQ2100 [HL] |DeWalt-M1 |GP3300 | | Cell Count | 6 | 6 | 8 | 25 | | Pack Wgt oz | 19.5 | 13.3 | 20.0 | 56.3 | | Cell Volts | 3.75 | 3.75 | 3.15 | 1.25 | | Cell mohm | 7.5 | 11.4 | 8.9 | 5.0 | | | | | | | | Motor KRPM | 8.94 | 8.66 | 9.46 | 10.45 | | Prop Watts | 625 | 568 | 740 | 999 | | Motor Watts | 753 | 687 | 898 | 1213 | | Motor Amps | 37.4 | 35.3 | 41.8 | 50.5 | | Motor Volts | 20.1 | 19.5 | 21.5 | 24.0 | | | | | | | | Battery Amps | 37.4 | 35.3 | 41.8 | 50.5 | | MAH | 2990 | 2060 | 2200 | 3210 | | % System Eff. | 74 | 72 | 70 | 63 | | % Motor Eff. | 83 | 83 | 83 | 82 | | | | | | | | Plane oz | 96 | 89 | 96 | 132 | | Wing oz/sqft | 21.3 | 19.7 | 21.3 | 29.2 | | Watts/pound | 126 | 123 | 150 | 147 | | | | | | | | Climb ft/min | 1588 | 1524 | 1821 | 1800 | | Cruise Minutes| 13.5 | 9.8 | 9.9 | 11.5 | | Stall MPH | 18 | 17 | 18 | 20 | | Max. MPH | 56 | 54 | 59 | 65 | | | | | | | | Thrust oz | 107 | 101 | 121 | 148 | | Drag oz | 11 | 10 | 11 | 13 | The first surprise is the cell count. 8 cells versus 6 might be expected with the difference in nominal voltage between LiPo and M1 chemistry. The surprise was how far off the rule-of-thumb was for NiMH. This is not due only to the increased weight that we're asking the motor to lift straight up, but also to the relatively high internal resistance of the NiMH. Even though the GP3300 has the lowest cell resistance, the rule-of-thumb says we need 2.5 or 3 to equal the other cells and this multiplies the equivalent resistance. 6 PQ2100's would be 68.4 milliohms. 8 M1's are a few milliohms more. 18 GP3300 would be 90 milliohms, 25 are 125 milliohms. It appears that the M1 setup has the performance edge over the LiPo setups. The same plane on 7 M1 cells would turn the prop about the same as the PQ2100 setup, but the slight weight difference gives the climb advantage to the LiPo. The performance of the NiMH setup is almost as good as the M1, but the wing loading is way too high for pleasant landings. The cruise time of the 3200 LiPo setup is by far the best. however, I fly 8-minute aerobatic flights plus taxi back on grass and put back about 1800 mah. So the cruise time number correlates pretty well with my experience. I suppose if my legs didn't get tired I might fly longer but I doubt it. Any of these setups would do the job for me. Now let's look at cost. Even at 25 cells, the NiMH comes in at only about $125. At the other end, the Kokam pack runs around $290. The PolyQuest runs about $160. Right now M1 cells cost $16 ($160 for DeWalt 36V pack, torn apart to get 10 cells). That puts the M1 pack at about $130. The M1 price will come down once the cells get to distributers. That's surface economics. The M1 cells are promised to last longer than LiPo and tolerate more abuse. They should last way longer than NiMH. Another real savings consideration is in charge times. Since the M1 cells can be charged essentially as fast as NiCads, you get to fly more often. To fly more often with LiPo, you must buy more packs. With NiMH, you either buy more packs or shorten their life by charging at greater than 1C. The M1 setup weighs exactly the same as my current setup and provides better performance in everything but flight time. An M1 pack costs less than half as much, and charges twice as fast. Guess I'll be taking another DeWalt pack apart! It appears that, barring some undiscovered shortcoming of these new cells, they should prove to be the product we need to convince the average glow flyer that "40-size" electrics are affordable.