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New! End-to-end soldering tutorial
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I feel that pack building is an essential skill for an eheli pilot. It can also be used for pack reconfiguration and crash repair. |
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Assuming that you already know the basics of how to build a flight pack (tutorial here), here's a method that gives an electric heli flight pack which stands up well to physical abuse (survives some crashes). After all, a reliable battery is very important, especially if one uses a BEC such as the UBEC or one of Mikado's units. Two styles are described, the one for a LOGO 10 having an internal plywood keel which also serves as an insulator between the rows of bare cells. In this case, the keel provides so much support that the individual sticks of cells do not need to be shrink wrapped and the pack will cool off better without 2 layers of shrink wrap. One wouldn't expect this, but the LOGO 10 pack with the ply keel is lighter than a pack made in the usual manner with the labels left on. The packs with the inner shrink the entire length of the pack are a lot stronger than a pack made in the usual manner. I use Astro Zero Power Loss connectors on my packs. I like them because of the their sturdy construction, ease of insertion, and definite polarity. |
For all heli's:
For LOGO 20 and other heli's:
For LOGO 10:
For all heli's: finish off the pack in your usual manner and add outside layer of heat shrink. |
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Why is extra support needed for a LOGO 10 battery? About half of the battery hangs out of the frame unsupported. The battery gets quite hot during a flight and the softened heat shrink will easily stretch from G-forces alone. The pack on the right has each of the two long sticks of cells shrink wrapped underneath the outer shrink wrap. It still developed a "banana" profile over a series of flights. BTW, I don't like that hump configuration because the battery takes a LOT longer to cool down after the flight. I've since switched to single layer packs. |
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Here's the same pack reconfigured to a more practical shape. The sticks of bare cells were glued to a 1/16" aircraft ply keel and covered with clear heat shrink. It was a long road for these cells, originally used in 10 cell packs for my Voyager. Note that the insulators are fashioned from masking tape between the cell ends. In this particular pack the case of the cell on the end was soldered directly to the "+" button of the adjacent cell, instead of using a piece of braid. It's a little harder to do and I don't bother anymore. The two layers of factory heat shrink add a lot of weight. With them removed, this pack weighs no more than one built in the normal fashion, even with the keel. And it's a lot stronger. I've thought about doing this for all my packs because the bare cells are accessible for repair and trouble-shooting by removing just one layer of heat shrink. |
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An alternate battery brace method:
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These photos from Rick Conkling show a practical means and source for a brace that's easily applied to a pack aleady assembled. Rick writes: I was at the LHS and seen the Sig fiberglass pushrods hanging on the wall. I took a pack home and it seemed to be an easy lightweight modification to support the packs. I just cut off the original shrink wrap, cut the rod to length and re-applied two layers of the Parma shrink wrap that the LHS carried. The mills were thinner on the Parma than what originally came on the cells I got from Cyberheli so this was the reason for the two layers. I have not had any of my 5 packs droop a bit since the mod, my cells reach 160 degrees after a flight. |
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And of course, now that the Mikado battery brace is widely available: Photo's of this brace kindly provided by Dana Ferguson. Use a bit of servo tape to hold the brace in place on the frame rails. Just use a normal battery pack. He reminds us that one needs to use a rubber band to hold the end of the pack to the brace if flying inverted very often |
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Flight batteries perform noticeably better when charged just before the flight. Wether it's been a half-hour or a week since the pack has been sitting charged, just re-peak and fly without much concern. The battery will be warm to the touch when properly peaked. I often cheat and charge all the packs at home, using my 120V power supply to power the charger and topping them up just before the flight. This allows me to have less of a wait between the first few flights and has the added advantage of giving me a couple of extra flights in a day because there is only a finite amount of energy available from my charging batteries. The Orbit charger has a charge limiter which can be set to cut off after a specified m.a.h. charge has been done. This allows one to pre-charge the packs to only 80% or so before leaving for the field. It is best that a new or rebuilt pack is "equalized" before using it. This is done with a 16 hour slow-charge at C/10, C being the m.a.h. rating of the cell. In the case of our 2400 m.a.h. cells that would be a 250 m.a.h. charge rate. This charge rate is one that the cells can take for an indefinite period of time (because they can tolerate dissipating the heat generated), allowing the least charged cells in the pack "catch up" without ruining the fully charged ones. |
One reason that I still have an Astro charger (112D) is that its "test mode" can be used to equalize a pack without interference from the peak detection that would shut down the (over)charging session early. Just hold the start button when powering up the charger, attach the battery, push the button again, and dial in the amps. As the pack charges the dial will have to be adjusted to keep the rate at the desired amount. Be careful to monitor the pack throughout this manual operation. You can ruin your pack. There should be a nice stable heat coming off the pack - nothing extreme. The best charge rate from then on is about 4C, or 10 amps. Most chargers do no more than 5 or 6 amps. I manage to drain my flight batteries in about 6 minutes, which is 1/10 of an hour which means that the average current is 24 amps. One of the fellows that developed the Schulze 35 Ho ESC said that they were recording up to 90 peak amps during hard application of load to the motor. Soooooooooo, a good discharge rate for the RARE time that you might feel like discharging them would be up to 20 amps:) The best way to "cycle" your battery pack is to go fly! |
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Here's some "real-life" data about charging battery capacity... I started with a single 34 AH gel cell wheelchair battery and then increased capacity by putting two of them in parallel for charging my LOGO 20's 20 cell packs. I would arrive at the field with the packs "pre-charged" for the first round to get close to 8 flights in a day. When I changed to 24 cells, I purchased an Optima "yellow top" deep cycle battery. The smaller batteries were still needed though. This season I proved reduced capacity with the Optima and got another one for free. This made it possible to get lots of flights, but I still could drain both Optima's in a day. To survive a weekend fun fly, now that I camp instead of staying in a motel where I could use 120V to charge up my flight and charging batteries, I have to rely on the kindness of locals to charge up the Optima's overnight. When Ralf and Inge from Mikado were coming here to spend a week with me and "my" Rockies, I ran out and bought a Sears Diehard "marine starting and deep cycle battery". It is wet, but maintenance-free (minimizes spillage), has 750 marine cranking amps and is group 27, 160 minute reserve. It has at least twice the capacity at half the cost of an Optima. The drawback is that it as not an absorbed electrolyte type, which means that it could spill in an auto accident. ANYWAY, Ralf and I managed to drain the Optima's and the Diehard in two days at two fun fly events, flying lots, but mostly the LOGO 10s. |
I estimate that at least 6-10 flights were done with LOGO 20s, though. The weekend before this, I was in heaven at another fun fly with all those batteries. I flew and flew and flew, but still managed to drain them. People commented that every time they turned around, I was flying, hehe. If you have a truck, then the Sears battery is a no-brainer. Otherwise, the increased risk of transporting it in the passenger compartment needs to be considered. I have two Schumacher "Ship 'n Shore" deep cycle battery chargers, purchased at Wall Mart at a low price. They can be plugged in at my parking stall at home, so I do not have to fuss much with charging or lifting the deep cycle batteries. They are automatic, 10 amp chargers that one can plug in and forget. I have Astro connectors on the fused leads from the charging batteries as well as the chargers and anything else (no alligator clips). I just "plug and play". I've increased the auto-cutoff voltage from 10v to 10.5V on my Orbit Microlader Pro charger to help preserve the batteries. Charging at 8 amps tends to suck down the voltage sooner and I can squeeze in a little extra charging by reducing the charge rate to 2-4 amps on the 24 cell packs. Charging 24 cells at 8 amps pulls 25 amps out of the charging battery! |
