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Most people are pulling A123 cells from Dewalt or Makita battery packs since A123 does not sell directly to the hobby market as far as I know.
A123's have 4 major benefits though over standard lithium-cobalt chemistry, first one being longer cycle life, second being high (30C+) continuous discharge rating, third, they can be speed charged at rates twice as fast as lipos, and lastly, they're made of a much safer chemical composition (lithium-nano-phosphate). Thermal runaway is almost non-existent with these cells, whereas a punctured lipo cell can be like a small hand grenade, and the higher their density, the more dangerous they become.
However, there are some notable downfalls with A123's. First off is their peak charge voltage is I believe only 3.6 volts vs 4.2 with standard lipo. This means you need more cells to make up the same voltage as a standard lipo battery pack, which obviously equates to more weight.
And, naturally, because they are a cylindrical cell form, they are heavier than a standard bagged lithium cell.
Regardless of which chemistry you choose, connecting them to a car battery is not the best method for charging. First off, a 15 aH pack will take a huge chunk of power from your car battery. Most car batteries are I think around 50 aH for a good quality one (if i'm understanding this bizzare "reserve capacity" rating right anyways) at peak charge. Since your car battery is not likely going to be at peak charge, you don't want to be draining that much juice from your car. You may not have a problem if the car is running, but it's hard to say. I've read about guys flying larger helicopters and using their car to charge batteries only to find their car won't start when they're ready to go home! Not the best situation to find your self in.
The other reason is that lithium chemistry, regardless of which one you use, requires dedicated charging hardware built specifically for it. One thing you definitely do NOT want is overcharged lithium cells, else you'll be treated to a spectacular fireworks show. I encourage you to read up very closely on the safety information of lithium batteries. It's great technology but you need to exercise a lot of caution with them as it's a fairly dangerous chemistry and NOT tolerant to negligent abuse.
In your case, you'll be working with high voltage and high capacity, which means you've probably only got 2 choices for charging. One is that you find someone to design a dedicated charger for the massive bank of cells you'll be using (likely expensive). The other is that you configure the packs so that they can be separated in chunks (i.e 22 volt packs at 5000 mAh x3 or something) at which point you can use a good commercially available lithium charger, though it'll take you longer to charge all 3 banks unless you buy multiple chargers to do them all at the same time.
I would suggest either purchasing an external car battery for charging (that you can then later on charge at home and then bring to the field), or just a small gas generator of some sort to power the charging equipment at the field.
Given the amount of cells you'll be working with, I would strongly encourage you to read all the proper handling procedures for lithium cells as I said above. Things like minimum discharge, charge, max current draw, cell balance etc. are all really important not only for the lifetime of the battery, but to also keep that large bank of packs from igniting during use (something you DON'T want happening when they're strapped to your back and you're 2000 feet up!). Specifically, because of the amount of cells, cell balance will be extremely important, so having a charger capable of balancing the cells is an absolute requirement. The batteries will be the most critical part of your setup and you need to put serious forethought into how the setup is going to work, and work safely.
Understand that you're basically trying to use commercially available hardware for an application it was never really intended for, so a large amount of the brunt work is left up to you to ensure things not only work correctly but to a safe enough degree. What you plan to do should be considered "highly experimental" considering the sourcing of the components. 
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Daniel
True Northern Hobbies, Inc.
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All Forums >> RC Helicopters >> RC Helicopter General Discussions >> Need a 3800 Watt or larger brushless outrunner 
Need a 3800 Watt or larger brushless outrunner - 
. They also do another one that I think produces close to 12kW or something. 
. For a motor that costs $40, I can't believe it's still going! 