High-flying with A123
By Gary Goodrum
We're not driving electric cars powered by A123 advanced lithium ion batteries... yet, but we are flying them in helicopters; remote-controlled electric helicopters, that is. And if you're curious about how good they are, read on.
I was very happy to read that A123 Systems will be involved in developing a new battery for GM’s Chevy Volt E-Flex program. I became familiar with them because of a side business/hobby that I have, selling and flying remote controlled electric helicopters.
Normally, we use high capacity lithium polymer packs but these are extremely volatile and hugely expensive. The packs required to fly a typical model that has 1.3m rotor disc size and that weighs about 8 pounds typically cost about $450-$500. This "sticker shock" has kept quite a number of people flying gas-powered models from converting over to electric, even though our electric versions can match the durations they get, while providing double the available horsepower. They just can’t see carrying around "a year’s worth of Nitro…" on every flight.
Early last year, in our never-ending quest to bring these poor souls over from the "dark side", a few of us started using the lithium ion cells out of a Milwaukee 28V "v28" tool pack, which could be had for about $85 on eBay. Each pack had seven cells that could be assembled into the form factor we needed for the various models. Although these proved to be a great entry level solution for someone on a budget, the cells themselves were heavy and they didn’t have the current delivery capabilities that the better lithium polymer (LiPo) packs can provide. The LiPo cells we use typically have capacities that vary from about 3.7 to 5.0 Ah, with nominal voltages of 3.7-3.8V. Most setups use packs that have 8-12 of these in series. These are rated by what multiple of their capacity they are capable of delivering continuously. A high-end LiPo cell right now has a "C" rating of 20-25C, which means a 20C 4Ah cell can deliver 80A continuously (4.0 x 20…). In helicopters, we don’t fly with continuous loads but we might hit peaks that are much higher. Most of the latest LiPos can supply about 30C for bursts up to 5 seconds, or so.
Then about six months ago, DeWalt finally released their new line of Lithium-powered tools that are based on a new cell chemistry developed by A123Systems, which is a spin-off of MIT. These new cells have some very unique characteristics that are really nothing short of revolutionary. Using a patented "nanotechnology" process that dramatically reduces the particle size for the lithium material, they come up with a cell that is incredibly robust and completely safe. Because of the small particle size, all of the Lithium gets converted during charging and discharging. What this means is they don’t blow up if you inadvertently overcharge them, something that is definitely an issue with LiPos. I have personally had two cases where faulty chargers cause a pack to explode in a fireball that is pretty spectacular. This has caused us to now only charge these in "battery bunkers". I’ve also over discharged packs in flight and have had models completely destroyed in fiery explosions. These A123 cells just can’t do that, either charging or discharging.
Soon the same eBay dealers that were selling v28 tool packs, started offering these new DeWalt 36V packs. Each pack contains 10 A123-M1 cells. Although these cells are about the same size as the "eMoli" cells in the v28 packs (both have cylindrical cans, 26mm in diameter; the eMoli cells are 70mm in length and the A123-M1s are 65mm long), but the aluminum-cased M1s are quite a bit lighter (70 gm vs 100 gm…). These M1s have some other differences as well. The nominal voltage is lower, at about 3.0V per cell, vs about 3.6V for the eMolis. Capacities are similar (The M1s are 2.3 Ah vs 2.8 Ah for the eMolis…) but the "C" rating for the M1s is off the charts. The eMolis are good for about 10C continuous and no more than about 15C burst. About three years ago that’s where we were with LiPos. The M1s are rated about 30C continuous and a whopping 52C burst. That works out to 70A and 120A, respectively.
Anyway, it took us awhile to optimize our helicopter setups to be able to use these (we have to run more in series to get equivalent voltages that we have with LiPo setups, but these can deliver more current), but now that we have, I think these will "fuel" a new wave of converts. Now we can have setups that don’t cost your first-born, without sacrificing performance, which has been my own personal "holy grail" pursuit. In addition to the cost and safety factors, one other huge benefit we get with these is that with the right charger, these cells can be completely recharged in as little as 5 minutes! In practical terms for us, where most of the high-end chargers we use top out at a 10A charge rate, this means recharge times of 15 minutes, which is still great compared to the hour a LiPo pack needs. Some of the newer LiPos can be charged at a 2C rate (i.e.—a half hour…) but people are finding out this cuts down typical cell life from 150-200 cycles, down to about 50-75 cycles. The A123-M1s are supposedly good for over 1000.
Anyway, I thought you might be interested in our experiences with these remarkable new batteries. I know A123 has been working in the defense world, so I’m sure these will make their way into a number of exotic electric powered UAVs and ground vehicles, but I hadn’t heard until this last week about any forays into the EV domain. If this technology can be "ported" over to the automobile application with anywhere near the level of performance we seen with these in our little world, I predict that we won’t need to wait anywhere near as long as some of the so-called pundits are claiming it will take to develop a battery that will allow GM to bring the Volt and its "cousins" to market.
Gary also runs a part-time business providing hobbyists with battery packs.
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