Ground Loop
05-08-2004, 07:13 PM
I've been making some investments in batteries for radio-controlled electric airplanes and thought you folks might get a kick of how the battery scene is shaping up on that side of the internet.
For many years, NiCd packs were all there was. Transmitters, receivers, and even electric motors strung together huge packs of NiCd cells. Sizes and shapes were all over, from extremely miniature to huge lethal packs of dozens of cells. NiCd was great because you could really abuse it -- charge in 15 minutes, discharge at hundreds of amps -- no problem. If you could handle the current, they could provide it.
Then NiMH came along and usurped some applications.. a little more expensive, and a lot slower to charge, but the densities were attractive. For the size and weight of a 600mAh pack, you can get around 1000mAh. Except where cost or current is a huge issue, NiMH has pretty much taken the place of NiCd.
Lithium Polymer is reshaping everything. The packs are super-light, a fraction of NiCd weight. They can be shaped into squares and prisms for more dense configurations. But they're Evil! They will explode if mistreated. You can't discharge them too much or they will go sour. You can't overcharge them, not even a little. You can't fast-charge them too much. You can't puncture them, short them, or dent the packs. Just about anything will result in EXTREME disaster. I watched a model jet burst into a ball of fire from a battery short. The only thing left was ashes. People have lost garages, homes, and in one case, a Lexus SUV to a battery going bad without supervision. It's a very hot and very fast fire, not easily predicted or extinguished. If you crash the plane, you get to sit and babysit the battery for half an hour to see if perhaps it explodes. For the first time, it's becoming common for people to build a stone, sand, or drywall "battery bunker" for charging. The threat is that significant.
But oh the capacities! Dancing with the devil gets you something like 2200mAh cells in 42 grams! (each 3.7v compared to NiCd 1.2v) Think about that.. 8.1 watt-hours in 42 grams. Our SAFT NiMH cells are 58g and 3.6 Watt-hours (3000mAh, 1.2v). I'm getting over THREE TIMES the energy density in these Li-Poly cells.
And the best news is that they're getting even better. 2000mAh cells simply didn't exist a few years ago. While NiCd densities haven't changed for a decade, LiPoly state of the art is tacking on about 5% every six months.
It's just a matter of time before the hurdles are overcome and we can pack two or three times the range into the same battery weight and size.
But oh the cost.. right now, to match the Segway pack's 3000mAh * 60 cells @ 1.2v (216 Watt Hours) would take about 23 of the very largest 2600mAh packs and cost around $550 just for the cells. BUT.. it would weigh only 1.2kg versus our 3.5kg and fit into a much smaller space.
Or for the same weight, we could buy $1500 in cells to get about three times the range. Less bleeding-edge cells are smaller and closer to 100mAh per dollar for a more realistic pack. More cells means more charging complexity and more potential for failure.
Then there's the small bit about fire and explosion risk, extreme cost, and the charging circuitry that would be an order of magnitude more complicated. But these are all solvable problems. Someone sufficiently motivated (and well-heeled) could build such a pack today and an off-board charging rig. It's not sci-fi, it's just difficult. Any engineer geek has to find these exciting times.
For many years, NiCd packs were all there was. Transmitters, receivers, and even electric motors strung together huge packs of NiCd cells. Sizes and shapes were all over, from extremely miniature to huge lethal packs of dozens of cells. NiCd was great because you could really abuse it -- charge in 15 minutes, discharge at hundreds of amps -- no problem. If you could handle the current, they could provide it.
Then NiMH came along and usurped some applications.. a little more expensive, and a lot slower to charge, but the densities were attractive. For the size and weight of a 600mAh pack, you can get around 1000mAh. Except where cost or current is a huge issue, NiMH has pretty much taken the place of NiCd.
Lithium Polymer is reshaping everything. The packs are super-light, a fraction of NiCd weight. They can be shaped into squares and prisms for more dense configurations. But they're Evil! They will explode if mistreated. You can't discharge them too much or they will go sour. You can't overcharge them, not even a little. You can't fast-charge them too much. You can't puncture them, short them, or dent the packs. Just about anything will result in EXTREME disaster. I watched a model jet burst into a ball of fire from a battery short. The only thing left was ashes. People have lost garages, homes, and in one case, a Lexus SUV to a battery going bad without supervision. It's a very hot and very fast fire, not easily predicted or extinguished. If you crash the plane, you get to sit and babysit the battery for half an hour to see if perhaps it explodes. For the first time, it's becoming common for people to build a stone, sand, or drywall "battery bunker" for charging. The threat is that significant.
But oh the capacities! Dancing with the devil gets you something like 2200mAh cells in 42 grams! (each 3.7v compared to NiCd 1.2v) Think about that.. 8.1 watt-hours in 42 grams. Our SAFT NiMH cells are 58g and 3.6 Watt-hours (3000mAh, 1.2v). I'm getting over THREE TIMES the energy density in these Li-Poly cells.
And the best news is that they're getting even better. 2000mAh cells simply didn't exist a few years ago. While NiCd densities haven't changed for a decade, LiPoly state of the art is tacking on about 5% every six months.
It's just a matter of time before the hurdles are overcome and we can pack two or three times the range into the same battery weight and size.
But oh the cost.. right now, to match the Segway pack's 3000mAh * 60 cells @ 1.2v (216 Watt Hours) would take about 23 of the very largest 2600mAh packs and cost around $550 just for the cells. BUT.. it would weigh only 1.2kg versus our 3.5kg and fit into a much smaller space.
Or for the same weight, we could buy $1500 in cells to get about three times the range. Less bleeding-edge cells are smaller and closer to 100mAh per dollar for a more realistic pack. More cells means more charging complexity and more potential for failure.
Then there's the small bit about fire and explosion risk, extreme cost, and the charging circuitry that would be an order of magnitude more complicated. But these are all solvable problems. Someone sufficiently motivated (and well-heeled) could build such a pack today and an off-board charging rig. It's not sci-fi, it's just difficult. Any engineer geek has to find these exciting times.