Don Stackhouse @ DJ Aerotech
May 25, 2003, 04:03 AM
This message from "Don Stackhouse @ DJ Aerotech" <djaerotech@erinet.com> brought to you by EFLIGHT!
With all the discussion of Lithium-polymer batteries going around various
e-mail lists these days, it looks like a good time for me to "spill the
beans" on our own fairly extensive tests of these batteries in our
"Roadkill Series" profile indoor/backyard/parkflyer electrics.
We've had a LOT of inquiries in this year about the use of various types of
lithium batteries in our models, but until recently we were reluctant to
endorse anything in particular. We had concerns about how well suited the
available batteries were for our particular kits, how well they would
perform in general, and about the availability of simple and reliable but
economical chargers for lithium cells (we had serious philosophical
objections to having to tell someone they needed a complex
multi-hundred-dollar charger to charge the batteries in their relatively
cheap and very simple model). There were also the obvious safety concerns,
based on all the rumors of fires, etc..
As far as chargers go, we've had outstanding results with the Plantraco
LPC-400. It can charge 1, 2 or 3 cells at 100, 200 or 400 milliamps, comes
with an AC adapter, has internal checks to make sure you connected
everything properly and made the correct settings in two switches on the
front for the charge rate and number of cells, has a built-in cooling fan,
is extremely simple to use, has a very nice charging profile that takes
excellent care of the health of the batteries, and does all of this for a
very economical price. We've used it very successfully for everything from
45 mah cells to 3300 mah 3-cell packs.
The only drawback to using the LPC-400 for the 3300 cells was the several
hours it needed to charge them at its max rate of 400 milliamps. However,
we've found that at least for Roadkill sized models with their cells of
1200 mah or less, you get so much flight time per charge that it's rarely
necessary to recharge during a flying session. In addition, the Li-poly
batteries hold their charge literally for years, unlike nickel-based cells.
It is not necessary to re-peak them just before flight. The Li-poly
batteries I flew for the entire evening flying session at the Bowling Green
indoor demo at Toledo this year were not recharged at all during the
evening, still had most of their charge left at the end of the evening, and
had last been charged as much as a week or more before that evening.
We have some other chargers we're investigating as well, including some of
the new ones from E-tec, and it looks like there are now a variety of
cheap, simple and safe chargers for at least the smaller sizes of Li-poly
cells.
The fire safety issues we've heard of for the most part seem to be related
either the older Lithium-metal cells, or to folks charging Li-ion and
Li-poly cells with nicad chargers, or using older chargers with inadequate
error checking and then setting the number of cells and/or charge rate
settings incorrectly. The newer chargers seem to do a much better job of
addressing these issues, such as the checking to see if the charge and
number of cells settings agree with the electrical characteristics they
sense for the pack being charged. Nothing is perfect of course, we still
need to be diligent with our charger setups and we need to monitor packs
while they are being charged, but from what we've seen it appears to now be
a manageable issue. I've heard of nicads exploding and even causing fires
too, so the issues folks seem to be so afraid of with the lithium cells are
not as new or unique as a lot of you seem to believe.
As far as maintaining balance between the cells in a pack, It does not seem
to be a significant issue in actual practice, at least in our experience on
these models. I've charged individual cells one at a time, but I don't
bother with that any more. I do occasionally check individual cell voltages
with a voltmeter, but so far they've always matched with each other within
0.01 volts, even on packs that see high discharge rates such as in the Ryan
ST during one of my aerobatic workouts.
So what about cells for these models?
The old Lithium-metal cells simply were not suited for our size of models
(much too big and heavy), as were the lithium-ion cells we investigated.
Regarding the currently available sizes of Kokam cells:
Two cell packs of the little 145 mah cells don't seem to have the power
output and the necessary max current capacity, except for the most
efficient MPS-1A models that don't need more than about 40% throttle for
cruise. The Cub, A6M2 Zero and the Curtiss-Wright Junior may be OK based on
this. Flight times might be acceptable, as long as you're willing to put up
with a significant loss of full-throttle power. The weight reduction helps
a lot, but the power loss more than offsets this. The max current limits on
the Kokams indicate that on models that use higher power settings than that
for a significant percentage of the flight, the cells might be damaged.
We're still investigating that, but so far it appears that the best
application for the Kokam 145 cells is as a single cell in an RFFS-100
equipped plane, or similar, which rules out most of the Roadkill models for
the present time.
Two cell packs of the larger 500 mah KoKam cells do OK in our twins and
MPS-2A powered models, about the same performance as the 230 mah NiMH cells
but with more flight time. The shorter-winged models with a single MPS-1A
do not seem to like the weight of the KoKam 500 mah cells. The Me109 Joe's
son Terry was flying at Bowling Green had one of our new MPS-2A twin-motor
systems with a pair of Kokam 560 cells. It's lighter than that same
airplane with a 230 mah NiMH pack but with almost the same power at full
throttle, and as anyone there could see it's a very happy airplane with
that setup.
Note, the Kokam cells do seem to be good quality products from what we can
tell, it's just that the presently available cells do not seem to have the
best electrical properties for the needs of our models, at least in
comparison to the other presently available options. We've heard of some
new Kokam cells that should be available soon and that should be a better
fit, and we're looking forward to testing them as soon as they are available.
Regarding the new E-tec cells that are now available:
These seem to have significantly more current output than the Kokam cells,
both in terms of what they can tolerate without damage and in what they
will actually produce in service (officially about 4-6C max continuous and
up to about 7C for brief periods), and come in sizes that seem better
suited to our company's current models.
A two cell pack of the E-tec 250 mah cells is fully 1/2 ounce lighter than
a 7-cell 110 mah NiMH pack (our standard NiMH recommendation for indoor
flying with a single MPS-1A). Full-throttle power at the beginning of the
run is slightly less (about 90%), but with the weight savings the overall
performance seems to be about the same or slightly better, and the flight
times are far better. Power at cruise is about the same as good 110 mah
NiMH cells, and power available at all throttle positions near the end of
the flight is a little better. I recently flew the Roadkill Series
Curtiss-Wright Junior with a 2-cell 250 mah E-tec pack outdoors in measured
winds averaging 10 knots with frequent gusts to 12 and occasional gusts to
15. This required about 75% on the throttle stick position (we can normally
cruise the Junior at about 1/4-1/3 throttle stick position in calm air),
and with the old 110 mah NiMH pack I would have expected about 7-8 minutes
at the most at these power settings. The E-tec cells lasted a very solid 22
minutes! It appears that the E-tec 250 mah 2-cell pack may be the best
presently available all-around choice for most of our models powered with a
single MPS-1A.
Flying this same Junior in calmer conditions resulted in a 32 minute
flight, most of that at about 40% throttle.
Earlier this week we tried the RK Series Sopwith Camel and Fokker Triplane
with the 250 mah 2-cell E-tec batteries. Two Li-poly cells is a little less
voltage than a 7-cell NiMH pack, so that's a little less rpm from the prop
and therefore a little less power. For most of our single-motor models the
1/2 ounce weight reduction more than makes up for this, but in the case of
the Camel and the Triplane there is a lot of stuff being dragged through
the air on not very much wingspan, so there is a lot of both induced and
parasite drag. Both of these models, especially the Triplane, like lots of
power and end up spending most of the flight at pretty high throttle
settings. Their short span also makes them very sensitive to weight, which
can offset some of the extra power from going to an MPS-2A and a larger
NiMH battery. Performance with the 2-cell 250 mah E-tec batteries was very
promising. There was still no great surplus of power, but we got flight
times of over 20 minutes, where with the 110 mah NiMH batteries times of
4-6 minutes were more typical. The Triplane seemed to be especially happy
with the new setup, where that 1/2 ounce weight savings made a huge
difference for its tiny wingspan (the smallest span of the entire RK
Series). Handling was noticeably livelier, and its already tight turning
radius was even tighter.
(continued in part 2)
Don Stackhouse @ DJ Aerotech
djaerotech@erinet.com
http://www.djaerotech.com
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With all the discussion of Lithium-polymer batteries going around various
e-mail lists these days, it looks like a good time for me to "spill the
beans" on our own fairly extensive tests of these batteries in our
"Roadkill Series" profile indoor/backyard/parkflyer electrics.
We've had a LOT of inquiries in this year about the use of various types of
lithium batteries in our models, but until recently we were reluctant to
endorse anything in particular. We had concerns about how well suited the
available batteries were for our particular kits, how well they would
perform in general, and about the availability of simple and reliable but
economical chargers for lithium cells (we had serious philosophical
objections to having to tell someone they needed a complex
multi-hundred-dollar charger to charge the batteries in their relatively
cheap and very simple model). There were also the obvious safety concerns,
based on all the rumors of fires, etc..
As far as chargers go, we've had outstanding results with the Plantraco
LPC-400. It can charge 1, 2 or 3 cells at 100, 200 or 400 milliamps, comes
with an AC adapter, has internal checks to make sure you connected
everything properly and made the correct settings in two switches on the
front for the charge rate and number of cells, has a built-in cooling fan,
is extremely simple to use, has a very nice charging profile that takes
excellent care of the health of the batteries, and does all of this for a
very economical price. We've used it very successfully for everything from
45 mah cells to 3300 mah 3-cell packs.
The only drawback to using the LPC-400 for the 3300 cells was the several
hours it needed to charge them at its max rate of 400 milliamps. However,
we've found that at least for Roadkill sized models with their cells of
1200 mah or less, you get so much flight time per charge that it's rarely
necessary to recharge during a flying session. In addition, the Li-poly
batteries hold their charge literally for years, unlike nickel-based cells.
It is not necessary to re-peak them just before flight. The Li-poly
batteries I flew for the entire evening flying session at the Bowling Green
indoor demo at Toledo this year were not recharged at all during the
evening, still had most of their charge left at the end of the evening, and
had last been charged as much as a week or more before that evening.
We have some other chargers we're investigating as well, including some of
the new ones from E-tec, and it looks like there are now a variety of
cheap, simple and safe chargers for at least the smaller sizes of Li-poly
cells.
The fire safety issues we've heard of for the most part seem to be related
either the older Lithium-metal cells, or to folks charging Li-ion and
Li-poly cells with nicad chargers, or using older chargers with inadequate
error checking and then setting the number of cells and/or charge rate
settings incorrectly. The newer chargers seem to do a much better job of
addressing these issues, such as the checking to see if the charge and
number of cells settings agree with the electrical characteristics they
sense for the pack being charged. Nothing is perfect of course, we still
need to be diligent with our charger setups and we need to monitor packs
while they are being charged, but from what we've seen it appears to now be
a manageable issue. I've heard of nicads exploding and even causing fires
too, so the issues folks seem to be so afraid of with the lithium cells are
not as new or unique as a lot of you seem to believe.
As far as maintaining balance between the cells in a pack, It does not seem
to be a significant issue in actual practice, at least in our experience on
these models. I've charged individual cells one at a time, but I don't
bother with that any more. I do occasionally check individual cell voltages
with a voltmeter, but so far they've always matched with each other within
0.01 volts, even on packs that see high discharge rates such as in the Ryan
ST during one of my aerobatic workouts.
So what about cells for these models?
The old Lithium-metal cells simply were not suited for our size of models
(much too big and heavy), as were the lithium-ion cells we investigated.
Regarding the currently available sizes of Kokam cells:
Two cell packs of the little 145 mah cells don't seem to have the power
output and the necessary max current capacity, except for the most
efficient MPS-1A models that don't need more than about 40% throttle for
cruise. The Cub, A6M2 Zero and the Curtiss-Wright Junior may be OK based on
this. Flight times might be acceptable, as long as you're willing to put up
with a significant loss of full-throttle power. The weight reduction helps
a lot, but the power loss more than offsets this. The max current limits on
the Kokams indicate that on models that use higher power settings than that
for a significant percentage of the flight, the cells might be damaged.
We're still investigating that, but so far it appears that the best
application for the Kokam 145 cells is as a single cell in an RFFS-100
equipped plane, or similar, which rules out most of the Roadkill models for
the present time.
Two cell packs of the larger 500 mah KoKam cells do OK in our twins and
MPS-2A powered models, about the same performance as the 230 mah NiMH cells
but with more flight time. The shorter-winged models with a single MPS-1A
do not seem to like the weight of the KoKam 500 mah cells. The Me109 Joe's
son Terry was flying at Bowling Green had one of our new MPS-2A twin-motor
systems with a pair of Kokam 560 cells. It's lighter than that same
airplane with a 230 mah NiMH pack but with almost the same power at full
throttle, and as anyone there could see it's a very happy airplane with
that setup.
Note, the Kokam cells do seem to be good quality products from what we can
tell, it's just that the presently available cells do not seem to have the
best electrical properties for the needs of our models, at least in
comparison to the other presently available options. We've heard of some
new Kokam cells that should be available soon and that should be a better
fit, and we're looking forward to testing them as soon as they are available.
Regarding the new E-tec cells that are now available:
These seem to have significantly more current output than the Kokam cells,
both in terms of what they can tolerate without damage and in what they
will actually produce in service (officially about 4-6C max continuous and
up to about 7C for brief periods), and come in sizes that seem better
suited to our company's current models.
A two cell pack of the E-tec 250 mah cells is fully 1/2 ounce lighter than
a 7-cell 110 mah NiMH pack (our standard NiMH recommendation for indoor
flying with a single MPS-1A). Full-throttle power at the beginning of the
run is slightly less (about 90%), but with the weight savings the overall
performance seems to be about the same or slightly better, and the flight
times are far better. Power at cruise is about the same as good 110 mah
NiMH cells, and power available at all throttle positions near the end of
the flight is a little better. I recently flew the Roadkill Series
Curtiss-Wright Junior with a 2-cell 250 mah E-tec pack outdoors in measured
winds averaging 10 knots with frequent gusts to 12 and occasional gusts to
15. This required about 75% on the throttle stick position (we can normally
cruise the Junior at about 1/4-1/3 throttle stick position in calm air),
and with the old 110 mah NiMH pack I would have expected about 7-8 minutes
at the most at these power settings. The E-tec cells lasted a very solid 22
minutes! It appears that the E-tec 250 mah 2-cell pack may be the best
presently available all-around choice for most of our models powered with a
single MPS-1A.
Flying this same Junior in calmer conditions resulted in a 32 minute
flight, most of that at about 40% throttle.
Earlier this week we tried the RK Series Sopwith Camel and Fokker Triplane
with the 250 mah 2-cell E-tec batteries. Two Li-poly cells is a little less
voltage than a 7-cell NiMH pack, so that's a little less rpm from the prop
and therefore a little less power. For most of our single-motor models the
1/2 ounce weight reduction more than makes up for this, but in the case of
the Camel and the Triplane there is a lot of stuff being dragged through
the air on not very much wingspan, so there is a lot of both induced and
parasite drag. Both of these models, especially the Triplane, like lots of
power and end up spending most of the flight at pretty high throttle
settings. Their short span also makes them very sensitive to weight, which
can offset some of the extra power from going to an MPS-2A and a larger
NiMH battery. Performance with the 2-cell 250 mah E-tec batteries was very
promising. There was still no great surplus of power, but we got flight
times of over 20 minutes, where with the 110 mah NiMH batteries times of
4-6 minutes were more typical. The Triplane seemed to be especially happy
with the new setup, where that 1/2 ounce weight savings made a huge
difference for its tiny wingspan (the smallest span of the entire RK
Series). Handling was noticeably livelier, and its already tight turning
radius was even tighter.
(continued in part 2)
Don Stackhouse @ DJ Aerotech
djaerotech@erinet.com
http://www.djaerotech.com
*** Any complaints or problems? Send an email to monitor@ezonemag.com
*** For help with list commands go to http://www.ezonemag.com/pages/mailhelp.htm
*** For the list rules go to http://www.ezonemag.com/pages/mailrule.htm