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Michael,
Being
one yourself, you should know the average, very-independent-minded experimental
aircraft builder holds strong opinions, especially in the area of safety. Many share
their experiences in that area bluntly and tersely, with the opinion the best
approach to guarding safety is “tough love, early and often.” I don’t
really know whether or not that approach is the most effective or not, but I DO
know being involved with a community that’s concerned with your personal safety
is a rare and valuable thing, one that can strongly and positively affect the risks
associated with dangerous activities like experimental aviation.
So,
despite the seemingly intentional sharp edges of some of the comments you’ve
gotten, I hope you are able to round the corners a bit, so to speak, and view them
as encouraging words of caution. Speaking for myself, I hope someday to benefit
in one way or another from your continued experimentation, I just want you to hold
safety as your paramount concern while doing so.
Regards,
Mark
Sletten
From: Michael McMahon
[mailto:afm528@gmail.com]
Sent: Monday, January 24, 2011 1:54 PM
To: lml@lancaironline.net
Subject: Re: [LML] Re: Lithium Batteries
I never claimed to be an
expert, and I've never advocated that people start using these batteries; I
said I was interested in them and had been doing a lot of research.
Tostino is far from the only person reporting that he does not balance his
cells. The SECOND link in my post is a thread with several hundred posts
on designing, building, and testing cells. Have any of the naysayers read
the thread, in depth, or done similar research? As I've written before,
I've been researching this for a long time, I'm not relying on one post from
one guy who did a few tests, it was just the first I pulled up of many, many
similar reports.
I also have read, but won't bother to post 15 links, that deep discharging,
such as Hamid had done, is different than the minimal discharge we would expect
from an engine start. The small discharge contributes to less need to
balance the cells. Quite different from the total-loss system in an RC or
solar-powered environment.
I'm an experimenter. I can't just sit on the sidelines and wait for
"someone else" to do the research, testing and development, then jump
on the bandwagon. That's why I'm building an experimental aircraft.
I thought this forum would be a good place to discuss development and testing
of new systems rather than quickly declare it "too risky" for
intelligent discussion.
Michael
On Mon, Jan 24, 2011 at 4:57 AM, <vtailjeff@aol.com> wrote:
The two worst places to have a fire...boat and plane. A IVP
was seriously damaged due to a battery fire. Risk v. Benefit.
It seems there may be some discrepancies between theoretical
extremes and real-world experience. Here's a link to one of the forums on
which people are reporting on their results (highlights mine):
Error! Filename not
specified.by tostino » Fri Sep 17, 2010 6:05 am
By doing discharges on the bench with the cells exposed, and
being able to tell that each string holds almost exactly the same voltage all
the way through the discharge, I am pretty sure I can tell they are not
horribly out of balance. I did IR matching of the cells, but no capacity
matching when I built the pack, so there is a little room for deviation i'm
sure, but it is not much I assure you, or I would have a really out of balance
pack at this point. I did a few 100% dod
cycles on the bench to test and see how out of balance they got... It went
right down to 3.2v/cell (resting) (2.9v under load) without them going out of
balance at all, and they then charged right back up to full and stayed
balanced.
You seem so certain that i'll kill my cells if I didn't have a cell level LVC
(with out the buffer space I provide it). My LVC is set to 46v with a 14s pack.
That means that it is 3.28v/cell. The resting voltage for each cell when I stop
the discharge is about 3.5v. I would have
killed them by now if it were going to happen, and they would have gone out of balance
now if that were going to happen too.
On Fri, Jan 21, 2011 at 8:35 AM, Hamid Wasti <hwasti@lm50.com> wrote:
Michael McMahon wrote:
I do not want to start an argument, but it is frustrating for people to
say they don't like ideas based on out-of-date or incomplete data.
It is even more frustrating when people proposed ideas based
on
incomplete data. Your "EV guru" friends are correct, you do not HAVE
to
cell balance a pack, but only as long as you are willing to live with
the limitations imposed by that choice. Do you know what those
limitations are?
When you have a number of cells of any chemistry in series in a battery
pack, they all receive exactly the same current when charging. Some
cells are a little more efficient than others so they get fully charged
before their colleagues in the string. If at that point they continue to
receive charging current, they will over-charge. Some chemistries are
able to handle this over-charging. Other chemistries like LiPo are very
intolerant of this over charging and quite literally blow up. Cell
balancing attempts to make sure that the charge on each of the cells in
the string is identical so they all get fully charged at the same time,
maximizing the charge that the pack can hold. That maximum number is the
one everyone throws around and that is the number you will expect to get
from your battery pack.
As I mentioned, you do not HAVE to cell balance. For a LiPo or LiFe
pack, you can just monitor the voltage on each individual cell and stop
charging when one of the cells gets fully charged. Over time, the
discrepancy in th charge state between the most efficient and the least
efficient cell in the string will keep increasing, with the usable
capacity of the entire pack being controlled by the charge in the least
efficient cell. Taking this to the theoretical extreme, at some point
the pack will not be able to deliver any energy because one cell will be
fully charged and another will be fully discharged. In real life, you
will declare the pack useless and stop using it before you get to that
point. If you are willing to live with this diminishing capacity, then
cell balancing is indeed not required. Just remember that your pack is
no longer going to have the same capacity as the pack that has cell
balancing and you must design the rest of your system to account for that.
Quoting the late Paul Harvey: Now you know the rest of the story.
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