On 10/17/2012 7:35 AM, Evan Ludeman wrote:
On Oct 17, 9:39 am, wrote:
Eric and Evan: you can't have it both ways, either the LiFePo
chemistry is safe or it isn't. If you agree that there is no
safety difference between the cheap HK cells and those you find in
the referenced motorcycle batteries, the fact that HK is used in
"toys" should not make a difference. Apropos toys, what do you
think the average American Joe is calling our gliders? I will of
course encase the HK battery and provide a 5A fuse inside that
enclosure before trying it in my glider. I'll let you all know how
it goes.
Herb: there's a lot to go wrong here that has nothing whatsoever to
do with chemistry, and this was my point. A fuse external to the
pack will do nothing to protect you from an internal short or a
short across one of the cells. Be especially critical of those
balance leads, I've had issues with those on LiPo packs.
We're talking about low probability events here... but event with
potentially high impact. If you install this battery and use it in
your glider, chances are 99%+ that it will work out fine for the
next year.
Here's my second point: this will not prove that this is safe.
Humans are extraordinarily bad at assessing risks in the 0.01 - 1%
range. We do something a little edgy and get away with it 20 times
(thermalling off a ridge with 150' of clearance, let's say) and think
"we've got this figured out"... when statistically we'll almost
certainly see it go wrong if we do that same thing 1000 more times.
If ten thousand of us install those HK batteries, I am confident that
we *will* have problems. Those batteries are designed for high
discharge rate performance and the compromise is safety. This is
acceptable for toys. Much less so for human carrying vehicles.
LiFePO4 cells designed for safety will be far more robust
structurally, and the necessary compromises will be lower
performance and higher cost (more material for the same capacity,
better QC, circuitry for internal charge/discharge regulation and
cell balancing, etc.). It looks like this is what the K2 guys are
pursuing.
My $0.02. I'm not a battery engineer, just a skeptical self
educated end user. Sorry if this came off as rock throwing.
Evan Ludeman / T8
Herb, I completely agree with Evan. The HK batteries were not designed
for our use, there is no evidence they've been tested for our particular
pattern of usage, nor is there a history of successful use for our
purposes.
Given the safety and low cost of SLA batteries, I don't see a compelling
reason to use HK batteries. If a pilot needs more capacity, I suggest
finding a way to add another SLA, or reducing the battery drain so the
present SLA is adequate. At the very least, wait another six months
until the soaring season is near, and see what is on the market then. I
bet there will be better, maybe much better, choices than we have now,
as battery situation is changing quickly.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz