On Oct 17, 9:39*am, wrote:
On Tuesday, October 16, 2012 12:40:54 PM UTC-5, Eric Greenwell wrote:
On 10/16/2012 8:06 AM, Evan Ludeman wrote:

Dear Unknown, Here is the battery I just ordered with the intent to

replace my 10Ah NiMH battery dedicated to the transponder. *It is

ready for usage, no soldering except for the fuse and the

plug.http://www.hobbyking.com/hobbyking/s...dproduct=20645

* This is a 8400mAh 4S cell with a 30C discharge rate, you can fry your

* eggs on the discharge of that thing (over 200A).

I've fooled around with RC model batteries enough to know that I

will never put one of these in my full scale glider. *In addition to

chemistry -- we'll presume for the moment that the HK LiFePO4

batteries are discharge & overcharge safe -- there's the issue of

short circuit protection *in* the battery pack. *K2 puts all the

balance circuitry and such inside a sealed box where the connections

can be strain relieved, solidly anchored and protected in a way that

simply cannot be done in a hobby pack like this.

Electrically, I like your idea. *But mechanically I think it needs

to be more robust.

THe HobbyKing pack is designed toys that don't have people in them. I'd

have more confidence in it if it had a battery management system built

into it and there was a datasheet listing the manufacturer, charging

requirements, cycle life, etc.

--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)

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