[NSRCA-discussion] FW: Resistance and retirement

[Ed White]

Hi Mike,

Yes you can monitor IR under discharge conditions.  First I'll tell you
how, then I'll offer an opinion on why it's probably not worth the effort.
You calculate IR of a cell as cell voltage divided by current.  So put a
cell meter on the battery that displays voltage of each cell and do a
ground run up with your watt meter.  Record the cell voltages and the
current and calculate the IR's.  IR's under 20 or 30C discharge will likely
be quite different than under 1C charge (what your charger tells you) and
the IR's will change as the battery heats up and discharges.  So I'm not
sure what all this will tell you other than which cells are bad, and you
don't need IR to tell you that. Bad cell(s) will sag in voltage (which will
make them look like low IR even though this is not a good thing).

The only use for this that I can think of is if you plan on trying to
replace bad cells in the pack.  That's doable but generally not worth the
trouble.  I've done that once, on a new pack where I screwed up and
discharged 2 cells in a 5 cell pack (left a cell meter plugged in for 2
weeks).  I wouldn't go to the trouble for a used pack since most likely the
next weakest cell will soon need replacing and you'll just end up chasing
your tail replacing cells.  Better to just get a new pack.

Ed

On Mon, Nov 10, 2014 at 7:08 PM, Michael Cohen via NSRCA-discussion <
nsrca-discussion at lists.nsrca.org> wrote:

> Ooops, I actual meant to send this e-mail to the group!
>
> Mike C
>
> ------------------------------
> From: precisionaero at hotmail.com
> To: vincebrc at gmail.com
> Subject: RE: [NSRCA-discussion] Resistance and retirement
> Date: Mon, 10 Nov 2014 18:07:30 -0700
>
> The issue I am having is with some packs I use for an EDF. Under the
> "load" of takeoff, two of the three packs I use noticeably sag.  I took off
> after the sag the first time and was able to make it around  for a landing,
> but I don't think I took a breath the whole time.  I topped off the packs
> and ran them on the ground and could hear the sag then too.  I did not hook
> up my watt meter in line because the watt meter has Deans connectors and
> the EDF has 5.5mm bullets.  I did cycle both packs on the chargers and they
> had about 3 to 4 mOhm resistance on 5 of the 6 cells with one pack having a
> 6 mOhm resistance and the other 12.5 mOhm resistance on an individual
> cell.  I guess I am surprised the higher resistance on one particular cell
> could cause a problem, but I know the EDF systems hit the batteries really
> hard.  Any thoughts on why I am seeing an issue?  Is it related to the
> higher resistance?  Is there some device out there that tests individual
> cells under a discharge load?
>
> Mike C
>
> ------------------------------
> Date: Mon, 10 Nov 2014 11:02:18 -0600
> To: chuenkan at comcast.net; nsrca-discussion at lists.nsrca.org
> Subject: Re: [NSRCA-discussion] Resistance and retirement
> From: nsrca-discussion at lists.nsrca.org
>
>
> I use TP.  The IR is usually around 2-3 when new.  I usually use them for
> practice and contest until around IR gets around 5.  Over 5 I start to feel
> the power loss and I don't use those anymore.  I don't like to take the
> smoke risk.  In general, I get around 80-100 cycles.
>
> Vicente "Vince" Bortone
>
> On Mon, Nov 10, 2014 at 9:18 AM, Phil S. via NSRCA-discussion <
> nsrca-discussion at lists.nsrca.org> wrote:
>
>  Yes, Dan, these things work on the Smooke Principle -- if you let the
> white smoke out, they quit working...
>
> Phil Spelt, KCRC Emeritus, Secretary
> AMA 1294 Scientific Leader Member
> SPA L-18, Board Member
> (865)435-1476v, (865)604-0541c
>
>
> On 11/9/2014 5:47 PM, Dan Curtis via NSRCA-discussion wrote:
>
>  Ah, flag, that is where I was making my mistake.  I thought it was white
> smoke.
>
>  Dan
>
>
>    On Sunday, November 9, 2014 4:42 PM, Ed White via NSRCA-discussion
> <nsrca-discussion at lists.nsrca.org> <nsrca-discussion at lists.nsrca.org>
> wrote:
>
>
>   I don't go by IR differences. I eventually reach a point that in a
> practice flight I pull up for the last maneuver in the sequence and the
> battery puts up a little white flag and says, "not gonna happen".  Then I
> retire the pack.
>
> Ed
>
> On Sat, Nov 8, 2014 at 1:56 PM, Michael Cohen via NSRCA-discussion <
> nsrca-discussion at lists.nsrca.org> wrote:
>
>  Is there any rule of thumb for how much difference between individual
> cell resistances is OK before a back should be retired?
>
>  Regards,
> Mike C
>
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