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'[EE] Battery Shelf Life Testing?'
2007\10\19@193538 by Brooke Clarke

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Hi:

I'd like to test the Sanyo eneloop Ready To Use (long shelf life) Ni-MH AA
cells to see what their shelf life really is.  Has someone already made up a
small PIC based tester to do this?  Or have any ideas on how to proceed?
http://www.prc68.com/I/RTU-Batt.shtml <- my eneloop page

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

2007\10\19@201546 by Russell McMahon

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> I'd like to test the Sanyo eneloop Ready To Use (long
> shelf life) Ni-MH AA
> cells to see what their shelf life really is.  Has someone
> already made up a
> small PIC based tester to do this?  Or have any ideas on
> how to proceed?
> http://www.prc68.com/I/RTU-Batt.shtml <- my eneloop page


Postscript - I've added a better way at the end :-)

Post-postscript.
Just reread your page.
You need to sort your tester out first.


______________

You've already done it :-)

Your page nicely describes the only available "quick test"
you are liable to be able to do.

You bought batteries that appeared to be 16 to 17 months old
(rather than the 15 you suggest) and measured the amount of
energy needed to restore them to their charged state.

Your test indicated that you inserted 15% of their nominal
energy to bring them back to a fully charged state. If your
charger is getting it "about right" then -

- NimH cells usually have a lower than nominal capacity for
the first few cycles. This MAY mean that your inserted
energy (hereafter = "charge") was a greater percentage than
nominal ratings suggest.

- Charge energy is not 100% assimilated - a figure of 80% is
used by some. Will vary with technology, maker and wind
direction.

If decay rate was linear (makes sums slightly easier) then
you lost 300/2000 in 16 months or 225 mAh/year ~= 11.3%.
If you allow charge efficiency of 80% that's a loss of only
11.3 x 0.8 =~ 9%.

That's so much lower than their claimed 15% loss per year
that I'd say that it's "extremely likely" that their claims
can be relied on in typical cases.

YMMV.
Check my assumptions.
The initial few cycles low takeup may catch you out.

___________________

BETTER TEST:

DISCHARGE a set of new batteries with known dates (as
above) - measure discharge energy.
Charge - measure charge energy.
Repeat say 10 times and plot the results.

This will allow you to refine most of the above assumptions.

Now put this set aside (preferably several sets) for say 3
months at 20 degrees C and then repeat.
Longer is better.

Even 1 month *may* do but the 1% or so loss over that period
would be low enough that you'd need to know the specific
battery history to sort result from production spread.

       Russell






2007\10\19@204513 by Stephen R Phillips

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--- Brooke Clarke <spam_OUTbrookeTakeThisOuTspampacific.net> wrote:

> Hi:
>
> I'd like to test the Sanyo eneloop Ready To Use (long shelf life)
> Ni-MH AA
> cells to see what their shelf life really is.  Has someone already
> made up a
> small PIC based tester to do this?  Or have any ideas on how to
> proceed?
> http://www.prc68.com/I/RTU-Batt.shtml <- my eneloop page
>
First you cannot draw any current from the battery if you want to
measure it's shelf life while it's on the shelf (given).
Second NiMh chemistry is very difficult to measure the capacity left in
without discharging the battery completely, the only method to measure
capacity therefore is to perform a complete discharge and periodic
intervals after the battery has been fully charged.
Third charging the battery to determine lost capacity is not a valid
method to determine lost capacity. This is because total capacity of
batteries varies between cells.

If you are just testing these for your own use .. do what's cheapest
and gives you an idea.  However if it's for any product you should make
a fully quantifiable and verifiable method and procedure  to perform
your tests.  In other words you need it to be documented very throughly
to the point anyone can repeat the test and get similar results.
Otherwise any conclusion you make is going to be questioned due to
methodology of testing.

Best idea I can think of is charge the batteries then discharge them
through a constant current sink (200ma is fine).  Look for the battery
knee (sudden voltage drop during discharge) and watch precisely when
the battery gets below 1V stop discharge at that point (do not further
discharge said cell either or you will ruin it).  You should then begin
the charge cycle afterward and wait whatever interval you want before
discharging it again.  This gives you the capacity loss over a fixed
period of time (IE say 604800 second multiples (604800 == 1 week in
seconds).  It's best to do this on multiple batteries simultaneously.
Likely 2 4 6 8 10 12 week intervals should give you enough information
on the self discharge. You might consider first qualifying each
batteries known capacity by charging and discharging each battery. This
value can be represented as a battery agnostic quantity known as C and
the curves can be scaled based on that.

That's a bit of a start.  Testing batteries requires a bit more than a
ni mh charging system and voltmeter I guess.  The only time a valid
measurement of voltage over time is available is during discharge.

Stephen

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2007\10\20@120451 by Brooke Clarke

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Hi Russell & Stephen:

I make a number of battery adapters and want a batter understanding of how
these cells work.

Yesterday I found that the Maha C777Plus has a problem with the eneloop cells.
When I put 4 cells in a battery holder on the 777 and press the discharge
button the charger does it's normal test charge then never switches over to
discharge, but instead keeps charging the battery.

The Maha FAQ for the C777Plus mentiones this behavior when a battery pack uses
a series diode to prevent charging from the load terminal.  But there's no
diode in these cells.

I like the idea of just discharging and measuring the actual capacity after
various amounts of shelf time.  But need to come up with a way to do that.  A
Watt's Up with an external 9 Volt battery will work down to 0 volts so is good
for a single cell, but then I need to make a load that disconnects at a very
repeatable 1.000 Volts.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

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