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'[EE] NIMH wierdness'
2005\09\26@163611 by David Van Horn

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Anyone have an idea what's going on here?

I've hit a very significant anomaly here, and I completely can't explain
it.

I have a population of Sanyo 2200 mAH batteries.
They are pedigreed cells, no chance of them being anything other than
that.

In a recent firmware change, we switched from a 1C to a 0.5C charge
rate.

Now the batteries charge for 2 hours, with normal temperature and
voltage curves, but they discharge in 1H at 1A.
(!)

I use a 1A CCS, and I've reverified that it is pulling 1.997A.


I can't explain it at all.
I've never heard of the coulometric efficiency going that far down,
especially at a lower charge rate.

I'm going to make some trials back at 1C charge, but this will take some
time.



2005\09\26@170429 by Howard Winter

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Dave,

On Mon, 26 Sep 2005 15:36:07 -0500, David Van Horn wrote:

>...<

> I use a 1A CCS, and I've reverified that it is pulling 1.997A.

Errr - did you mean 0.997A?  If not, then I've misunderstood what you were saying...

Cheers,



Howard Winter
St.Albans, England


2005\09\26@172111 by olin piclist

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David Van Horn wrote:
> I have a population of Sanyo 2200 mAH batteries.
> They are pedigreed cells, no chance of them being anything other than
> that.
>
> In a recent firmware change, we switched from a 1C to a 0.5C charge
> rate.
>
> Now the batteries charge for 2 hours, with normal temperature and
> voltage curves, but they discharge in 1H at 1A.

Charging at .5C for 2 hours won't top off the battery since there is some
inefficiency.  NiMH are usually charged in phases:

1 - Slow charge .2C or so until voltage reaches a minimum, like 1V.  Fast
charging an empty cell can damage it.

2 - Fast charge at .5C to 1C looking for negative dV/dt, temperature rise,
or a time limit in case either of the other two fail.  Overcharging at this
rate can damage a cell.

3 - Top off at .2C or so for a fixed time of maybe 30-60 minutes.  This is
low enough current to not cause damage by overcharging the cell and it's for
a limited time anyway.  The reason for this step is that the cell is not
quite fully charged at the end of step 2.

Consult the data sheet for details since there is some difference between
brands.  For example, some manufacturers say it's OK to trickle charge long
term at .1C while other specifically say not to do that.  In general don't
unless you know for sure it's OK.

Your batteries are not being fully recharged, and they may have been damaged
when charged at 1C when nearly empty.


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2005\09\26@174914 by David Van Horn

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> > I use a 1A CCS, and I've reverified that it is pulling 1.997A.
>
> Errr - did you mean 0.997A?  If not, then I've misunderstood what you
were
> saying...

Fumblefingers.. 0.997A.. I have two CCS's the other is 2A, but no, they
aren't mismarked :)




2005\09\26@175526 by David Van Horn

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> Charging at .5C for 2 hours won't top off the battery since there is
some
> inefficiency.  NiMH are usually charged in phases:

Aware of that, and what you're describing would be a subset of what I'm
actually doing.

In this version, we are skipping the topping charge because the 0.5C
charge should bring us close enough to full, as not to matter.

> Consult the data sheet for details since there is some difference
between
> brands.  For example, some manufacturers say it's OK to trickle charge
> long term at .1C while other specifically say not to do that.  
> In general don't unless you know for sure it's OK.

Yup. These are Sanyo HR-AUC's designed for near-nicad-like trickle rate,
even though I don't actually use that.  What I do, is a 1C pulse every
so often, with an algorithm to hold the voltage within limits and expand
the pulse width as needed. If the load pulls the battery too far down,
then we go back through main charge.  We've run this past Sanyo, and a
few others with their approval.

> Your batteries are not being fully recharged, and they may have been
> damaged when charged at 1C when nearly empty.

You'll love this:  Sanyo cleared us to use an initial charge phase we
call "Boot to the head" as 1C for 30-90 seconds.  We have to do this
because the system applies a short 0.5C load when it first comes out of
reset.  Without that, we'd end up never actually making any headway on
energy when the system boots.


I'm checking now to see if the logging/graphing software is lying to me.





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